DNA replication Flashcards
By what process does DNA replicate?
Semi-conservative replication
What bases are purines?
Adenine (A)
Guanine (G)
What bases are pyrimidines?
Thymine (T)
Cytosine (C)
What are the 2 stages to DNA replication?
Initiation and Elongation
What is the first step of initiation?
Unwinding DNA supercoil by topiosomerases
What is the second step of initiation?
- Separation of DNA strands, breaking the hydrogen bonds between bases using helicase
- Single stranded DNA binding proteins (SBBs) bind to prevent re-annealing
What are SSBs?
Single stranded DNA binding proteins (SBBs) that bind to the DNA to prevent re-annealing once it has been seperated
Describe the initiation process of DNA replication?
- Unwinding DNA supercoil by topiosomerases
- Separation of DNA strands, breaking the hydrogen bonds between bases using helicase
- Single stranded DNA binding proteins (SBBs) bind to prevent re-annealing
- creating 2 replication forks
At each replication fork, what are the names given the the 2 types of strands being synthesised?
Leading strand
Lagging strand
How is the leading strand synthesised?
Synthesised continuously from 5’ to 3’ end
-RNA primer added, then DNA polymerase on a sliding clamp moves along the strand adding bases
How is the lagging strand synthesised?
Synthesised discontinuously
- Multiple RNA primers placed along the lagging strand.
- DNA polymerase joins at the RNA primer and then dissociates when it reaches the next- forming Okazaki fragments
- RNase H removes the RNA primers and DNA ligase synthesises phosphodiester bonds between the fragments to join them
What is the potential problem when the RNA primers are removed in the synthesis if new strands?
The primers are removed so the ends are unreplicated, meaning chromosomes get shorter with each cell division
What is the solution to stop chromosomes getting shorter when the RNA primers are removed?
Adding telomeres, which are stretches of DNA with no informational role
-means if they are replicated it doesn’t matter
Do we run out of telomeres?
When we are born our telomeres are longer, each time a cell divides and replicates, the telomeres shorten. This continues until the telomeres run out, and the cell becomes inactive or dies, which leads to disease
What are telomeres?
Stretches of DNA that have no informational role
How do some immortal cells (e.g cancer cells) maintain telomere length?
By using telomerase, which is a reverse transcriptase that is responsible for the synthesis of telomeres
Why is the error rate for DNA replication so low?
- Structural differences of purines/pyrimidines
- Proof reading activity of DNA polymerase
- Mismatch repair
How does DNA polymerase proof read?
DNA polymerase has 3’ to 5’ exonuclease activity (backwards)
-checks the last nucleotide added, and if it is correct adds another nucleotide or if it is wrong chops it off
What is the mismatch repair of DNA?
Where newly synthesised DNA is compared and matched to the template DNA it was copied from
-if there is a change that segment is cut and replaced with the correct segment
What are the 3 types of point mutations?
Silent
Missense
Nonsense
What is a silent mutation?
Change in base, but codes for the same amino acid
What is a missense mutation?
Change in base that codes for a different amino acid, but doesn’t change the function of the protein
What is a nonsense mutation?
Change in base that produces a stop codon in the midst of the mRNA, stopping translation early, which can produce a non functional protein
What are the types of chromosomal mutations?
Deletions
Duplications
Inversions
Translocations
What is a deletion mutation?
A mutation in which one or more pairs of nucleotides are removed from a gene
What is a duplication mutation?
A mutation that involves duplication of a region of DNA on the same strand
What is an inversion mutation?
A mutation in which a chromosome piece reattaches to the original chromosome but in reverse orientation
What is a translocation mutation?
A mutation in which one part of one chromosome breaks off and attaches to another
-can be reciprocal or Robertsonian translocation
What are the 2 types of translocation?
Reciprocal
Robertsonian
What is reciprocal translocation?
A form of translocation where portions of two chromosomes break off and are exchanged
-no net loss of genetic information
What is Robertsonian translocation?
(also known as non-reciprocal)
A form of translocation where a portion of a chromosome breaks off and attaches to another
