Genetics

DNA

DNA is made up of lots of nucleotides joined together, called a polynucleotide.
Each nucleotide is made from a pentose sugar, a phosphate group, and a nitrogenous base.
The sugar is called a deoxyribose sugar.
DNA nucleotides join together to form polynucleotide strands.
They join between the phosphate group of one nucleotide and the sugar of another, forming a sugar-phosphate backbone.
Two polynucleotide strands join by formation of hydrogen bonds between the bases, this is specific base pairing.
DNA molecules are long and coiled tightly, so lots of information is stored in a small space.
DNA molecules have a paired structure, which makes it easier for it to copy itself.
The double helix structure means that DNA is very stable in the cell.
Eukaryotic cells contain linear DNA molecules which exist as chromosomes.
- DNA is really long, so must be wound up to fit in the nucleus.
- DNA is wound around proteins called histones, which help to support the DNA.
- DNA and histones are coiled up tightly to make a compact chromosome.
Prokaryotes also carry DNA as chromosomes, but the DNA molecules are shorter and circular.
The DNA isn't wound around the proteins; it condenses to fit by supercoiling.

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The order of the nucleotide bases in a gene determines the order of amino acids in a protein.
In eukaryotic DNA, introns don't code for amino acids; exons do.
Introns are removed during protein synthesis.
Eukaryotic DNA also contains regions of multiple repeats which don't code for anything either.

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Genes can exist in different forms called alleles.
Alleles code for different versions of the same characteristic.
Alleles coding for the same characteristic will be found at the same position on each chromosome.

Mutations are changes in the base sequence of DNA.
Mutations can produce new alleles of genes.
If the sequences of bases in a gene changes, a non-functional or different protein could be produced.
All enzymes are proteins. A mutation could prevent the enzyme from folding up properly.
- This may produce an active site that's the wrong shape, and so a non-functional enzyme.

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Gametes have a haploid (n) number of chromosomes, so normal body cells have the diploid (2n) number of chromosomes.

DNA unravels and replicates so that there are two copies of each chromosome, called chromatids.
DNA condenses to form double-armed chromosomes.
Meiosis I: Chromosomes arrange into homologous pairs.
The homologous pairs are then separated, halving the chromosome number.
Meiosis II: The pairs of sister chromatids in each chromosome are separated.
Four genetically different haploid cells are produced.

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Crossing Over Of Chromatids:
- This occurs in Meiosis I.
- It means that the 4 daughter cells that are formed from meiosis contain chromatids with different alleles.

Independent Segregation Of Chromosomes:
- The four daughter cells have completely different combinations of chromosomes.
- All of your cells have a combination of chromosomes from your mother (maternal) and father (paternal).
- When gametes are produced, different combinations of those maternal and paternal chromosomes go into each cell.
- This is called independent segregation.

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Genetic Bottlenecks Reduce Diversity:
- A genetic bottleneck is an event that causes a big reduction in a population.
- e.g. when a large number of organisms die before reproducing.
- This reduces the number of different alleles in the gene pool, and so reduces genetic diversity.
- The survivors reproduce, and a larger population is created.

The Founder Effect Is A Type Of Genetic Bottleneck:
- The founder effect describes what happens when a few organisms from a population start a new colony.
- Only a small number of organisms have contributed their alleles to the gene pool.
- There's more inbreeding, which can lead to a higher incidence of genetic disease.

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Selective Breeding Involves Choosing Which Organisms Reproduce:
- This leads to reduction in genetic diversity, as only organisms with desired characteristics will be bred.
- It reduces the number of alleles in the gene pool.

ADVANTAGES:
- It produces high yielding animals and plants.
- It produces organisms with high resistance to disease.
- This means that fewer drugs and pesticides will be used.

DISADVANTAGES:
- It can cause health problems.
- It reduces genetic diversity, which increases genetic diseases and also increases the susceptibility to new diseases due to the lack of alleles.

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