[FMS] MCG - DNA replication and Mutagenesis Flashcards
a nucleotide is made up of a nitrogen base, phosphate group, and deoxyribose sugar. Which position is the nitrogen base and phosphate group attached to?
nitrogen base attached to the 1’ position
phosphate group attached to the 5’ position
REMEMBER ‘ph’ in phosphate sounds like ‘f’ in ‘five’
fosphate is on carbon five
in DNA the sugar is deoxyribose, what does that mean?
that lacks an OH at the 2’ position.
what is a nucleoside
combination of the nitrogen base and the sugar.
what bond stabilises the 2 DNA strands
hydrogen bonds
what are the 2 classes of RNA
coding RNA and non-coding RNA
what is an example of coding RNA
mRNA RNA copies of genes (messenger RNA) that are produced via the process of Transcription
what are 5 examples of non-coding RNA?
rRNA RNA components of the ribosome
tRNA transfer RNA involved in the Translation of mRNA and guiding the incorporation of the correct amino acid in the synthesis of proteins
snRNA small nuclear RNA involved in the splicing of eukaryotic mRNA
miRNA microRNA involved in the regulation of gene expression
Xist involved in the inactivation of X-chromosome
where is replication initiated?
origin of replication
outline the process of DNA replication
- DNA helicase unwinds the DNA
- as a result 2 replication forks form
- SSB (single strand binding) prevents bonds from reforming
- TOPOISOMERASE prevents supercoiling
- DNA polymerase then synthesises new strands using old templates from 5’ to 3’ direction
- as a result theres a LEADING and LAGGING strand = OKOZAKI FRAGMENTS
- HOWEVER, DNA polymerase cannot initiate DNA synthesis so DNA synthesis begins with synthesis of a short RNA primer via PRIMASE
- The RNA primer is degraded by an exonuclease, leaving a gap
- DNA LIGASE joins the gap between the okozaki fragments
in DNA replication the immediate re-formation of the double helix is prevented by what?
single strand binding proteins (SSB)
whats an okozaki fragment?
leading strand and lagging strand formed in DNA replication
what does DNA polymerase require from the RNA primer to initate synthesis?
Free 3’OH
in DNA replication, when okozaki fragments are formed, how is this combatted? why is DNA polymerase not involved?
DNA polymerase cannot initiate DNA synthesis, it can only add nucleotides to pre-existing chains
RNA polymerase can initiate RNA synthesis, so DNA synthesis begins with synthesis of a short RNA primer
how many nucelotides long is the RNA primer and how is it synthesised in DNA replication?
8-10 nucleotides long, is synthesized by primase (an RNA polymerase)
role of helicase
Unwinds DNA strands
role of topoisomerase
Releases supercoils in DNA and PREVENTS SNAPPING
role of SSB
Stabilises single stranded DNA and prevent bonds forming on the strand when DNA helicase unwinds it during replication
role of primase
Makes RNA primer so that DNA synthesis can begin
role of DNA polymerase
Synthesises DNA
role of exonuclease
Removes RNA primer
role of DNA ligase
Joins adjacent Okazaki fragments
how many origins of replication in eukaryote vs prokaryote chromosomes?
eukaryotic chromosomes = multiple origins
Bacterial chromosomes = single origin
what prevents the loss of functional parts of the genome following DNA replication
during end replication paradox, end of linear chromosomes is lost, so Telomeres are added at the end
what are telomeres made up of
5’ TTAGGG 3’
what complex is formed when proteins bind to telomeres?
Shelterin complex
what does Shelterin complex do?
complex prevents end to end chromosome fusions
what happens in stem cells and gametes when the cells too short?
enzyme telomerase adds telomere repeats to the end of chromosomes
What is telomerase made of?
a reverse transcriptase enzyme and an RNA telomere repeat template
what are the 3 types of mutations?
point
indel
chromosomal
what is point mutation? what are the 3 point mutations
point mutation – single base is changed
Silent mutation (synonymous) – change does not affect the encoded amino acid
Missense mutation – the encoded amino acid is changed
Nonsense mutation – change from encoding an amino acid to a stop codon
what is an indel mutation? what are the 2 indel mutations
insertion
deletion
remember the word ‘indel’ is combination of insertion and deletion
Indel mutations can occur due to errors during DNA replication, repair, or recombination processes, or they can be induced by external factors such as mutagenic agents.
IF ANYTHING OTHER THAN 3 NUCLEOTIDES ADDED/REMOVED THEN FRAMESHIFT OCCURS
What are the 4 types of chromosomal mutations?
Inversions (inverts, flips)
Deletions
Duplications
Translocations (changes location)
what disease is caused by translocation?
CML
chronic mylegenous leukemia
chromsomes 9 and 22 affected
what disease is caused by deletion?
cri du chat syndrome
limited mental and physical development
what are the 7 causes of DNA mutagensis?
- Spontaneous errors in DNA replication – not detected via proof reading
-
Replication slippage – associated with repetitive regions
^ 2 types of this, reverse slippage (gain of repeats)
forward slippage (loss of repeats)
- Spontaneous deamination of nitrogen bases
- UV induced formation of dimers between adjacent Thymines
- Ionising radiation causing single of double-strand breaks (DSB)
- Free radicals causing single strand breaks
- Alkylating agents causing addition of alkyl groups to Guanine
how is aklylation repaired in damaged DNA?
- Direct repair of alkylation caused by methyl-nitrosourea by specific enzyme that removes the methyl group
- Removal of damage region following by re-synthesis (nucleotide excision repair NER)
how is damaged DNA repaired in homologous recombination of eukaryotes?
Repair of DOUBLE STRAND BREAK
Exonuclease removes a few bases leaving protruding 3’ single-strand ends
Free strand invades homologous intact duplex, promoted by protein Rad51
Donor strand used as template for the re-synthesis
Complex junctions resolved to leave two duplexes
3 drugs that affect cell division
Methotrexate – inhibits enzyme involved in the synthesis of thymine
Doxorubicin – binds to DNA and prevents action of topoisomerase
5-fluorouracil – inhibits enzyme involved in synthesis of thymine
How does end replication paradox occur?
- DNA polymerase only extends 3’ OH ends.
- This results in the end replication paradox - i.e. a small amount of DNA is lost from each end of a linear chromosome after each round of DNA replication.
End replication paradox = Ends of linear chromosomes are lost after each round of replication.
when are telomeres switched off?
- In somatic cells telomerase is switched off.
- So with every division telomeres get shorter.
- Eventually the telomeric sequences are lost, and useful DNA will be lost due to the end replication defect.
- This is thought to make a significant contribution to ageing.
what is an example of a monosomy
TURNER SYNDROME
which type of CFTR is dominant and which is recessive
Wild-type CFTR is Dominant - one wild-type copy is able to maintain the Cl- gradient
CFTRD508 is Recessive – no CFTR at the cell membrane, so no Cl- gradient
in a pedigree for x linked disease, would the children be affected and would there be any carriers
Will occur most frequently in males.
Cannot be passed from father to son.
All daughters of affected fathers are carriers.
What is g0 phase
A phase where a cell is quiescent
what is g1 phase
Signal committing the cell to replicate DNA received in G1
What is s phase
DNA synthesis
What is g2 phase
Preparation for mitosis
