Genetics Flashcards
What is the relationship between the degree of crossing over and the distance between two genes?
It is direct; as the distance increases, the frequency of recombination increases.
In a three-point mapping experiment what three general classes of offspring are expected (assuming crossovers occur)?
Non-crossovers
Single crossovers
Double crossovers
In a three point mapping experiment how many different genotypic classes are expected? (assuming crossovers occur)
8
What are the main checks performed by the G2/M checkpoint?
DNA replication has been completed and any DNA damage has been repaired
Do all cells that enter G0 come back to the cell cycle at some point?
No, most differentiated cells in multicellular organisms remain in G0 phase indefinitely. Some, such as cardiomyocytes or motoneurons, once they are terminally differentiated in the adult never re-enter the cell cycle.
What would be the consequence of a defective M checkpoint?
The most likely outcome will be unequal separation of chromosomes, with a daughter cell receiving more chromosomes than the other.
What do you think is the point of having a G0 in pluricellular organisms?
It allows for terminal specialization of cells or for some type of cells re-entering the cell cycle under special circumstances, for example when regeneration is required, as we explained is the case with satellite cells and skeletal muscle.
What would be the consequence of a defective G1/S checkpoint?
It may allow DNA replication to proceed even if DNA is damaged, leading to an accumulation of mutations in the daughter cells. If these mutations affect the very genes that control the regulation of the cell cycle, cells may divide uncontrollably. This is the underlying cause of many forms of cancer.
Assume that the G1 nuclear DNA content of a species is 30 picograms. What would be the expected DNA content in a somatic cell going through metaphase?
In metaphase the chromosomes have already replicated and ready to be split at the centromere, but the cell has not yet divided and therefore contains double the amount of the DNA, i.e. 60 picograms
There are two rounds of division during meiosis, and yet the amount of DNA is only halved.
When has DNA replication occurred?
Interphase, before prophase I
When does synapsis between homologous chromosomes occur?
Paquinema.
What structure is required for the chromosomes to synapse?
The synaptonemal complex.
What are chiasmata and when are they visible?
Chiasmata are places where non-sister chromatids remain bound and are thought to represent the place where DNA exchange took place during the process of crossing over. They are visible during diplonema.
Define zygonema
Zygonema is the sub-stage where synapsis between homologous chromosomes begins.
In a prokaryotic gene, what is an operator?
The region where RNA polymerase binds
What is allostery?
Changes in the structure and function of a protein triggered by the binding of a small molecule
What mutation could give rise to constitutive expression of the lac operon?
Deletion of the lacI gene. If the gene is deleted then the repressor protein will be absent and so there will be no repression of the lac operon (i.e there will be constitutive expression)
What is chromatin?
The complex of DNA and proteins that make up a eukaryotic chromosome
Why is chromatin remodelling so important for eukaryotic gene activation?
Because Eukaryotic DNA is highly packaged and DNA is not accessible to the transcription machinery when packaged in regularly spaced nucleosomes
If you compared the promoter sequences of two human genes that are both expressed in the same cell type, would you expect them to be identical?
No - they may share some promoter elements that direct cell-type specific expression but there would be other elements that would be different
Splicing takes place in the nucleus. True or False?
True - splicing takes place in the nucleus and is thought to occur co-transcriptionally.
Euchromatin has a more open chromatin structure than heterochromatin. True or False?
False - the correct answer is true. Heterochromatin is densely packaged with little transcriptional activity. Euchromatin is still packaged into nucleosomes but has a more open structure.
Each exon of a eukaryotic gene will have a start and stop codon. True or False?
False - Each exon does not have its own start and stop codon. Exons are the regions of eukarotic genes that are present in the mature mRNA. When spliced together they will form an open reading frame but they are not translated independently of each other and each exon does not have its own start and stop codon.
List the events associated with eukaryotic gene expression in the correct order.
- ) Opening up of chromatin at the promoter region
- ) Transcription initiation
- ) Splicing
- ) mRNA export from the nucleus
- ) Translation
A gene has three exons. How many introns will it have?
It will have 2 as a gene with three exons will have two introns. Organised exon1-intron-exon2-intron-exon3
Which of the following is true for the process of splicing?
- The spliceosome is composed of proteins and RNA
- Exons have to be joined with great accuracy.
- Alternative splicing can increase the coding capacity of genomes such that single genes can code for multiple protein products
- All of the above
All of the above
During transcription to which end of the transcript are ribonucleotides added?
The 3’ end
What enzyme is responsible for transcription?
RNA Polymerase - Take care not to confuse transcription with DNA replication.
The sugar component of RNA is?
Ribose
What is an Open Reading Frame?
The coding sequence of a gene, beginning with a start codon and ending with a stop codon - Open Reading Frame (ORF) refers only to the region that codes for (or is predicted to code for) a protein.
Ribosomes are composed of what?
RNA and Proteins
How many of the 64 triplet codons can be made from the three nucleotides A, U and C?
27 there are 3 positions in a triplet codon and 3 possible nucleotides at each. 3 x 3 x3 = 27
Where are anti-codons found?
Transfer RNA - Anti-codons are found on tRNA molecules and base-pair with codons on the mRNA.
What charge does DNA have?
DNA is negatively charged
DNA will move towards the positive electrode during gel electrophoresis. True or False?
True - DNA is negatively charged and so will move towards the positive electrode during electrophoresis.
Longer DNA fragments will travel faster than smaller fragments during gel electrophoresis. True or False?
True - It is the smaller fragments that will travel fastest
How can DNA be visualized following gel electrophoresis?
Using a fluorescent dye that will bind to DNA. There are various dyes that will bind to DNA. In the practical we will be use a dye called Sybr Safe that fluoresces under blue or UV-light.
What are restriction enzymes made of?
Protein
If a 4 kb plasmid contains two sites for the restriction enzyme EcoRI located 1 kb apart, what will be the products of EcoRI digestion?
Two linear DNA fragments: 3 kb and 1 kb
Define the Shine-Dalgarno sequence
Ribosome binding site on prokaryotic mRNA
Define a Poly(A) tail
A chain of ~200 adenine nucleotides that are added to the 3’ end of eukaryotic mRNAs
Define a constitutive mutant
A mutation that results in the constant expression of a gene
Define Polycistronic
An mRNA that codes for multiple proteins
Define Peptidyl Transferase
Catalytic activity of the ribosome that forms peptide bonds during translation.
