Bridle Genetics Flashcards
Why does evolution happen by natural selection?
- Overproduction (too many offspring and finite resources)
- Variation between individuals in ability to use these resources
- Some of this variation can be passed between generations
When did Eukaryotes evolve?
1.5-2 billion years ago
What are the four levels of packing of DNA so it fits in a nucleus?
- Nucleosomes (subunit of chromatin)
- 30nm looped fibre
- Looped domains (basic structural unit of eukaryotic chromatin
- Metaphase chromosome
What percentage of DNA is in functional genes?
5%, the rest is ‘junk’ DNA
How does the p53 protein regulate the cell cycle?
p53 levels are increased in damaged cells
It blocks the cell cycle if DNA is damaged, giving time to repair the DNA
If the DNA damage is severe the p53 protein can cause apoptosis
This prevents damaged cells skipping the resting stage and replicating without control
A p53 mutation is the most frequent mutation leading to cancer
What are the 6 steps of mitosis?
Prophase Prometaphase Metaphase Anaphase Telophase Interphase
What is prophase?
Condensation and spindle formation. Chromatic begins to condense and becomes visible as chromosomes. The nucleolus disappears and centrioles begin moving to opposite poles of the cell. Some fibres cross the cell to form the mitotic spindle
What happens in prometaphase?
Migration to centre
The nuclear membrane dissolves, and proteins attach to the centromeres creating kinetochores. Microtubules (spindle fibres) attach at the kinetochores and the chromosomes begin moving
What happens in metaphase?
End of migration
Spindle fibres align the chromosomes along the middle of the cell. This line is the metaphase plate and it helps to ensure that when the chromosomes are separated each new nucleus will receive one copy of each diploid chromosome
What happens in anaphase?
Separation and migration to the poles of the cell
The pie chromosomes separate at the centromeres and move to opposite sides of the cell
What happens in telophase?
Splitting of cells
Chromatids arrive at opposite sides of the cell and new membranes form around the daughter nuclei. The chromosomes and spindle fibres disperse and are no longer visible under a light microscope. Cytokinesis may begin
What happens in interphase?
The G1, S and G2 of the cell cycle. Chromosomes not visible
Not part of mitosis
Who discovered meiosis and when?
Oskar Hertwig
1876
Cost of meiosis
Sexual reproduction affects short term fitness
Risk of increased predation
Risk of time lost
Risk of energy lost
Each allele only has 50% chance of being passed on
What happens in prophase I of meiosis?
DNA replication has already occurred, the homologous chromosomes pair and synapse to form bivalents/tetrads (4 chromatids). Chiasma form and recombination occurs at the equatorial plate. Pairs of chromosomes and genes are exchanged
What happens in late prophase I of meiosis?
The nuclear membrane disappears. One kinetochore forms per chromosome, rather than per chromatid in mitosis. The chromosomes begin to move towards the metaphase plate
What happens in metaphase I of meiosis?
Bivalents align at the metaphase plate. Orientation is random so either parental chromosome of each homologous pair can be on either side of the metaphase plate. The pairs separate and migrate to opposite ends
What happens in anaphase I of meiosis?
The chiasma separate. Chromosomes move to separate poles of the cell. Daughter cells will now be haploid
What happens in telophase I of meiosis?
Nuclear envelope reforms and cell goes on to cytokinesis, or the cell me go straight onto meiosis II. The cell splits into two cells with two haploid dyads (double chromosomes)
What happens in meiosis II?
Each dyad splits in two within the two daughter cells, generating four haploid cells. Meiosis II is just like mitosis, except there is no DNA synthesis and each haploid cell is unique because of recombination
How many chromosome combinations do humans have? (ignoring recombination)
2^n = 2^23 = 8.4 million
Problems with meiosis
- Nondisjunction - homologues don’t separate properly in meiosis I, can be lethal
- Aneuploidy - gain or loss of chromosomes e.g. Downs sydrome
- Translocation or deletion - transfer a piece of one chromosome to another, or loss of a fragment of a chromosome
- Non-reciprocal crosses - chromosomes exchange unequal-sized segments of DNA, so one partner loses more genes than it receives
What is the benefit of gene duplication?
Sub-functionalisation can follow where the new gene is free to take on a new function