DNA replication

Question 1

why do cells divide

Answer 1

to produce more cells needed for growth and repair of tissues

Question 2

what is DNA replication

Answer 2

• the process where the DNA double helix separates and each strand serves as a template for the creation of a new double-stranded DNA molecule
• complementary base pairing rules ensure that the 2 new strands are identical to the original

Question 3

semi-conservative replication

Answer 3

• double helix structure unwinds and separates into 2 strands
• hydrogen bonds are broken
• free DNA nucleotides will pair with their complementary bases which are exposed as the strands separated
• hydrogen bonds form between them
• the new nucleotides join to their adjacent ones with phosphodiester bonds in condensation reactions
• 2 new molecules of DNA are produced
• each one consists of one old strand and one new strand

Question 4

what does the enzyme DNA helicase do

Answer 4

• unwinds and separates the 2 strands of DNA double helix
• travels along the DNA backbone, catalysing reactions that break the hydrogen bonds between bases
• ‘unzipping’

Question 5

what does the enzyme DNA polymerase do

Answer 5

• catalyses the formation of phosphodiester bonds between free nucleotides that pair with the newly exposed bases of the template strands.

Question 6

Continuous replication

Answer 6

• the strand that is unzipped from the 3’ end can be continuously replicated as the strand unzips
• this is the leading strand which undergoes continuous replication

Question 7

discontinuous replication

Answer 7

• the strand that is unzipped from the 5’ end
• DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand
• this results in DNA being produced in sections called Okazaki fragments
• these have to be joined
• this strand is the lagging strand which undergoes discontinuous replication

Question 8

direction of DNA polymerase replication

Answer 8

• DNA polymerase moves along the template strand in the same direction
• it travels in the direction of 5’ to 3’
• DNA only unwinds in 1 direction so DNA polymerase has to replicate each of the template strands in opposite directions

Question 9

what does the enzyme DNA ligase do

Answer 9

• catalyses the joining of the Okazaki fragments into a single strand

Question 10

what is a mutation

Answer 10

when a random and spontaneous error occurs which lead to a change in the sequence of bases in a newly-copied strand.

Question 11

what is the genetic code

Answer 11

how DNA codes for a sequence of amino acids

Question 12

what is a triplet code

Answer 12

• the code in the base sequence along the chain of nucleotides that make up the strands of DNA
• a sequence of 3 bases is called a codon
• each codon codes for an amino acid

Question 13

what is a gene

Answer 13

a section of DNA that contains the complete sequence of bases(codons) to code for an entire protein

Question 14

what is a start codon

Answer 14

• it comes at the beginning of a gene and signals the start of a sequence that codes for a protein
• this ensures that the codons are read ‘in frame’ so the genetic code is non-overlapping
• if it is in the middle of a gene, it codes for methionine

Question 15

what are stop codons

Answer 15

they do not code for amino acids
they signal the end of the sequence

Question 16

why is the code known as degenerate

Answer 16

• there are a lot more codons than amino acids
• as there are only 20 amino acids
• therefore, many amino acids can be coded for by more than one codon