ALL OF BIOLOGY explained in 17 Minutes

Summary

This script takes a fascinating journey through the origins of Earth and the fundamentals of biology. It explains the transformation from a fiery ball to a habitable planet, the emergence of life from hydrothermal vents, and the basic building blocks of life, including cells, enzymes, and genetic material. It delves into the complexity of genetics, the process of evolution, and the human body's intricate systems, highlighting the importance of understanding our biological makeup.

Takeaways

• 🌌 The Earth began as a ball of flaming rocks bombarded by space debris, which contributed water that turned into steam as the planet cooled.
• 💧 Hydrothermal vents played a crucial role in early Earth, providing heat and chemicals necessary for the emergence of life's building blocks.
• 🔬 Biology is essentially chemistry in action, with life forms being complex arrangements of molecules capable of metabolism, growth, reproduction, and response to stimuli.
• 🌿 Enzymes are special proteins that act as catalysts to speed up chemical reactions, essential for life processes.
• 🔬 Eukaryotic and prokaryotic cells differ in their structure, with eukaryotes having membrane-bound organelles and prokaryotes lacking these features.
• 🔬 The classification of life forms into kingdoms is based on taxonomic ranks that reflect the relationships among different species.
• 🔬 Homeostasis is the process by which organisms maintain stable internal conditions, essential for the proper functioning of enzymes and cellular processes.
• 🚰 The cell membrane's selective permeability allows for the regulation of what enters and exits the cell, maintaining necessary chemical balances.
• 🔋 ATP, or Adenosine Triphosphate, is the energy currency of the cell, derived from the chemical bonds between phosphate groups.
• 🌱 Plants, as autotrophs, produce their own glucose through photosynthesis, while humans, as heterotrophs, consume food to obtain glucose.
• 🧬 DNA stores genetic information through the sequence of its nitrogenous bases, which is transcribed into RNA and then translated into proteins.

Q & A

How did the Earth initially form according to the script?

The Earth initially formed as a big ball of flaming rocks about 4.5 billion years ago, constantly bombarded by more rocks from space, which likely contained water that turned into steam.

What is the role of hydrothermal vents in the early Earth?

Hydrothermal vents played a significant role as they were piping hot and filled with chemicals that could create interesting substances, contributing to the early chemical processes on Earth.

What is the basic unit of life according to the script?

The basic unit of life is the cell, which can be classified into two main categories: Eukaryotes and Prokaryotes.

How do Eukaryotes and Prokaryotes differ in terms of their cellular structure?

Eukaryotes have organelles bound by membranes, including a nucleus containing DNA, while Prokaryotes lack these organelles and have DNA that floats freely within the cell.

What is the function of enzymes in living organisms?

Enzymes are special proteins that act as catalysts to speed up chemical reactions by breaking down or combining specific substances, enabling life processes to occur more efficiently.

How does the cell maintain homeostasis?

Cells maintain homeostasis by keeping certain conditions, such as pH levels and chemical concentrations, in check to ensure that enzymes and other cellular processes function properly.

What is the role of the cell membrane in maintaining the cell's internal environment?

The cell membrane, being a semipermeable phospholipid bilayer, controls what substances enter and exit the cell, allowing small molecules to pass through while requiring larger particles like ions to use specific channels.

What is the process by which cells balance out gradients known as?

The process by which cells balance out gradients is known as 'diffusion,' which can occur passively or actively with the use of energy from ATP.

How do plants produce their own glucose?

Plants produce their own glucose through a process called photosynthesis, where chlorophyll in chloroplasts absorbs light energy to split water and carbon dioxide into glucose and oxygen.

What is the genetic material of an organism and how is it replicated?

The genetic material of an organism is DNA, which consists of two strands of nucleotides. It is replicated when an enzyme called DNA polymerase synthesizes a new strand using each original strand as a template.

What is the process of transcription and translation in genetics?

Transcription is the process where the information in a gene is copied onto mRNA. Translation is the process where the mRNA is used to build proteins by assembling a chain of amino acids, with the help of ribosomes and tRNA.

How do mutations in DNA affect organisms and can they be beneficial?

Mutations in DNA can change the protein coded by a gene, potentially affecting the organism's traits. While many mutations are neutral or harmful, some can be beneficial, leading to increased fitness and natural selection.

What is the difference between bacteria and viruses?

Bacteria are prokaryotic single-cell organisms that can reproduce on their own and are treated with antibiotics. Viruses are non-cellular entities that require a host to reproduce and cannot be treated with antibiotics, relying on the host's immune system.

How do neurons transmit signals in the nervous system?

Neurons transmit signals as electrical impulses called action potentials, which travel along the axon. These signals can be modulated in strength and frequency to convey different types of information to the brain.

What is the function of neurotransmitters in the nervous system?

Neurotransmitters are signaling molecules that dock onto ion channels on the axon, causing ions to flow and change the electric potential, which can either block or trigger further action potentials in the connected neuron.

Outlines

00:00

🌏 Earth's Formation and Early Biology

This paragraph delves into the origins of Earth, starting as a fiery mass bombarded by space rocks that contributed to the planet's water through steam. It highlights the cooling process that led to Earth's first rain and subsequent flooding, creating environments like hydrothermal vents. The script introduces the concept of biology as chemistry in action, with a focus on the molecular composition of living beings, including carbohydrates, lipids, proteins, and nucleic acids. Enzymes are described as biological catalysts that facilitate life-sustaining chemical reactions. The distinction between living organisms and non-living matter is explored, emphasizing the characteristics that define life, such as metabolism, growth, reproduction, and response to stimuli. The paragraph concludes with a discussion on the cellular structure of life, differentiating between eukaryotes and prokaryotes, and introduces the taxonomic classification of organisms.

05:02

🧬 DNA, Genetics, and Cellular Life

The second paragraph explores the intricacies of DNA, the genetic material that carries the code for life. It explains the structure of DNA, including its four nucleotide bases and how they pair up through hydrogen bonds. The concept of genes as sections of DNA that code for specific traits is introduced, along with the role of proteins in performing various biological functions. The paragraph discusses the process of transcription, where DNA information is copied into mRNA, and translation, where this information is used to build proteins. It also touches on the vastness of the human genome, the presence of non-coding DNA, and the idea of genetic dominance and recessiveness in determining traits. The discussion on cell division includes mitosis for growth and repair, and meiosis for gamete production, highlighting the importance of genetic diversity and the formation of zygotes.

10:05

🔬 Cellular Functions and Genetic Inheritance

This section delves deeper into the mechanisms of cellular function and genetic inheritance. It starts by explaining the cell cycle, including interphase and the various checkpoints that ensure cellular health. The paragraph discusses apoptosis as a self-destruction mechanism for unhealthy cells and introduces the concept of cancer as uncontrolled cell replication due to gene mutations. Mutations are further explored as potential drivers of evolution, with both negative and positive outcomes for species survival. The distinction between bacteria and viruses is clarified, with an emphasis on their different treatment methods. The paragraph also examines the symbiotic relationship between humans and beneficial gut bacteria, and concludes with an overview of the human body's complex organ systems, particularly the nervous system, and how electrical signals are transmitted through neurons.

15:05

🚀 Neurotransmission and Educational Resources

The final paragraph focuses on the process of neurotransmission, detailing how neurons communicate through electrical signals and neurotransmitters. It describes the resting state of a neuron, the initiation of action potentials, and the propagation of these signals along the axon. The role of the myelin sheath in speeding up signal transmission is highlighted, as well as the synaptic cleft and the release of neurotransmitters for inter-neuronal communication. The paragraph concludes with a promotional mention of Brilliant, an educational platform offering interactive lessons in various subjects, including math and its applications in biology. The offer includes a free trial period and a discount on an annual premium subscription for viewers interested in enhancing their scientific understanding.

Keywords

💡Biology

Biology is the scientific study of life and living organisms, encompassing their structure, function, growth, origin, evolution, and distribution. In the video, it is presented as the study of life that is essentially chemistry in disguise, highlighting the molecular basis of life. The script uses the term to introduce the audience to the fundamental concepts of life, such as energy production, growth, and response to the environment.

💡Enzymes

Enzymes are special proteins that act as biological catalysts, speeding up chemical reactions in living organisms by breaking down or combining specific molecules. In the context of the video, enzymes are crucial for life as they facilitate the myriad of chemical reactions necessary for metabolism, allowing organisms to produce energy and carry out essential life processes.

💡Eukaryotes

Eukaryotes are organisms whose cells contain a nucleus and other organelles bound by membranes. The video explains that eukaryotic cells are more complex than prokaryotic cells and can form multicellular organisms like plants and animals. This distinction is important for understanding the diversity of life and the cellular organization within higher life forms.

💡Prokaryotes

Prokaryotes are single-celled organisms that lack a nucleus and membrane-bound organelles, such as bacteria and archaea. The script mentions prokaryotes to contrast their simpler cellular structure with that of eukaryotes, emphasizing the diversity of life at the cellular level and the different modes of existence among living organisms.

💡Homeostasis

Homeostasis refers to the ability of an organism or cell to maintain a stable internal environment, even when external conditions change. The video script illustrates this concept by explaining how the human body sweats when warm and shivers when cold to maintain a constant temperature. This concept is central to understanding how living organisms regulate their internal conditions to ensure survival.

💡Cell Membrane

The cell membrane, also known as the plasma membrane, is a semipermeable phospholipid bilayer that surrounds and protects the cell, controlling the movement of substances in and out. In the video, the cell membrane's role in maintaining homeostasis by regulating the passage of molecules and ions is highlighted, showcasing its importance in cellular function and life processes.

💡ATP (Adenosine Triphosphate)

ATP is the primary energy currency of the cell, used to power various cellular processes. The video explains that ATP is generated through cellular respiration in the mitochondria, where glucose and oxygen are converted into water, carbon dioxide, and ATP. This concept is key to understanding how organisms derive energy from food and other metabolic processes.

💡Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. The video script describes how plants use chlorophyll in chloroplasts to absorb light energy and produce glucose and oxygen, illustrating the fundamental role of photosynthesis in the energy cycle of life on Earth.

💡DNA (Deoxyribonucleic Acid)

DNA is the hereditary material in cells, containing the genetic instructions for the development, functioning, and reproduction of all known living organisms. The video script delves into the structure of DNA, its role in storing genetic information, and the process of transcription, where DNA information is copied into RNA, highlighting the centrality of DNA in the biology of life.

💡RNA (Ribonucleic Acid)

RNA is a molecule similar to DNA that plays a crucial role in protein synthesis, acting as a messenger that carries genetic information from DNA to the ribosomes for translation into proteins. The video explains the process of transcription and translation, emphasizing RNA's function in linking genetic information to the production of proteins, which are essential for the structure and function of cells.

💡Mutation

A mutation is a change in the DNA sequence that can result in a variation of the genetic information. The video script discusses mutations in the context of gene mutations and chromosomal mutations, explaining how they can lead to changes in the traits of an organism, such as eye color, and can also result in conditions like Down syndrome. This concept is integral to understanding genetic variation and evolution.

💡Natural Selection

Natural selection is the process by which certain heritable traits become more or less common in a population over successive generations due to their effect on the reproductive success of individuals. The video uses the example of grasshoppers changing color to illustrate how natural selection can lead to adaptations that increase an organism's fitness in its environment, driving the evolution of species.

💡Nervous System

The nervous system is a complex network of nerves and cells, known as neurons, that transmit signals between different parts of the body. The video script explains how neurons generate electrical signals called action potentials, which are crucial for communication within the body. This concept is central to understanding how the body processes sensory information and coordinates responses.

💡Neurons

Neurons are specialized cells that transmit nerve impulses and are the fundamental units of the nervous system. The video describes how neurons conduct electricity along their axons, creating action potentials that enable communication within the body. Understanding neurons is essential for grasping how the nervous system functions and how we perceive and interact with the world.

Highlights

Earth's formation began 4.5 billion years ago as a ball of flaming rocks, with water-bearing rocks contributing to early steam.

The emergence of hydrothermal vents during Earth's cooling phase created environments rich in chemicals, crucial for early biological processes.

Biology is essentially chemistry, with organisms being complex molecules capable of metabolism, growth, reproduction, and environmental response.

Enzymes, as biological catalysts, are essential for speeding up life-sustaining chemical reactions by breaking down or combining specific substances.

Life is differentiated from non-living matter by its ability to metabolize energy, grow, reproduce, and maintain homeostasis.

Cells, the basic units of life, are categorized into Eukaryotes with membrane-bound organelles and Prokaryotes lacking such organelles.

Eukaryotes form complex organisms like animals and plants, while Prokaryotes are limited to single-celled life forms.

Taxonomic ranks, such as 'kingdoms,' classify and relate different species based on shared characteristics.

Each species has a unique scientific name composed of genus and species, aiding in precise identification.

Homeostasis is vital for life, maintaining stable internal conditions for enzymes and cellular functions.

The cell membrane's phospholipid bilayer selectively controls the passage of molecules, maintaining internal balance.

Diffusion and active transport are mechanisms by which cells manage the movement of molecules across the cell membrane.

ATP, derived from glucose through cellular respiration, is the energy currency for biological processes.

Plants, as autotrophs, perform photosynthesis to convert light energy into chemical energy stored in glucose.

DNA, composed of nucleotides, carries genetic information through base pairs of Adenine-Thymine and Cytosine-Guanine.

Genes, sections of DNA, code for traits by specifying sequences of amino acids in proteins through the process of transcription and translation.

RNA plays a crucial role in protein synthesis, transferring genetic information from DNA to ribosomes for translation.

Mutations, alterations in DNA sequences, can lead to changes in protein structure and function, potentially impacting an organism's survival and evolution.

Natural selection favors beneficial mutations, driving evolution as better-adapted species pass on advantageous traits.

The human body's complex organ systems work in concert to maintain life, with the nervous system facilitating rapid communication through electrical signals.

Neurons transmit information via action potentials, which are electrical signals modulated by ion flow across the neuron's membrane.

The myelin sheath in some neurons accelerates signal transmission, contributing to faster and more efficient nervous system responses.

Neurotransmitters are key in intercellular communication, bridging the gap between neurons and modulating subsequent neuronal activity.

Transcripts

Hi. You’re on a rock, floating  in space. Have did we get here? 

Well, about 4.5 billion years ago, the earth  was big ball of flaming rocks, constantly  

bombarded by even more rocks from space. Fun  fact! Those rocks probably had some water  

inside them, which has now turned into steam. Breaking news! The earth is cooling down. Oh yeah,  

did I mention tha- [it’s raining.] Whoops, everything’s flooded, but hey,  

at least there’s some cool stuff at the bottom,  like hydrothermal vents, which are piping hot  

and filled with a bunch of chemicals, that can  make some very interesting stuff. Wait a minute,  

what the heck is going on here? [Biology] 

Biology is the study of life, but really,  it’s just chemistry in disguise. I mean  

you and I are basically just a big ball  of molecules that can make funny sounds. 

Carbohydrates give you quick energy, lipids store  long term energy and make membranes, proteins make  

up tissues and nucleic acids make DNA. Also, to  make all the chemical reactions possible, living  

beings, have inside of them a bunch of enzymes. They’re special proteins that act as catalysts,  

which just means they help chemical reactions  speed up by either breaking down or combining  

one specific thing. For example, lactase  breaks down lactose, the sugar found in milk. 

Ok, so enzymes make life possible  by speeding up chemical reactions,  

but what even is…life? Scientists don’t really  seem to agree, but obviously a cat is different  

from a rock. The cat can produce energy by  metabolizing food, it can grow and develop,  

reproduce, and it responds to the  environment, whereas the rock does not. 

Also, unlike rocks, every living thing on  earth is made of cells, of which there’s  

two main categories: Eukaryotes and prokaryotes. Eukaryotes have fancy organelles which are bound  

by membranes, like the nucleus, inside of which is  DNA. Prokaryotes, have none of those organelles,  

and the DNA is just kind of chilling  there, like freely floating around. 

This is why Prokaryotes are just  single cell organisms like bacteria  

and archea whereas eukaryotes can form  complex organisms like protists, fungi,  

plants and animals. These are what’s known  as “kingdoms”, which is a taxonomic rank,  

so basically, how we classify different living  things and how they’re related to one another. 

Because there are quite a few species of  life on this planet, and naming them cat,  

dangerous cat and water cat wouldn’t really be  all that helpful, we also give every species  

a unique and unambiguous scientific name  consisting of the genus and the species. 

One thing every species has  in common is homeostasis, aka,  

keeping certain conditions in check, so ya don’t  die. If you feel warm, your body will sweat,  

if you’re cold, your body will shiver. A cell does kind of the same thing just  

that it balances out concentrations of certain  chemicals. You see, enzymes for example, only  

work in a very specific environment, let’s say at  some specific pH value. If this changes too much,  

the enzymes will denature and won’t work anymore.  To counter this, the cell needs to constantly keep  

up this specific pH value, which is controlled  by the concentration of acid and base molecules. 

Ok. But like, how does the cell do that? The secret lies in the cell membrane. You see,  

it’s a semipermeable phospholipid bilayer,  okay that’s way too many words, all it is,  

is two layers of these funky looking molecules  with a polar head and a nonpolar tail. 

This allows small molecules like water  and oxygen to slip right through,  

whereas larger particles like ions need special  channels that can be opened or closed, which  

gives the cell control of what goes in and out. Naturally, particles move with the gradient,  

so from a place of high concentration  to a place of low concentration. Or,  

in the case of water, it can also move to a place  of high solute concentration, so for example salt. 

Welcome to Biology Pro Tips Season 1, tip  of the day: do not drink too much saltwater.  

There’s a bunch of salt in saltwater, in  fact, more salt than inside of a cell,  

which means it will draw water from your cells and  dehydrate you. Yeah that’s it have a great day. 

The process of balancing out gradients is known  as “diffusion” and happens automatically, but,  

by using a little bit of energy, particles  can actively be moved against the gradient. 

The energy comes from Adenosine  Triphosphate or ATP. To be exact,  

the highly energetic chemical bonds between the  phosphate groups can be broken to obtain energy. 

This is kind of important, as  in, every organism and every cell  

needs to make ATP for example, through cellular  respiration which happens in the mitochondria: 

Together with oxygen, glucose, so sugar, is  turned into water, carbon dioxide and ATP. 

This is nice, but it only works if you already  have glucose. Humans are “heterotrophs”. They  

eat food, inside of which is sugar,  which is then broken down into glucose. 

Plants on the other hand are “autotrophs”.  Simply put, they said “screw food, I’ll just  

make my own glucose by staring at the sun”. You  see, plant cells have small organelles called  

“chloroplasts” inside of which is chlorophyll,  which absorbs red and blue light but reflects  

green light, which is why most plants look green. The absorbed energy from light is used to split  

water and make a special form of carbon dioxide  which can then be turned into glucose and oxygen.  

Okay quick recap, once you have glucose, either  from food or photosynthesis, you can do cellular  

respiration, to get energy in the form of ATP. Chemically, ATP is what’s known as a nucleotide.  

It has a phosphate group, a five carbon sugar and  a nitrogenous base. You know what else is made of  

nucleotides? Deoxyribonucleic acid, or DNA. It consists of two strands of nucleotides,  

with the sugar and phosphate groups, but the  actually important part is the nitrogenous base,  

which comes in four flavours: Adenine,  Thymine, Cytosine and Guanine. 

These bases can form base pairs through  hydrogen bonds, where Adenine goes with Thymine,  

and Cytosine goes with Guanine. These bonds  are what holds the two strands of DNA together. 

Okay, but, how the heck does that store  genetic information? I’m glad you ask! 

A “gene” is a section of this DNA  that codes for a special trait,  

by carrying a certain sequence of base pairs,  which is like a recipe for making a protein. 

Why proteins? Because they’re like really  important, they transport molecules,  

act as enzymes and determine the way you look.  For example, the difference between brown and  

blue eyes is the amount of a pigment called  “melanin” in the cells of the iris. The OCA2  

Gene codes for “P-Protein” which we believe  controls the amount of melanin in cells,  

meaning that the proteins made from this gene,  could be what determines your eye colour. 

Cool! There’s just one issue: Your DNA  and its information is in the nucleus,  

but proteins are made in organelles  called the ribosomes. How do we get the  

information from A to B? The answer is RNA. It’s kind of like DNA, just that it’s most  

often a single strand, it uses a ribose instead of  deoxyribose and instead of Thymine it uses Uracil,  

which makes it less stable, but that’s besides  the point, here’s what RNA actually does: 

Let’s say you want to make the protein  coded for by this gene. An enzyme called  

“RNA polymerase” will split the DNA and make  a strand of RNA with the complementary bases,  

essentially copying the information from the  DNA to the RNA. This is called “transcription”. 

The new strand is called messenger  RNA or mRNA, because it carries this  

message out of the nucleus to a ribosome. Remember how I said that a gene is like a  

recipe for a protein? Well, on the mRNA, which  carries the same base sequence as that gene,  

every group of three bases, which is called  a “codon”, codes for a specific amino acid,  

which are the building blocks for proteins. Welcome to Biology Pro Tips Season 1, if you want  

to decode a sequence of RNA, there is actually a  chart for that! Yeah that’s all have a great day. 

These amino acids are carried by special  molecules called transfer RNA or tRNA,  

which have a unique anticodon that can only  attach to its matching codon on the mRNA. 

The job of the ribosome is to read over codons on  the mRNA and attach the matching tRNA molecules,  

which then leave behind their amino acid. As the  ribosome moves along the mRNA and attaches more  

tRNA, which happens a couple thousand times, the  amino acids combine into a “polypeptide chain”,  

which is just a really long chain of  amino acids, that can be bunched up,  

creased, smacked and folded into a protein. Okay, let’s recap: A gene is copied onto mRNA,  

which is then used to build proteins  by assembling a chain of amino acids.  

Aka transcription and translation. Hey, this genetics stuff is pretty  

cool, can we learn more? Absolutely. Oh yeah did I mention that you have, like,  

a bunch of DNA? You have about 20000 protein  coding genes, each thousands to millions of  

bases long, and that only makes up around 1% of  your entire DNA, the rest is just non-coding. 

PLUS, almost every cell in your body contains your  entire genetic code, but genes can be turned on or  

off depending on the cell, which is good, because  otherwise your brain cells might just start  

making stomach acid, which would not be good. FUN FACT! If you were to stretch out all the  

DNA of just one single cell, it  would be about 2 meters long. 

Wait a minute, how does that fit into a  microscopic cell? Well, if you were to look inside  

the nucleus, you wouldn’t find the DNA floating  around like this or even this, no, you would  

actually find lots of these worm looking things. To be exact, DNA is coiled up around Proteins  

called “Histones”, which are then condensed into  strands of Chromatin, which are then coiled up  

even more to make tightly packed units of DNA  called “Chromosomes”, which kinda look like  

worms. Different sections on a chromosome carry  different genes, and the entire human genome is  

split amongst 23 different chromosomes, although  every body cell has 2 copies of every chromosome,  

one from the mother and one from the father. For most chromosomes, the two copies are  

said to be homologous, meaning that they carry  the same genes in the same location. However,  

the two versions of a gene can be different,  so the mother’s gene could code for brown eyes,  

while the father’s gene codes for blue eyes. These  different versions of a gene are called “alleles”. 

For most of your genes, you have 2 alleles, one on  each chromosome from either parent. These alleles  

can be dominant or recessive, which determines  which of them is expressed. For example,  

brown eye color is a dominant trait, which  is shown by an uppercase B, whereas blue  

is recessive, which is shown by a lowercase b. All this means, is that if you have the dominant  

brown allele, you will have brown eyes, no matter  what the second allele is. Only when there are  

two recessive alleles will it be expressed. With this knowledge, we can predict the future! 

Let’s look at how this trait is  inherited from parents to children: 

Both of these parents have brown eyes, but  also have a recessive blue allele in their  

genotype. Every child receives one allele  from each parent randomly, so these are the  

possible combinations for the children. Most combinations contain the dominant  

brown allele, so the child will have brown eyes.  But, there is a small chance that a child gets  

two recessive alleles and has blue eyes, even  though both parents had brown eyes! You see,  

it’s what’s on the inside that counts. Alright, that’s cool, but reality is not always  

so simple. Some genes are not fully dominant, but  not fully recessive either, which means that the  

phenotype, so the appearance, appears to mix. Crossing a red and a white snapdragon, where  

red is “dominant” and white is “recessive” gives  you a pink phenotype which is somewhere inbetween,  

aka intermediate inheritance. Or, crossing  a brown and a white cow where both colours  

are dominant could give you spotted cow, so both  phenotypes are expressed equally, aka codominance. 

Hey remember how I said almost all  chromosomes are homologous? Well,  

there’s one exception: the sex chromosomes. Females have two big X chromosomes, whereas  

males have one X and one smaller Y chromosome. These are partially homologous at the top,  

but since the Y chromosome is so small,  it’s missing genes that are present  

on the lower part of the X chromosome.  These genes are called “X-linked genes”. 

If one of these genes is a recessive trait like  colour blindness, males are stuck with that trait,  

whereas females probably have another  dominant allele, to override it. This  

is why most colourblind people are male. Now, for genes to even be passed on,  

the body has to make new cells which can  inherit the genes. There’s two main mechanisms: 

Mitosis, which is how the body makes identical  copies of body cells to grow and repair tissues,  

and Meiosis, which is how the body  makes gametes, so sperm and egg cells. 

Mitosis starts with a diploid cell, so a cell with  two sets of chromosomes. These chromosomes consist  

of one chromatid, which has to be replicated  for the new cell. After replication is when  

you see the familiar X shape consisting of  two identical sister chromatids. These are  

split into two identical diploid cells, with two  sets of chromosomes consisting of one chromatid. 

Meiosis also starts with a diploid cell, but  after replication, the chromosomes comingle  

and exchange genetic information in a process  called “crossing over”. The cell is then split  

into two non-identical haploid cells. These  have one set of chromosomes, but they still  

consist of 2 sister chromatids. These cells split  again into 4 genetically different haploid cells,  

where each chromosomes has one chromatid. Meiosis produces haploid cells, so that when two  

gametes combine into a fertilized egg or “zygote”,  it again has the correct number of chromosomes. 

This is cool, but, cell division is only a tiny  part of a cell’s entire life cycle. Most of its  

time is actually spent in interphase, aka just  chilling. All it does here, is grow and replicate  

all of its DNA, so that it actually has enough  genetic material and size to divide in M-Phase. 

There’s multiple checkpoints in the cell  cycle which are controlled by proteins  

like p53 or cyclin to check if the cell is  healthy and ready to reproduce. If a cell  

is not quite right, it’s either fixed  or it destroys itself, which is called  

“apoptosis”…or at least, that’s what it should do. Normal cells replicate until there’s no need to,  

but some cells just keep going. This is because  they don’t respond correctly to these checkpoints  

and end up replicating out of control and  functioning wrong, which is also known as cancer. 

This damaging behaviour is often a result of a  gene mutation, which is a change somewhere in the  

base sequence of a gene. This can happen during  DNA replication, when a single base is changed,  

left out or inserted into the original sequence. This often changes the protein coded for by that  

gene and let’s just say that  change is often not optimal. 

Another type of mutation happens in chromosomes,  where entire sections of DNA could be duplicated,  

deleted, flipped around or transferred between  chromosomes. The most famous chromosomal mutation  

is probably when the 21st pair of chromosomes  has an additional copy, so that there’s 3  

instead of 2. The result? Down syndrome. Mutations might seem like a bad thing,  

but actually, they can also be neutral  or even beneficial. For example,  

a species of yellow grasshoppers might  mutate and become green, which makes them  

blend in with the grass and get eaten less. Over time, you can expect to see more and  

more green grasshoppers, as their fitness  has increased. Not that kind of fitness,  

fitness as in, they can have more  offspring, because they get eaten less. 

This is natural selection and the driving  factor behind evolution, as the poorly adapted  

species gets selected against and the fittest  species, which has adapted to the environment,  

survives and and has the most offspring,  passing down the trait that made them survive. 

If you think adaptation is cool, yes,  but also it kind of sucks. You see,  

humans can get sick from bacteria or viruses,  but nowadays, we have medicine that works. Good! 

However, what if the bacteria mutates and  suddenly, the medicine doesn’t work anymore? Well,  

that’s kind of exactly what is happening,  aaand we have no clue how to fix it. So, yeah. 

Oh yeah by the way, one thing many people confuse  is bacteria and viruses, and NO, they’re not the  

same. Bacteria are prokaryotes, so they consist  of a single cell which can reproduce on its own,  

and we treat bacterial infections such as  strep throat and tetanus with antibiotics. 

Viruses are not made of cells, in fact,  we’re not even sure they’re alive. They  

share some signs of life, but they can only  reproduce inside a host, and they don’t grow,  

so it’s not really alive, but it’s not dead  either, it’s more of non-living kind of thing.  

Also, you cannot treat viral infections with  antibiotics, most of the time you just have to  

chill out and let your immune system do its thing. Now you might think bacteria are a bad thing, but  

actually, you have millions good bacteria inside  your gut. The live in symbiosis with you, so you  

give them food, and they help you digest it. Speaking of digestion, your body is made of  

many complex organ systems that work  together to make sure you don’t die,  

and I know what you’re thinking. Actually  I don’t, but I know how you’re thinking. 

The nervous system, consisting of nerves,  which connect to the spinal cord and lead  

to your brain, is made of cells called  “neurons” which can conduct electricity  

along this long tube called the “axon”. Anything you see, think and feel, it’s  

all just electrical signals going to your brain,  and your brain telling your body how to respond. 

To be exact, the signals are called “action  potentials” and happen at the same strength  

and the same speed every time, so  the only difference between “hey,  

I’m a little cold” and “OMG I AM ON FIRE” is  where it happens and how frequent the signals are. 

When a neuron is just chilling, the axon is  more negative on the inside than on the outside,  

because there’s an unbalanced amount ions. This  causes an electric potential of about -70mV. 

When there is a stimulus, signalling molecules  called neurotransmitters dock onto ion channels on  

the axon and open them, letting the ions flow and  changing the electric potential around that area. 

Now, action potentials are all or nothing.  A small stimulus won’t really do anything,  

but, if the potential exceeds about  -55 mV, boom, action potential. 

Ion channels around the stimulus  open and ions rush into the cell. 

This causes the charge distribution in that  section of the axon to reverse for a split second,  

which is called “depolarisation”. The ion channels that are next to  

this area are influenced by this and open as well,  

which causes a chain reaction and sends  the signal all the way down the axon. 

Some neurons have a myelin sheath made  of Schwann cells, which insulate the  

axon and only leave tiny gaps called nodes of  ranvier. If there’s a stimulus, the charges  

can “jump” across the nodes which transmits  the signal way faster than a normal neuron. 

But either way, at the bottom, the electric signal  reaches a terminal button, which connects the  

current neuron to the dendrites of the next. If  you zoom in, you’d notice that the two cells don’t  

even touch, there is actually a small gap. This  is once again where neurotransmitters come in: 

Once the button is depolarized, tiny packages  of neurotransmitters get released, and bind  

to receptors of following dendrite, either  blocking it from doing anything or causing  

another action potential, which repeats the cycle. Hmmm. Something in my brain’s telling me that you  

should definitely subscribe, and also, if you  want to stimulate your neurons and find out  

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