4.3 Genetic diversity can arise as a result of mutation or during meiosis Flashcards
What is a gene mutation?
- A change in the base sequence of DNA (on chromosomes)
- Can arise spontaneously during DNA replication (interphase)
- Involves base deletion / substitution
How can a mutation lead to the production of a non-functional protein/enzyme?
- Change in base / triplet sequence of DNA / gene
- Changes sequence of codons on mRNA
- Changes sequence of amino acids in the primary structure of the polypeptide
- Changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein
- Changes tertiary structure / shape of the protein (and active site if enzyme)
- (if enzyme) substrate can’t bind to active site and form an enzyme-substrate complex
What happens during base deletion?
- One nucleotide/base removed from DNA sequence
- Changes triplet/codon sequence from the point of mutation (frameshift)
- Changes sequence of codons on mRNA after point of mutation
What are the two different types of base substitution?
- Changes one mRNA codon and one amino acid -> sequence of amino acids in primary structure of polypeptide changes
- Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid so the sequence of amino acids in the primary structure of the polypeptide remains unchanged
What are the two different types of base substitution?
- Changes one mRNA codon and one amino acid -> sequence of amino acids in primary structure of polypeptide changes
- Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid so the sequence of amino acids in the primary structure of the polypeptide remains unchanged
What are mutagenic agents?
- Increase the rate of gene mutation (above the rate of naturally occurring mutations)
Eg/ ultraviolet light or alpha particles
What are mutagenic agents?
- Increase the rate of gene mutation (above the rate of naturally occurring mutations)
Eg/ ultraviolet light or alpha particles
What happens before meiosis starts?
DNA replicates so there are two copies of each chromosome, called sister chromatids, joined by a centromere
- 2 x 2n
What happens before meiosis starts?
DNA replicates so there are two copies of each chromosome, called sister chromatids, joined by a centromere
- 2 x 2n
What happens in meiosis I?
Separated homologous pairs
What happens in prophase 1?
- Chromosomes condense and become visible
- Chromosomes come together in homologous pairs (bivalent) (synapsis)
What happens in metaphase 1?
- Chromosomes line up across the equator in their homologous pairs
- The orientation of the pair is random (not influenced by any other pair), each member of the pair faces on opposite pole of the cell
- The chromosomes attach to the spindle by their centromeres
What happens in anaphase 1?
- Spindle pulls homologous pairs apart (opposite ends of the cell) (centromere don’t divide)
What happens in telophase 1?
- Cytokinesis
- Nuclear envelope may reform
What happens in meiosis II?
Separate chromatids
- Creates 4 haploid cells that are genetically varied
What happens in prophase 2?
- Chromosomes condense
- Nuclear envelope breaks down
- Centrioles replicate and new spindle starts to form
What happens in metaphase 2?
- Chromosomes line up across the centre of the cell and attach to the spindle
What happens in anaphase 2?
Spindle pulls centromeres apart
What happens in telophase 2?
- Nuclear envelope reform
- Cytokinesis
How does meiosis create genetic variation?
Crossing over between homologous chromosomes (prophase 1)
- Alleles exchanged between chromosomes
- Creates new combinations of maternal and paternal alleles on chromosomes
Independent segregation of homologous chromosomes (metaphase 1)
- Random alignment of homologous pairs at equator→random which chromosome from each pair goes to each daughter cell
- Creates different combinations of maternal and paternal chromosomes and alleles in daughter cells
Random fertilisation when two gametes fuse to form a zygote
What is the importance of meiosis?
- Two divisions – creates haploid gametes (half number of chromosomes)
- Diploid number restored at fertilisation
- Maintains chromosome number from one generation to the next
- Independent segregation and crossing over creates genetic variation
What is the importance of meiosis?
- Two divisions – creates haploid gametes (half number of chromosomes)
- Diploid number restored at fertilisation
- Maintains chromosome number from one generation to the next
- Independent segregation and crossing over creates genetic variation
What is non-disjunction mutation?
- Homologous chromosomes fail to separate during meiosis I OR sister chromatids fail to separate during meiosis II
- One gamete has an extra copy of this chromosome and the other has none
- Upon fertilisation, zygote has one fewer (dies) or one extra chromosome (survives)
- Arises spontaneously
- Causes genetic diseases e.g. down’s syndrome in humans – extra copy of chromosome 21
What is non-disjunction mutation?
- Homologous chromosomes fail to separate during meiosis I OR sister chromatids fail to separate during meiosis II
- One gamete has an extra copy of this chromosome and the other has none
- Upon fertilisation, zygote has one fewer (dies) or one extra chromosome (survives)
- Arises spontaneously
- Causes genetic diseases e.g. down’s syndrome in humans – extra copy of chromosome 21
