CGCC Flashcards
Describe the four stages of mitotic cell division.
G1 - Signal committing the cell to replicate DNA is received. Contents of the cell is duplicated (excluding chromosomes).
S - 46 chromosomes are duplicated.
G2 - Chromosomes are checked for errors and spindles form.
Mitosis - The chromosomes are separated and the cell splits (cytokinesis).
What happens if a cell doesn’t receive the signal to replicate?
It enters the quiescent G0 phase.
What is semi-conservative replication?
Where each new DNA molecule contains one parent and one daughter strand.
What is an origin of replication?
The point on the double helix where DNA replication is initiated. This is done by initiator proteins.
What unwinds DNA and what prevents immediate reformation of the double helix?
DNA helicase and single strand binding proteins.
What prevents supercoiling and how does it do this?
Topoisomerase breaks a phosphodiester bond in one of the parental strands ahead of the replication fork.
Which strand of DNA is the leading strand?
3’ to 5’
How does DNA synthesis begin and why?
It begins with a short DNA primer (synthesised by RNA primase/polymerase).
This is because DNA polymerase is unable to initiate DNA synthesis but can initiate RNA synthesis.
How do Okazaki fragments join up?
The RNA primer that precedes a fragment is degraded by an exonuclease and the gap left is filled by DNA polymerase. The missing phosphodiester bond is put in place by DNA ligase.
How are single base changes removed from DNA replication?
An exonuclease found within DNA polymerase goes backwards and removes the mistake when the movement of DNA polymerase stops as a consequence of the mistake.
What is the end replication paradox?
Where the ends of linear chromosomes (i.e. eukaryotic) are lost after each round of replication. The free 5’ end of the template.
How is the end replication paradox prevented?
The ends of chromosomes are capped by telomeres which are repeated bases. These are formed by telomerase.
Telomerase extends DNA without using a chromosomal template because it carries its own template (a short stretch of RNA).
What is thought to be a significant contribution to ageing?
The loss of telomeric sequences due to somatic cell replication. This is because telomerase is switched off in somatic cells.
What do 85% of all human primary tumours test positive for?
Telomerase activity.
What are 1) Silent 2) Missense and 3) Nonsense mutations?
1) Where a single base is changed but the same amino acid is coded.
2) Where a point mutation leads to a different amino acid being coded for.
3) Where a stop codon is created leading to premature truncation.
What are indels?
Small scale insertions and deletions.
If this occurs in a multiple of 3, the reading frame is maintained. If this does not occur in a multiple of 3, then a frameshift occurs.
What are inversions and translocations?
- Where a chunk of DNA removes itself, turns 180 degrees then reforms.
- Where a piece of DNA moves from one chromosome to another.
What are examples of chromosomal deletions and translocations?
Deletion - Cri du chat syndrome.
Translocation - Chronic myelogenous leukaemia.
What is deamination and give examples.
Deamination is the removal of an amine group from a molecule.
- C turns to U (can now pair with A)
- A turns to Hypoxanthine (now pairs with C)
What are the effects of UV-C and UV-B?
- UV-C is the most lethal form but the ozone layer absorbs it.
- UV-B is the major mutagenic fraction in sunlight and induces chemical bonds between adjacent thymines distorting DNA and causing problems during DNA replication often resulting in point mutations.
Give examples of physical and chemical mutagens.
Physical: ionising radiation (single or double strand breaks).
Chemical: nitrous acid (C to U), alkylating agents (guanine modification) and free radicals (strand breaks and base modifications).
Describe nucleotide excision repair (NER).
The region where the error has occurred is unwound by DNA helicase and then removed by nuclease. The gap is then filled by DNA polymerase.
What is the error rate of DNA polymerase and the overall error rate of DNA replication?
- 10^-7
- 10^-10
What is the Central Dogma?
An explanation of the flow of genetic information within a biological system. Information moves from nucleic acid to protein, but this flow cannot be reversed.
Where is the promoter region found on DNA?
Upstream of the transcription initiation site. Its nucleotides are marked with negative numbers.
How is prokaryotic transcription initiated?
The pribnow box upstream of the transcription start site is recognized by the Sigma factor protein. The sigma protein then allows RNA polymerase to bind to the DNA. Once transcription is initiated, sigma factor dissociates.
How is the 5’ end of mRNA capped?
GTP reacts with the 5’ end of mRNA resulting in the addition of phosphate groups. The guanine is then methylated. A 7-methylguanosine cap is formed.
What to RNA Polymerase I, II and III produce?
- Pol I = rRNAs (long)
- Pol II = mRNAs, microRNAs
- Pol III = tRNAs, rRNAs (5S)
What turns genes on and off?
When genes are switched on, the chromatin is unwound, the cytosines are unmethylated and histones are acetylated.
When genes are off, chromatin is condensed, cytosines are methylated and histones are deacetylated.
What is Rett syndrome?
A developmental brain disorder that is caused by mutations in methyl CpG binding protein (MECP2). This leads to a loss or reduced levels of protein within the brain.
What is the mutation that causes sickle cell anaemia?
Glutamate (GAG) to Valine (GTG). Point/missense mutation.
What is ‘wobble pairing’?
This is pairing between two nucleotides in RNA molecules that does not follow the traditional Watson-Crick base pair rules (A-T, G-C). This occurs between mRNA and tRNA.
What makes up a 70S complex within an E-coli ribosome?
Large subunit (50S) and small subunit (30S).
Describe the process of initiation.
The initiation complex is formed comprising of a ribosome, mRNA and initiator tRNA. Initiation factor proteins IF1, 2 and 3 are required. IF2 is activated by binding to GTP.
What is the Shine-Dalgarno sequence?
A sequence upstream of AUG which binds to 16S rRNA. It ensures translation along the correct reading frame.
Found only in prokaryotes.
Describe the process of elongation.
The activated amino acid binds to elongation factor EF-Tu-GTP and enters the A site of the ribosome. Peptide bonds form between amino acids in P and A sites using the energy from EF-Tu-GTP hydrolysis. EF-G-GTP then binds and ribosome translocases using GTP hydrolysis so that the A site is free again.
Describe the process of termination.
When the stop codon of mRNA reaches the A site, a release factor (RF1 or 2) binds. This causes hydrolysis of the protein from tRNA. The ribosome complex is then disassembled using IF3, ribosomal recycling factor and GTP hydrolysis.
How do some antibiotics work? Give some examples.
They distinguish between eukaryotic and prokaryotic transcription and translation.
Examples:
- Streptomycin
- Tetracyclines
- Cycloheximide
What is recombinant DNA technology also known as and why?
Gene cloning because copies (or clones) are made up of unique pieces of DNA.
Where are restriction enzymes derived from and why?
They are derived from bacteria because they use these enzymes as a defence against viral infections.
What is a method used to replicate and amplify DNA fragments?
The use of plasmid cloning vectors.
How can harvesting mRNA from different cells help genetic studies?
Because not all cells express the same genes, harvesting mRNA from different cells allows you to see which genes are expressed by which cells and how this changes in disease.
What is cDNA and why is it used?
cDNA is called complementary DNA. It is a DNA copy of mRNA made by using an enzyme called reverse transcriptase. cDNA is used because you cannot clone RNA. The introns are removed and the clone size is smaller.
Describe the steps needed to make cDNA.
- Add oligo(dT) primer to the poly-A tail of the mRNA to be copied.
- Add reverse transcriptase which extends the primer forming a complementary strand.
- Partially digest the RNA with RNase H leaving small sequences behind.
- Add DNA polymerase I which recognises the small RNA sequences and synthesises a complementary strand.
- Add DNA ligase to seal the gaps.
How do you add cDNA strands to bacterial vectors?
- Protect EcoRi sites in cDNA from digestion using EcoRi methylase.
- Add ligate linkers containing EcoRI sites to the ends of the strand.
- Digest with EcoRI and clone into the vector.
What is the main difference between cDNA and a genomic DNA library?
cDNA is copied from mRNA so only contains the DNA that the cell was expressing at that point; the genomic DNA is every gene that the organism has within its genome regardless of what is being expressed.
What is dideoxynucleotide chain-termination sequencing?
A method of DNA sequencing where DNA polymerase is used to copy single stranded DNA, but stops when it encounters a dideoxy nucleotide.
This method can be automated and allows for rapid sequencing of large amounts of DNA.
How do dideoxynucleotides stop DNA sequencing?
ddNTPs do not have an -OH on the 3’ carbon which means that DNA polymerase cannot incorporate any further nucleotides after ddNTP as there is no 3’ OH which is needed to form a phosphodiester bond.
What methods does automated DNA sequencing utilise?
- The use of fluorescent tags on each of the different ddNTPs. (ddATP is green, ddTTP is blue etc.).
- The use of a capillary gel to separate the fragments - uses a laser to stimulate the fluorescent tags to emit wavelengths.
What is the 100,000 Genome Project?
A project that is sequencing genomes of humans from various backgrounds, with rare diseases or cancer to investigate the basis of variation in humans and help to identify the genetic basis of disease, explain individual susceptibility to disease, response to drugs or reaction to environmental factors, and identify single base pair differences (SNPs) between individuals.