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Topic 3- DNA
The Structure of DNA
DNA is a polynucleotide made up of monomers called nucleotides.
Each nucleotide is composed of three parts- a deoxyribose sugar, a phosphate
group and an organic base.
There are four bases- adenine, thymine, guanine and cytosine.
Adenine and guanine are purines and thymine and cytosine are pyramidines.
The sugar phosphate backbone is held together by bonds between the phosphate of
one nucleotide and the deoxyribose of another.
Bases of the 2 polynucleotide strands pair up A-T and C-G by forming hydrogen bonds
between molecules. This joins the 2 strands together forming a double-helix shape.
The bases only pair up A-T and C-G because this creates the right distance between the sugar phosphate backbones.
A section of DNA is a chromosome. A gene is a section of DNA that codes for a particular polypeptide.
Each sequence of three bases codes for an amino acid and a sequence of amino acids make up a polypeptide.
The code for the polypeptide is just on one strand but there are two strands to provide stability and to allow DNA replication.
DNA is divided into chromosomes. Humans have 46 chromosomes, or 23 pairs. A gene occupies a fixed position on a strand of DNA. This
position is called a locus.
1. The enzyme DNA helicase breaks the hydrogen bonds between the base pairs, unzipping the DNA into two single polynucleotide
2. Each strand acts as a template for free nucleotide bases to attach to. The free nucleotides match up by base pairing A-T and C-G.
3. DNA polymerase joins up the sugar-phosphate backbone of new strands. Hydrogen bonds form between new base pairs.
Evidence for Semi-conservative Replication- The Meselson and Stahl
1. Bacteria are grown on nutrients containing the heavy isotope of
2. When the DNA of these bacteria is centrifuged it forms a layer near the
bottom of the centrifuge tube because the DNA is heavier.
3. The bacteria are transferred to light N14 nutrients for one generation.
4. The centrifuged DNA settles near the middle of the centrifuge
tube because it is half heavy and half light.
5. The next generation is grown on N14 nutrients again.
6. When the DNA is centrifuged it settles in 2 bands- one near the middle
and one near the top.
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Introns and Multiple Repeats
Introns are lengths of non-coding DNA.
Introns have to be removed from mRNA before the protein is synthesised.
Multiple repeats are repeated base sequences between genes.
Meiosis is the process of cell division that produces gametes.
Gametes are produced every time one cell divides by meiosis.
As humans inherit one set of genes from each parent, gametes only
need one set of chromosomes. Meiosis halves the number of
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Further variation occurs due to fertilisation being a random process- any sperm can fertilise any egg.…read more