Chapter 9 - Gene Diversity And Adaptation Flashcards
What is a mutation
Any change to the quantity or the base sequence of the DNA of an organism
What is a gene mutation
A change in the sequence of bases that results in the formation of a new allele
3 types of gene mutations
- base deletion (known as point mutation as it occurs at a single point)
- base substitution (also known as point mutation as it occurs at a single point)
- base insertion
When does gene mutation occur
- spontaneously during dna replication
What is a chromosome mutation
Any changes in the structure or number of whole chromosomes
When do chromosome mutations occur
During the process of meiosis
Two forms of chromosome mutations and how the occur
Polyploidy - This is when organisms have more than two sets of chromosomes, and is mostly seen in plants.
Non-disjunction - This occurs when homologous chromosomes fail to separate in meiosis, resulting in gametes with extra or missing chromosomes, and leads to individuals with extra or missing chromosomes in all cells.
What happens during base substitution
- one base is replaced by another
Why might base substitution NOT have an effect
Due to degenerate coding as most amino acids have more than one codon that codes for the same amino acid so there will be no change in the polypeptide produced and so the mutation will have no effect
Why might base substitution HAVE an effect
The polypeptide produced will differ in a single amino acid which may form different bonds in different locations that would effect the tertiary structure of the final protein, causing the protein to be a different shape and therefore not function properly
What happens during base deletion
A base is removed from the sequence
Why can base deletion have a major effect
One deleted nucleotide causes all triplets in a sequence to be read differently as each has been shifted to the left by one base, which is called frame shift mutation to the left so this will change the amino acid sequence of the encoded polypeptide, therefore changing the primary structure of the protein causing bonds to form in different places in the tertiary structure which will cause the proteins structure and function to change
3 types of base substitution and what they do
Nonsense mutation - changes triplet codon to stop codon
Misense mutation - changes triplet codon to code for another amino acid
Silent mutation: when a different triplet codon codes for the same amino acid due to degenerate coding (or any change that does not have an effect)
What is produced in meiosis
- Daughter cells that are genetically DIFFERENT from each other
- 4 haploid daughter cells from a single diploid parent cell
What are haploid cells and diploid cells
Cells that contain only one set of chromosomes - haploid
Cells that contains two sets of chromosomes - diploid
Two mechanisms that introduce variation of cells in meiosis and when they occur
- Independent segregation of homologous chromosomes
- Crossing over between homologous chromosomes
- both happen during the first division of meiosis
What are homologous chromosomes
Pair of chromosomes with the same gene sequence, loci, centromere location and chromosomal length but different alleles (one chromosome comes from mum the other from dad)
What happens in independent segregation
1) in metaphase 1 the homologous pairs of chromosomes line up opposite each other at the equator of the cell
2) it is random which side of the equator the paternal and the maternal chromosomes from each homologous pair lie
3) these pairs are separated so one of each homologous pair ends up in the daughter cell
4) this creates a large number of possible combinations of chromosomes in the daughter cells produced
How can you calculate the number of possible combinations of chromosomes in the daughter cell produced in independent segregation
2^n
n = number of homologous pairs/ pairs of chromosomes
What happens during crossing over
Steps of crossing over:
1) During prophase I, the homologous chromosomes condense and come together to form bivalents (pair up)
2) The chromatids of each chromosome then twist around one another, forming a chiasmata.
3) When the chromosomes are separated during anaphase I, the chromatids break at the chiasmata and then reconnect to a chromatid from the other homologous chromosome, this creates a recombinant chromatid
4) This swaps alleles between the homologous chromosomes to produce different combinations of alleles on each chromosome.
Comparison of meiosis to mitosis
Meiosis
- two nuclear divisions
- produces haploid cells
- introduces genetic variation
Mitosis
- one nuclear division
- produces diploid cells
- creates genetically identical cells
How to identify meiosis in a cell cycle
- it’s the point of the cell cycle where the cells are turning from a diploid cell (2n) to a haploid cell (n)
How to calculate the possible combinations of genetically different zygotes that are possible from two parents as a result of independence segregation
(2^n)^2
N = number of chromosomes pairs
Stages of meiosis - meiosis 1
1) before meiosis starts the cell would have gone through interphase where the cell copies its chromosomes and the organelles
2) then the cell enters meiosis 1- Prophase 1 occurs - The chromosomes condense and homologous chromosomes pair up. Centrioles migrate to opposite poles of the cell where each centriole starts forming spindle fibres. The nucleolus disappears and the nuclear envelope starts to break down, leaving the chromosomes free in the cytoplasm. (Crossing over takes place)
3) Metaphase 1- the pair of homologous chromosomes line up at the equator of the cell and are each attached to the spindle fibres (independent segregation takes place)
4) Anaphase 1 - the spindle fibres shorten and the homologous chromosomes move to opposite poles (crossing over ends)
5) telophase 1 - the nuclear envelope starts to form around each set of chromosomes and the nucleolus reform and the chromosomes uncoil back to their chromatin state and at this point the cell undergoes cytokinesis dividing into two haploid cells
6) then the cells enter meiosis 2
