BMP: DNA replication

Question 1

What is a breif outcome of DNA synthesis/ replication?

Answer 1

The DNA molecule contains a polynucelotide chains in a double helix stucture

These strands seperate and form 2 parent strands. These parent strands act as templates for 2 new complementray daughter strands to be synthesised.

The outcome is 2 DNA molecules; both containing one parent and one duaghter strand

Question 2

Why is DNA replication referred to as semi-conserative?

Answer 2

• Each strand contains a parent strand with containing the original base sequence and a new complementary daughter strand

Question 3

What needs to happen before DNA replication can occur?

Answer 3

The strands need to be seperated

Question 4

What is the mechanism by which the daughter strand is synthesised?

Answer 4

• Formation of the daughter strand involves polymerisation.
• It involves the formation of phosphodiester bonds between the 3’OH of the nucleotide at the end of the growing chain and the 5’OH of the incoming nucleotide. There is a nucleophillic attack by the terminal 3’OH group on the a-phosphoryl of the incoming nucleotide

Question 5

Where is energy obtained?

Answer 5

From the liberation of the pyrophosphate group (PPi)

Question 6

What is the first step in DNA replication. What enzyme is involved?

Answer 6

Seperation of the parental DNA. This is carried out by helicase

Question 7

Where does replication begin?

Answer 7

Specific sites termed the origin of replication

Question 8

After DNA strands have been seperated what is the next step?

What is the purpose of this?

Answer 8

• Single stranded binding proteins transiently attach. They dissociate to allow synthesis
• Prevent premature annealing (recombination of strands to double stranded fom)
• Protect the single strands from being digested by nucleases
• Remove secondary structure from SNA to allow other enzymes to function effectively upon it

Question 9

What can occur as a result of seperation of parent strands? What enzyme is involved?

Answer 9

• Unwinding of DNA can lead to supercoiling downstream
• Topisomerase is used to break the phosphodiester bonds, relieve the supercoiling and reform the phosphodiester bonds

Question 10

Describe the process of DNA replication

Answer 10

• DNA replication occurs at the replication fork. It is DNA polymerase which catalyses replication. It only can only catalyse in the 5’ to 3’ therefore both strands are synthesised differently.
• The 5’-3’ daughter strand is synthesised continuously and termed the leading strand
• The 3’-5’ duaghter strand is synthesised in disocontinously in short segments which aere jonied by DNA ligase. This is called the lagging strand
• DNA polymerase needs a free 3’OH group to add the first nucleotide too. This is provided by a primer (short RNA sequence) which is subsequetly removed and the gap filled in.

Question 11

What are the main functions of DNA polymerase

Answer 11

• DNA replication - template directed synthesis
• 5’ to 3’ exonuclease - removes RNA primer
• 3’ to 5’ exonuclease - checks correct nucleotides inserted to maintain accuracy

Question 12

What is the problem with DNA replication

Answer 12

• •Eukaryotic chromosomes are linear and have exposed ends.
• •When the lagging strand approaches the end primase commonly cannot add primer at the very end.
• •This would leave a shorter 5’ end and there would be a 3’ overhang.
• •Successive replication would result in progressive chromosome shortening and potential loss of important genes.

Question 13

How can this problem be fixed?

Answer 13

• •The presence of telomeres at the 3’ overhanging end can resolve this problem.
• •Human telomeres have the sequence 5’ TTAGGG 3’ and up to 1000 repeats are present at the ends.
• •The enzyme telomerase contains RNA with the sequence 3’ AAUCCC 5’, being the template for telomere synthesis.
• •After suitable extension of the 3’ end, primase can then bind and lagging strand synthesis is completed.

Question 14

What is the importance of telomeres?

Answer 14

• Ptotect chromaomes for detoriation or fusion with neighbouring chromosomes
• telomeres are disposable buffers at the end of chromonses which are tuncated during cell division. Their presence protects genes before them on the chromonsome from being truncated instead