Nucleic Acids

OCR A Level Bio

structure of nucleotide

• nitrogenous base

• phosphate group

• pentose monosaccharide

what is a phosphodiester bond?

when the phosphate group at the fifth carbon of the pentose sugar of one nucleotide forms a covalent bond with the hydroxyl group at the third carbon of the pentose sugar of another nucleotide in a condensation reaction

purines

• pure like Ag

• adenine and guanine

pyrimidines

• CUT like pyramid

• cytosine, uracil and thymine

cytosine and ?

guanine, forms 3 hydrogen bonds

thymine and ?

adenine, forms 2 hydrogen bonds

what is semi-conservative replication?

When two molecules of DNA are produced, they consist of one old strand of DNA and one new strand with both acting as templates.

what is the genetic code?

the DNA must code for a specific sequence of amino acids

universal - all organisms use this code

non-overlapping - triplets are read ‘in-frame’

degenerate - can be coded for more than one codon

how does DNA replication occur?

1. DNA helicase breaks the hydrogen bonds between complementary bases

2. DNA double-helix unwinds and the two strands separate

3. Free floating nucleotide bases line up and form hydrogen bonds with the complementary bases

4. DNA polymerase synthesises phosphodiester bonds between 5’ and 3’ hydroxyl groups

• The leading strand going in the direction 5’ to 3’ is synthesised continuously as it is going in the direction of the replication fork

• The lagging strand going in the direction 3’ to 5’ is not synthesised continuously because it is not in the direction of the replication fork. Therefore Okazaki fragments form while the lagging strand is being synthesised. DNA ligase then seals these gaps by forming phosphodiester bonds between the phosphate and hydroxyl groups

how does transcription of protein synthesis occur?

1. RNA polymerase binds to the locus of the target gene

2. When it binds to the target gene, RNA helicase unzips and separates the DNA strands

3. Free-floating RNA nucleotides bind to the template strand (the strand whose base sequence is complementary to the base sequence is complementary to the base sequence of the target gene)

4. RNA polymerase forms phosphodiester bonds between the nucleotides

5. RNA polymerase reaches a STOP codon

6. mRNA is now removed from the template strand

7. the DNA strands join back together

8. mRNA leaves the nucleus

how does translation of protein synthesis occur?

1. mRNA that has been transcripted, binds to a ribosome

2. 2 codons can fit a ribosome at a time, so 1 molecule of tRNA binds to the first codon because it has an anticodon that is complementary to the mRNA’s codon

3. An amino acid is bound to the tRNA using ATP. Another tRNA binds to the second codon in the mRNA - now both amino acids form a peptide bond

4. The ribosome then moves along the mRNA until it reaches a STOP codon

5. The polypeptide chain is now released from the ribosome

6. The polypeptide chain is joined to other amino acids or prosthetic gorups.

structure of ATP

three phosphate groups, ribose sugar, adenosine