BMP: DNA replication Flashcards
What is a breif outcome of DNA synthesis/ replication?
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
Why is DNA replication referred to as semi-conserative?
- Each strand contains a parent strand with containing the original base sequence and a new complementary daughter strand
What needs to happen before DNA replication can occur?
The strands need to be seperated
What is the mechanism by which the daughter strand is synthesised?
- 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
Where is energy obtained?
From the liberation of the pyrophosphate group (PPi)
What is the first step in DNA replication. What enzyme is involved?
Seperation of the parental DNA. This is carried out by helicase
Where does replication begin?
Specific sites termed the origin of replication
After DNA strands have been seperated what is the next step?
What is the purpose of this?
- 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
What can occur as a result of seperation of parent strands? What enzyme is involved?
- Unwinding of DNA can lead to supercoiling downstream
- Topisomerase is used to break the phosphodiester bonds, relieve the supercoiling and reform the phosphodiester bonds
Describe the process of DNA replication
- 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.
What are the main functions of DNA polymerase
- DNA replication - template directed synthesis
- 5’ to 3’ exonuclease - removes RNA primer
- 3’ to 5’ exonuclease - checks correct nucleotides inserted to maintain accuracy
What is the problem with DNA replication
- •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.
How can this problem be fixed?
- •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.
What is the importance of telomeres?
- 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
