Topic 4.2 - Mutations and Meiosis Flashcards
What is a gene mutation?
Any changes in the bases of the genetic code
Why do mutations lead to changes in proteins?
A change in the base sequence causes a change the amino acids coded for in polypeptide. Depending on the changes, this may cause the bonds within the polypeptide to be different when the polypeptide folds, which can change the protein’s tertiary structure. This change in shape may affect the way the protein functions e.g. the active site of an enzyme.
Name some types of single base mutations.
Substitution, deletion, addition
Name and explain the types of substitution mutations.
Mis-sense- The change in base produces a codon that causes a different amino acid
Nonsense- The change in base produces a stop codon, which cuts the polypeptide short when translated.
Silence - The change in base produces a different codon that codes for the same amino acid due to the genetic code being degenerate (the peptide is unchanged)
What can happen when an addition or deletion mutation occurs?
A frameshift- the deletion or addition of bases causes the codons after the mutation to be read differently . This may cause all of the codons after the mutation to code for different amino acids which changes the protein a lot. However, a frame shift doesn’t occur if a full codon is deleted or added.
What is a chromosomal mutation?
When full sections of a chromosome (or whole chromosomes) are lost or added to a different chromosome. Usually occurs during cell division.
What is non-dysjunction?
When chromosomes are not separated properly during meiosis
What is polypolidy?
When non-dysjunction causes a daughter cell to have extra copies of chromosomes. It is rare in mammals, but common in plants. An example of polypolidy in humans is trisomy 21 (Downs syndrome) where people have an extra copy of chromosome 21.
What are the gametes in plants?
Ovum and pollen
Why do diploid cells (ones that divide into gametes) have to half their number of chromosomes?
Gametes have to fuse with another gamete to make a full set of genes to code for all the proteins
Why do diploid cells (ones that divide into gametes) have to half their number of chromosomes?
Gametes have to fuse with another gamete to make a full set of genes to code for all the proteins
Where does meiosis take place in?
Reproductive organs
Explain how sexual reproduction increases genetic diversity within a species?
Fertilisation of gametes is random & produces zygotes with different combinations of chromosomes to both parents
What happens before meiosis starts?
DNA unravels and replicates, forming 2 copies of each chromosomes = called chromatids
How are sister chromatids joined?
By centromere in the middle
Describe what happens in meiosis 1
- Prophase 1
- Chromosomes condense and super coil
- Metaphase 1
- Chromosomes line up along the centre of nucleus with their homologous pair (bivalents)
- Anaphase 1
- One of each homologous pair get pulled to either side of nucleus
- Telophase 1
- Nuclear envelope reforms to make 2 separate nuclei
Describe what happens in meiosis 2
- Prophase 2
- Chromosomes condense and super coil
- Metaphase 2
- Chromosomes line up along centre of nucleus still attached at centromere
- Anaphase 2
- Each chromatid gets pulled to either side of nucleus (centromere is divided)
- Telophase 2
- Nuclear envelope reforms to make 4 operate nuclei each with single copy of each chromosome
- End up with 4 haploid non-identical daughter cells (n)
What is genetic variation?
Differences in the genes of individual organisms
Describe what is happening in this diagram & what this means
- Crossing over
- Between chromatids of homologous chromosomes
- (Chromatid twist around each other & bits of chromatids swap over)
- Chromatids still contain same gene but now have different combination of alleles
Name 2 ways how meiosis produces cells that are genetically different
- Crossing over of chromatids
- Independent segregation of chromosomes
Describe how crossing over of chromatids increases genetic variation
Each of 4 daughter cells formed from meiosis contains a different chromatid with a different set of alleles
Describe how independent segregation increases genetic variation
- We don’t know which maternal or paternal chromosomes will get pulled to each side
- i.e. we don’t know which chromosome (from each pair) ends up in which daughter cell
- ∴ 4 daughter cells produced have different combinations of those maternal and paternal chromosomes
- (Leads to genetic variation in any potential offspring)
Describe and explain the appearance of this chromosome
- Chromsome is formed of 2 chromatids
- ∵ of DNA replication
- Sister chromatids are held together by centromere
What is the role of the centromere (2x)?
- Holds chromatids together
- Allows chromatids to be separated
This cell produces gametes by meiosis. Draw a diagram to show the chromosomes in one of the gametes.
3 chromosomes - one from each homologous pair
Name 3 advantages of variation (caused by meiosis) to species
- Different adaptations
- Passing on alleles/genes
- Allows for changing environment
Explain why Plant A (has 42 chromosomes) is fertile while Plant B (has 21 chromosome) is not
- Plant A’s cells are diploid - have 2 copies of each chromosome
- Plant A’s chromosomes can produce haploid gametes
- Plant A’s gametes receive a copy of every chromosome
Explain the role of independent segregation in meiosis
- To provide genetic variation
- Allows different combinations of maternal and paternal chromosomes
How do you calculate the number of different types of gametes that can be produced from a diploid cell?
Use this formula:
2 to the power of n
n = number of gene pairs
Calculate the number of different types of gametes that can be produced in a species with a diploid number of 24
2 to the power of 24 = 4096
Assuming random fertilisation, calculate the number of different combinations of maternal and paternal chromosomes in the zygotes of this species (with a diploid number of 24)
4096 x 4096 = 16 777 216
