- 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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- Genes are sections of DNA that code for proteins.
- Proteins are made from amino acids.
- 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.
- Enzymes control our metabolic pathways and so contribute to our development.
- All enzymes are proteins, and genes decide which types of protein are made.
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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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Meiosis & Genetic Variation
- Gametes join together at fertilisation to form a zygote.
- Gametes have a haploid (n) number of chromosomes, so normal body cells have the diploid (2n) number of chromosomes.
- Gametes are formed by meiosis:
- 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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Meiosis & Genetic Variation
- Meiosis produces genetically different cells:
- 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.
- 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.
- 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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