Genes Flashcards
The South African Clawed Frog
Description
- Xenopus laevis
- first cloned vertebrate
- produces eggs (oocytes) that are released and fertilised externally
- an unfertilised oocyte contains 10^12 ribosomes
- ribosomes are made in a few weeks
- synthesised, injected mRNA is very readily translated in unfertilised oocytes as they don’t translate their own proteins until after fertilisation
- following fertilisation, no new synthesis of ribosomes takes place until after the 4096 cell stage
The South African Clawed Frog
How long should it take to make 10^12 ribosomes?
- eukaryotic ribosomal RNA is a total of 45S transcript, ~12kb
- RNA polymerase binds every 100bp
- Xenopus genome contains 500 rRNA genes
- simultaneously, 120x500 = 60000
- to make 10^12 transcripts, this has to be repeated 16.7 million times
- RNA polymerase joins 20 nucleotides per second so 12kb takes 10 minutes
- this means it would take 317.5 years to make 10^12 ribosomes
The South African Clawed Frog
Gene Amplification
- during oogenesis, rDNA repeat unit is amplified
- after amplification, the cell contains 2x10^6 copies of the rRNA genes
- this means that it takes 28.9 days to make 10^12 transcripts
- rDNA repeat units are amplified as circular extrachromosomal arrays
- so they have no centromeres and telomeres so they aren’t replicated during division
- eventually the amplified extra sequences are lost by degradation
Drosophilia melanogaster
Description
- egg
- chorion (eggshell) made of protein encoded by four genes that are amplified during oogeesis
Drosophilia melanogaster
Amplification
- 4 genes on after the other the first three in one direction and the fourth is inverted
- in the middle is an origin of replication for the chromosome
- after the first round of replication there are two copies of each gene
- after the second round there are 4 copies of each gene and so on…
- this is called the onion skin model of DNA amplification
- this allows lots of chorion proteins to be made
Cellular Differentiation in Cyanobacteria
Description
- vegetative cells differentiate to form heterocysts in which nitrogen fixation takes place when nitrates are scarce
- this differentiation process involves DNA rearrangements
Cellular Differentiation in Cyanobacteria
Gene Layout
- nifKD, fdxN, 11kb, nifD, nifH, nifU, nifS, fdxN, 55kb, fdxN, nifB
- homologous recombination results in the deletion of the sequences between the nif genes
- this allows coregulation of the nif genes so that nif proteins can accumulate
Stages of the Immune Response
1) an antigen is introduced to the body
2) recognised by cells of the immune system
3) antibodies released by B-cells circulate in the blood
The Immune System
Lymphocytes
T cells - phagocytes
B cells - antibody secretion, antibodies release by B cells circulate in the bloodstream to immobilise antigens
Immune System
Recognition and Response
- antigen encounters and specifically binds to an antibody molecule bound to the surface of a B cell
- this stimulates the B cell to reproduce itself as a clone of cells
- each cell in the clone produces and secretes circulatory forms of the specific antibody
- for this to work there must be pre-existing cells displaying every conceivable antibody on the surface to ensure there exists a cell that can bind and respond to a novel antigen
Antibodies
Structure
- two heavy chains
- two light chains
- antigen binding domain that is very variable and different in each specific antibody
Somatic Recombination in Cells of the Immune System
Organisation of the Kappa-Light Chain Genes
- 350 variable regions with link regions between each
- length of DNA
- 5 junction genes
- constant region
- enzymes cut and rejoin the DNA discarding some sections so there are many combinations that can be formed from the same length of DNA
Somatic Recombination in Cells of the Immune System
Mouse Kappa Light Genes
- 350 L-V regions
- 5(4functional) J regions
- 1 constant region
- this provides 1400 combinations not including alternative splicing or mistakes in recombination
- additionally small variation in V-J recombination eventually generating a vastly increased number of variants
Somatic Recombination in Cells of the Immune System
Mouse Lambda Light Genes
- 2 L-V regions
- 4 J regions
- 1 constant region
Somatic Recombination in Cells of the Immune System
Mouse Heavy Chain Assembly
- 350 L-V regions
- 4 J regions
- 4 constant regions
- 12 D (diversity) regions
-this provides 67200 different combinations
