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What is DNA?

DNA stands for deoxyribonucleic acid. It's the chemical that all of the genetic material in the cell is made up from.
It contains coded information - basically all the instructions to put an organism together and make it work.
DNA is a polymer. It's made up of two strands coiled together in a shape of a double helix.


What are chromosomes?

DNA is found in the nucleus of animal and plant cells, in really long structures called chromosomes.
Chromosomes normally come in pairs.


What is a gene?

A gene is a small section of DNA found on a chromosome.
Each gene codes for (tells the cell to make) a particular sequence of amino acids which are put together to make a specific protein.


What is a genome?

Genome is just a fancy term for the entire set of genetic material in an organism.


Why is it important to understand the human genome?

1. It allows scientists to identify genes in the genome that are linked to different types of disease.
2. Knowing which genes are linked to inherited diseases could help us better understand them and treat them.
3. It allows scientists to trace the migration of certain populations around the world.


What is DNA made up of?

DNA strands are polymers made up of lots of repeating units called nucleotides.
Each nucleotide consists if one sugar molecule, one phosphate molecule and one 'base'.
The sugar and phosphate molecules in the nucleotides form a 'backbone' to the DNA strands. The sugar and phosphate molecules alternate. One of four different bases - A, T, C or G - joins to each sugar.


What is complementary base pairing?

In DNA, A always pairs with T, and C always pairs up with G.
It's the order of bases in a gene that decides the order of amino acids in a protein.
Each amino acid is coded for by a sequence of three bases in a gene.


What does mRNA do?

1. Proteins are made in the cell cytoplasm on tiny structures called ribosomes.
2. To make proteins, ribosomes use the code in the DNA. DNA is found in the cell nucleus and can't move out of it because it's really big. So the cell needs to get the code from the DNA to the ribosome.
3. This is done using a molecule called mRNA - which is made by copying the code from DNA. The mRNA acts as a messenger between the DNA and the ribosome - it carries the code between the two.
4. The correct amino acids are bought to the ribosomes in the correct order by carrier molecules.


What are the functions of proteins?

Enzymes - act as biological catalysts to speed up chemical reactions in the body.
Hormones - used to carry messages around the body. E.g. insulin is a hormone released into the blood by the pancreas to regulate the blood sugar level.
Structural proteins - are physically strong. E.g. collagen is a structural protein that strengthens connective tissue (like ligaments and cartilage).


What is a mutation?

Occasionally a gene may mutate. A mutation is a random change in a organism's DNA. They can sometimes be inherited.
Mutations occur continuously. They can occur spontaneously, e.g. when a chromosome isn't quite replicated properly. However, the chance of mutation is increased by exposure to certain substances or some types of radiation.
Mutations change the sequence of the DNA bases in a gene, which produces a genetic variant (a different form of gene). As the sequence of DNA bases codes for the sequence of amino acids that make up a protein that it codes for.


How can mutations effect proteins?

If the shape of an enzymes's active site is changed, its substrate may no longer be able to bind to it.
Structural proteins like collagen could lose their strength if their shape is changed, making them pretty useless at providing structure and support.


What are the 3 types of mutation?

1. Insertions are where a new base is inserted into the DNA base sequence where it shouldn't be.
2. Every three bases in a DNA base sequence codes for a particular amino acid.
3. An insertion changes the way the groups of three bases are 'read', which can change the amino acids they code for.
4. Insertions can change more than one amino acid as they have a knock-on effect on the bases further on in the sequence.

1. Deletions are when a random base is deleted from the DNA base sequence.
2. Like insertions, they change the way that the base sequence is 'read' and knock-on effects further down the sequence.

Substitutions mutations are when a random base in the DNA base sequence is changed to a different base.


What is sexual reproduction?

Where genetic information from two organisms is combined to produce offspring which are genetically different to either parent.
In sexual reproduction, the mother and father produce gametes by meiosis.
Sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture if their parents' genes.


What is asexual reproduction?

Where there's only one parent so the offspring are genetically identical to that parent.
Asexual reproduction happens by mitosis - an ordinary cell makes a new cell by dividing in two.
In asexual reproduction there's only one parent. There's no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring. The offspring are genetically identical to the parent - they're clones.


What is the process of meiosis?

1. Before the cell starts to divide, it duplicates its genetic information, forming two armed chromosomes - one arm of each chromosome is an exact copy of the other arm. After replication, the chromosomes arrange themselves into pairs.
2. In the first division in meiosis the chromosome pairs line up in the centre of the cell.
3. The pairs are then pulled apart so each new cell only has one copy of each chromosome. Some of the father's chromosomes and some of the mother's chromosomes go into each new cell.
4. In the second division, the chromosomes line up again in the centre of the cell. The arms of the chromosomes are pulled apart.
5. You get four gametes, each with only a single set of chromosomes in it. Each of the gametes is genetically different from the others because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them, at random.


What are the advantages of sexual reproduction?

The organism inherits genes (and therefore features) from both parents, which produces variation in the offspring.
Variation increases the chance of a species surviving a change in environment. Some individuals may be able to survive the new environment giving them a survival advantage.
Individuals with characteristics that make them better adapted to the environment have a better chance of survive, and therefore pass there genes on.
We can use selective breeding to speed up natural selection, which allows us to produce animals with desired characteristics.


What are the advantages if asexual reproduction?

There only needs to be one parent.
This means that asexual reproduction uses less energy than sexual reproduction, because organisms don'y have to find a mate.
This also means that asexual reproduction is faster than sexual reproduction.
Many identical offspring can be produced in favourable conditions.


What are examples of organisms reproducing asexually and sexually?

Malaria is caused by a parasite that's spread by mosquitoes. When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human. The parasite reproduces sexually when it's in the mosquito and asexually when it's in the human host.

Many species of fungus can reproduce both sexually and asexually. These species release spores, which can become new fungi when they land in a suitable place. Spores can be produced sexually and asexually. Asexually-produced spores form fungi that are genetically identical to the parents fungus. Sexually-produced spores form spores introduce variation and are often produced in response to an unfavourable change in environment, increasing the chance that the population will survive the change.


How do X and Y Chromosomes work?

There are 23 pairs of chromosomes in every human body cell. Of these, 22 are matched pairs of chromosomes that just control characteristics. The 23rd pair are labeled XY or XX. They're the two chromosomes that decide your sex - whether you turn out male or female.
Males have an X and a Y chromosome: XY
The Y chromosomes causes male characteristics.
Females have two X chromosomes: XX
The XX combination allows female characteristics to develop.
When making sperm, the X and Y chromosomes are drawn apart in the first division in meiosis. There's a 50% chance each sperm cell gets an X-chromosome and a 50% it gets a Y-chromosome. A similar thing happens when making eggs. But the original cell has two X-chromosomes, so all the eggs have one X-chromosome.