2.3 Nucleotides And Nucleic Acids

Question 1

Structure of DNA

Answer 1

• polynucleotide
• one purine and one pyrimidine bonded by hydrogen bonds:
  A and G are pyrimidine
  C and T are purine
• A to T or U with 3 hydrogen bonds
• C to G with 2 hydrogen bonds
• 2 polynucleotides are antiparallel
• 2 strands twist to make a double helix

Question 2

How is a polynucleotide formed?

Answer 2

Formation if phosphodiester bonds between sugar of one nucleotide and phosphate of another making a sugar-phosphate backbone

Question 3

Explain the process of semi conservative replication

Answer 3

An original stand of DNA is untwisted and H bonds between pairs are broken. This is catalysed by HELICASE (enzyme)

2 nucleotide xhains seperate exposing unpaired bases

Each strand used as a template to make a total of 2 new polynucleotides

New nucleotides complementary ro each strand of original DNA and the two chains are bonded together by the enzyme DNA POLYMERASE

Enzyme checks that base pairings are correct

Each new molecule twists to foem a double helix

Question 4

What is a mutation? Explain why it is imprtant that each new double helix is identical to the original

Answer 4

Mutation is when an inccorect base occurs changing the codon. These are random and spontaneous. New strand must be identical to original as, if there is a different base or codon, a new protein could be formed which coyld change the function of the protein or cause it to not function properly

Question 5

Give the 3 main features of the genetic code

Answer 5

Universal
Degenerate (some proteins are coded for by many codons)
Non overlapping triplets

Question 6

Explain transcription

Answer 6

1. RNA polymerase breaks H bonds between base pairs in one section of DNA
2. RNA and prescription factors (protein) bind to promoter (specific sequence in the gene)
3. 2 strands are pryed apart and one strand will serve as a template (antisense strand) and the other as a sense strand. This is INITIATION
4. Polymerase moves along template strand forming mRNA as it goes along, reading DNA from a 3’ to 5’ and forming mRNA from 5’ to 3’
5. RNA polymerase zips DNA back up as it goes along. Once the enzyme reaches the end of the gene, termination occurs and the ezyme detaches from the gene.
6. DNA back to normal but mRNA strand formed (with ribose sugar and uracil instead of thymine) and be modified during processing
7. mRNA leaves nucleus and enters the cytoplasm where it attaches to a ribosome

Question 7

Explain translation

Answer 7

mRNA codons are paired with anticodons of tRNA which are complementary to bases on mRNA

To each tRNA molecule an AA is attached

The initiator tRNA adheres to start codon and then tRNA with the correct anitcodons will bind to the codons on the mRNA

Peptide bond will form between these two AA and tRNA molecules will continue binding to mRNA in the order of the reading frame. Translation stops when stop codon reached

In the end a polypeptide is formed and is releasedd into cytoplasm and attaches to golgi body for process and packaging

Question 8

Why are nucleotides read in triplets?

Answer 8

Because codon contains 3 base pairs which give 4 cubed (64) variations. 2 base pairs wpuld gice 16 however there are 20 AA so number of variations of codons must be greater than this