DNA
- Stands for deoxyribonucleic acid - it’s the chemicals that make up all the genetic material.
- It contains coded info - all the instructions to form a working organism.
- The contents of our DNA determines our inherited characteristics.
- Found in the nucleus of animal and plant cells, in long structures called chromosomes, which normally come in pairs.
- DNA is a polymer made up of 2 strands coiled together in a double helix shape.
Gene
- A small section of DNA, found on a chromosome.
- Each gene codes for a particular sequence of amino acids, which are out together to make a specific protein.
- Only 20 amino acids are used, but they make up thousands of different proteins.
- Genes tell cells the order to put the amino acids together.
- DNA determines what proteins the cell produces, e.g. haemoglobin - this determines the type of cell e.g. red blood cell
Genome
- The entire set of genetic material in an organism, the human genome has been worked out by scientists.
- Understanding the human genome is important for science and medicine:
1. We can identify genes, which link to different diseases
2. We know which genes are linked to inherited diseases - this helps us to understand them better and could help us develop
treatments for them
3. With genomes, we can trace the migration of certain populations of people round the world - all modern humans descended
from a common African ancestor, but now we live all over the planet. The human genome is mostly identical, but as different
populations of people migrated out of Africa, tiny differences in their genomes developed. Investigating these differences
means we can work out when new populations split off and where in the world they went.
The Structure of DNA and Protein Synthesis
Nucleotides
- DNA strands are polymers made from many repeating units called nucleotides.
- Each nucleotide consists of 1 sugar molecule, 1 phosphate molecule and one ‘base’.
- The sugar and phosphate molecules alternate and form a ‘backbone’. One of the 4 different bases -
A,T,C or G - join to one sugar.
- Each base links to a base on the opposite strand in the helix.
- A pairs with T and C pairs with G - complementary base pairing.
- The order of bases in a gene decides the order of amino acids in a protein.
- Each amino acid is coded for by a sequence of 3 bases
- Amino acids are joined together to make different proteins, depending on the order of the gene’s
bases.
- Some parts of DNA don’t code for proteins, some of these control whether or not a gene is expressed (used to make proteins).
mRNA
- Proteins are made in the cytoplasm on ribosomes.
- To make proteins, ribosomes use the code in the DNA (which is found in the nucleus and can't leave the cell because it's too big) - so the
cell gets the code from the DNA to the ribosome.
- This is done using the molecule mRNA, which is made by copying the code from the DNA - it acts as a messenger between the DNA
and ribosomes and carries the code between the 2.
- The correct amino acids are brought to the ribosomes in the correct order by carrier molecules.
Proteins
, - When a chain of amino acids are assembled, they fold into a unique shape, allowing the protein to perform the task it’s meant to do:
1. Enzymes - biological catalysts to speed up bodily reactions
2. Hormones - carries messages around the body, e.g. insulin is a hormone released into the blood by the pancreas to regulate
the blood sugar level.
3. Structural Proteins - physically strong, e.g. collagen strengthens connective tissues (like ligaments and cartilage).
Mutations
- Genes mutate occasionally, which is a random change in an organism's DNA - can be inherited.
- They occur continuously and spontaneously (e.g. when a chromosome isn’t replicated properly). The chance of mutations is increased
by exposure to certain substances or radiation.
- They can change the sequence of the DNA bases, producing a genetic variant. The sequence of DNA bases code for the sequence of
amino acids that make up a protein, so mutations to a gene sometimes lead to changes in the protein it codes for.
- Most mutations are harmless to the protein, so it’s function or appearance is unaffected.
- Some however can seriously affect a protein - sometimes the mutation will code for an altered protein with a change in its shape,
potentially affecting its ability to function, e.g:
1. If the shape of the enzyme's active site changes, its substrate may no longer be able to bind to it.
2. Structural proteins could lose their strength if their shape is changed, so they lose their value of providing structural support.
- If a non-coding DNA mutates, then it can alter how genes are expressed.
Types of Mutation
There are different ways that mutations can change the DNA base sequence:
1. Insertions:
- Where a new base is inserted into the DNA sequence, where it shouldn't be. Every 3 bases in a DNA base sequence codes for a
certain amino acid.
- An insertion changes the way the groups of 3 bases are ‘read’ - this can change the amino acids they code for.
- Insertions can change more than 1 amino acid as they have a knock-on effect on the bases further on:
2. Deletions:
- When a random base is deleted from the base sequence, like insertions they change the way the sequence is ‘read’ and have
knock-on effects further down the sequence.
3. Substitutions:
- When a random base in the sequence is changed to a different base, e.g:
Sexual reproduction produces genetically different cells
- Genetic information from two organisms is combined to produce offspring which are genetically different to each parent.
- The mother and father produce gametes via meiosis. These are egg and sperm cells in animals.
- In humans, each gamete has 23 chromosomes - half the number of a normal cell. Instead of having two of each chromosome, a gamete
has one of each.
- The sperm and egg then fuse together ( fertilisation ), to form a cell with the normal amount of chromosomes.
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