DNA Replication

Question 1

Why does DNA replicate?

Answer 1

• DNA copies itself before cell division so that each new cell has the full amount of DNA.
• This is important for making new cells and for passing genetic information from generation to generation.

Question 2

How is DNA replicated?

Answer 2

1) DNA helicase (an enzyme) breaks the hydrogen bonds between the 2 polynucleotide DNA strands. Helix unzips to form 2 single strand.
2) Each original single strand acts as a template for a new strand. Free floating DNA nucleotides join to the exposed bases on orig. strand by complementary pairing.
3) DNA Polymerase joins nucleotides on the new strand. This forms the sugar-phosphate backbone. Hydrogen bonds form between the bases on the original and new strand.
4) Strands twist to form double-helix. Each new DNA molecule contains 1 strand from the original DNA molecule and 1 new strand.

Question 3

Semi conservative replication

Answer 3

• Half of the strands in each new DNA molecule are from the original piece of DNA.

Question 4

Continuous and discontinuous replication

Answer 4

• DNA polymerase always moves along template strand in same direction.
• It can only bind to 3’ (OH) end so it travels from 3->5.
• Strand that’s unzipped from 3’ can be continuously replicated as strand unzips - LEADING STRAND
• Other strand unzipped from 5’ end, and DNA Polym. has to wait until a section has unzipped and then work back along the stand. This results in DNA having OKAZAKI FRAGMENTS which are then joined - This strand is LAGGING STRAND and undergoes DISCONTINUOUS REPL.

Question 5

Replication errors

Answer 5

• Sequence of bases are not always matched exactly, and an incorrect sequence may occur in the newly copied strand.
• Errors occur randomly and spontaneously and lead to a change in sequence of bases - mutations.

Question 6

Genetic code

Answer 6

• Sequence of base triplets (codon) in DNA or mRNA which code for specific amino acids.
• Base triplets don’t share bases - the code is non-overlapping.
• Genetic code is DEGENERATE - there are more possible combinations of triplets than there are amino acids (20 a.a but 64 codons).
• This means some a.a are coded for by more than 1 codon.
• Not all code for a.a; some are ‘stop’ signals.
• Genetic code is universal - all organisms use this same code, although the sequence of triplets coding for each individual protein will be diff.