Meiosis and mutations Flashcards
What does mutation mean?
Any change to the quantity or the base sequence of the DNA of an organism
When do gene mutations occur?
can arise spontaneously during DNA replication
What are the two types of mutations? Describe what happens in each one
Substitution- A nucleotide in the DNA molecule is replaced by another nucleotide with a different base
Deletion- A nucleotide is lost from the normal DNA sequence
What are the different types of substitution mutations
Non-sense substitution: production of a stop codon on mRNA, the polypeptide would be too short so it will no longer be functional
Mis-sense substitution: different amino acid coded for, the polypeptide can still be function
Silent substitution: same amino acid coded for (because of degenerate code, some amino acids have more than one codon)
What happens during deletion mutation
Frame-shift:one deleted nucleotide causes all the triplets in a sequence to be read differently because each has been shifted to the left by one base
An addition of a base will also cause a frame shift
What are the effects of mutation?
- Production of new/superior protein:
Results: gain of reproductive advantage - Neutral mutation:
Result: No change - Production of inferior or no protein:
Result: Fatal and/or disease causing
What are mutagenic agents?
environmental factors that increase the mutation rate of cells
What are examples of mutagenic agents?
- High-energy radiation such as UV light
- Ionising radiation such as X rays
- Toxic chemicals such as peroxides
What are chromosome mutations?
changes in the structure of number of whole chromosomes
What are the two forms of chromosome mutations?
- Changes in whole sets of chromosomes: occur when organisms have three or more sets of chromosomes (polyploidy), occurs in most plants
- Changes in the number of individual chromosomes: If individual homologous pairs of chromosomes fail to separate during meiosis (non-disjunction) the resulting gametes have one more or on less chromosome than normal
What is polyploidy?
when organisms have three or more sets of chromosomes rather than the usual two
What is non-disjunction?
When individual homologous pairs of chromosomes fail to separate during meiosis
What happens during Prophase I?
DNA coils to form chromosomes, and chromosomes replicate to form sister chromatids (joined together by a centromere)
The nuclear envelope breaks down, the centriole divides into two and the two centrioles move to opposite poles of the cell and spindle fibres form.
Homologous chromosomes pair up. Crossing over occurs, where non-sister chromatids exchange sections of DNA with one another.
What happens during Metaphase I?
homologous pair of chromosomes line up in the middle of the cell and attach to spindle fibre via centromere.
The homologous chromosome pairs line up randomly on the equator (random assortment).
What happens during Anaphase I?
spindle fibres pull, homologous pair of chromosomes separate to opposite sides.
The chromosomes still exists as sister chromatids.
Due to crossing over, sections of DNA have been exchanged between the homologous chromosomes.
Due to the random nature of how chromosomes lined up in metaphase 1, they are now pulled to opposite sides of the cell randomly too (independent segregation)
What happens during Telophase I?
separated chromosomes reach the opposite poles of the cell.
The nucleus envelope forms round each set of chromosomes.
The nuclei that form now contain half the normal number of chromosomes (haploid).
The chromosomes still exist as two sister chromatic joined by a centromere.
What happens during Prophase 2?
Sister chromatids (which are no longer identical due to crossing over previously), condense again (become shorter and thicker).
The nuclear envelope breaks down.
The centrosome devices and the two centrioles move to opposite poles of the cell and spindle fibres form
What happens during Metaphase 2?
Chromosomes line up on the equator of the cell attached to spindle fibres via centrosome
chromatids arrange themselves randomly on the equator = random assortment
What happens during Anaphase 2?
The centromere splits so that the two joined chromatids are pulled apart so the chromosomes are pulled apart to opposite ends of the cell.
Due to the random nature of how chromatids lined up in metaphase 2, they are now pulled to opposite ends of the cell randomly (independent segregation)
What happens during Telophase 2?
separated chromosomes reach the opposite poles of the cell
A nuclear envelope forms around each of the four sets of chrmosomes
the four nuclei that form are all haploid but not genetically identical to one another.
describe the process of crossing over
When alleles originally on chromosomes from mum and dad can be swapper:
homologous chromosomes come together and pair up to form a bivalent.
The chromosomes in each homologous pair twist around each other.
Chromosomes break and rejoin so sections of one chromatid are swapped with the corresponding bits of the other chromatid
happens during prophase 1
What is chiasma?
the points where the chromatids cross over (during crossing over)
What is the formula for the number of possible combinations of chromosomes for each daughter cell, assuming NO random fertilisaiton happens?
2^n where n is the number of pairs of homologous chromosomes
What is the formula for the number of possible combinations of chromosomes for each daughter cell, assuming random fertilisaiton happens?
(2^n)^2
where n is the number of pairs of homologous chromosomes and ^2 represents the number of gametes
what is random assortment
homologous chromosomes line up randomly at the equator to increase genetic diversity
happen in the metaphase 1
what is independant segregation
when homologous chromosomes are separated randomly so the four daughter cells produced have completely different combination of those maternal and paternal chromosomes
happen in the metaphase 1
