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Summary CIE A level Biology notes on unit o6 - Nucleic Acids and Protein Synthesis
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Everything you need to know about unit 06 of the CIE A level Biology course, including diagrams and notes.
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6.1 - Structure of Nucleic acids and replication of DNA:• Genetic material needs to have some important features such as the ability to carry instructions for the construction of cells and that they can be
copied perfectly to pass on an exact copy to the nuclei of the daughter cells.
• DNA and RNA are nucleic acids - they are macromolecules made of many repeating monomers (nucleotides) to
form polynucleotide polymers.
• Nucleotides are made up of 3 smaller components which are a pentose sugar, a phosphate group and an organic
base, which are joined by condensation:
◦There are 5 di erent nitrogen-containing bases found in DNA and RNA.
◦In DNA there are adenine, thymine, guanine and cytosine.
◦RNA also has 4 bases, but instead of thymine, it has uracil.
◦The pentose sugar can either be ribose (in RNA) or deoxyribose (in DNA) - as the name shows, deoxyribose
has one fewer oxygen atoms in the molecule.
◦The Bases adenine and guanine are purines (GAP) which means that they have a double-ring
structure and Cytosine, thymine and uracil are pyrimidines which means that they have a single ring
structure.
DNA:
• Two nucleotides are linked together by covalent bonds (phosphodiester bonds) between the sugar of one
nucleotide and the phosphate group of another.
• Two polynucleotide chains (deoxyribose sugars) joined together by hydrogen bonds between the bases,
wound round each other into a double helix structure.
• The chains run in opposite directions (antiparallel) - 5’ to 3’ on one strand and 3’ to 5’ on the other strand.
• The bases in DNA always pair the same way (complementary pairing) - A-T via 2 hydrogen bonds and G-C via 3 hydrogen bonds (3 GREEN
CABBAGES).
• Charga ’s rules show that any sample of DNA will have equal amounts of thymine to adenine and equal amounts of cytosine to guanine.
Structure relates to the function...
• The coiled structure allows a large amount of information fo be packed into a nucleus as its very dense.
• Strong sugar phosphate backbone prevents the chain from breaking.
• the strands are held together by hydrogen bonds which allows them to separate for DNA replication and protein synthesis.
DNA Replication:
• DNA replication can be conservative (DNA remains intact and new daughter cell made from scratch), semi-conservative (DNA splits and each strand
replicated) and dispersive (parental DNA Broken down and nucleotides replicated).
• Meselson and Stahl:
◦When the centrifuging DNA from the bacteria was grown with N-15, the band of DNA was low down the tube as it is more dense.
◦As they repeated the experiments with N-14, the band of DNA it moves further up the tube as it is less dense and 2 bands appear as it is semi-
conservative.
◦It it were done conservatively, there would only be one band of DNA.
◦If it were done dispersively, the results would be more random and unpredictable.
The process...
• DNA replication is semi-conservative (half of the original molecule is kept) and takes place during the S phase of the cell cycle.
• Helicase unzips the DNA double helix by breaking the hydrogen bonds between bases to form a replication fork.
• The two seperated strands provide a template for the new DNA strands to be made.
• Primase makes a small RNA molecule called a primer which marks the starting point for the new strand.
• DNA polymerase adds the bases from the 5’ to 3’ end to form a leading strand.
• The other strand (the lagging strand) cannot be made in this way as it runs in the opposite direction, therefore the DNA polymerase can only make this
strand in small chunks called Okazaki fragments.
◦Starts by a primer giving the polymerase a start point to build on.
◦Another primer is made further down the fragment and another Okazaki fragment is formed by DNA polymerase.
• Finally, once the new strands have been made, exonuclease removes the primers from the strands and DNA polymerase lls the gaps with DNA.
• DNA ligase then seals the fragments to form 2 continuous polynucleotides.
RNA:
• RNA has the same structure as DNA, except the pentose sugar is ribose (has one more oxygen atom) and it has uracil rather than thymine.
• There are 3 types of RNA which are...
◦rRNA - large complex molecule which is a major component of ribosomes (over half their mass) - it makes the ribosomes and proteins.
◦mRNA - long strand with thousands of nucleotides in a single helix shape which is manufactured from a section of DNA - it leaves the nucleus
via the nuclear pore and acts as a template for protein synthesis.
◦tRNA - small molecule (about 80 nucleotides long) which is a single stand, folded into a clover-leaf shape with one chain extending beyond the
other - the extended chain always has the base sequence CCA where the amino acids attach - there is an anticodon at the other end which
links to the codon of mRNA - its use is to bring in the amino acids for protein synthesis.
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