Genetic Diversity can Arise as a Result of Mutation or During Meiosis

What is a gene mutation?

• A change in the base sequence of DNA on chromosomes

• Which arises spontaneously during interphase of DNA replication

What is a mutagenic agent?

A factor that increases rate of gene mutation e.g UV light

Explain how a mutation can lead to the production of a non-functional protein or enzyme

• Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA

• So changes sequence of amino acids in the polypeptide

• So changes position of hydrogen / ionic / disulphide bonds (between amino acids)

• So changes protein tertiary structure (shape) of protein

• Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form

Explain the possible effects of a substitution mutation

• DNA base / nucleotide (pair) replaced by a different base / nucleotide (pair)

• This changes one triplet so changes one mRNA codon

• So one amino acid in polypeptide changes

  • Tertiary structure may change if position of hydrogen / ionic / disulphide bonds change

OR amino acid doesn’t change

• Due to degenerate nature of genetic code (triplet could code for same amino acid) OR if mutation is in an intron so removed during splicing

Explain the possible effects of a deletion mutation

• One nucleotide / base (pair) removed from DNA sequence

• Changes sequence of DNA triplets from point of mutation (frameshift)

• Changes sequence of mRNA codons after point of mutation

• Changes sequence of amino acids in primary structure of polypeptide

• Changes position of hydrogen / ionic / disulphide bonds in tertiary

• structure of protein

• Changes tertiary structure / shape of protein

Describe features of homologous chromsomes

Same length and same genes at same loci, but may have different alleles

Describe the difference between diploid and haploid cells

Diploid- has two completes sets of chromosomes, represented as 2_n

Haploid- has a single set of unpaired chromosomes, represented as n

How does a cell divide by meiosis?

In interphase, DNA replicates so there is 2 copies of each chromosomes joined by a centromere (these are sister chromatids)

Meiosis I (first nuclear division)- separation of homologous chromosomes

• Homologous chromosomes arrange into pairs and their chromatids wrap around each other

• Crossing over between homologous chromosomes (portions of chromatids may be exchanged)

• Independent segregation of homologous chromosomes

Meiosis II (second nuclear division)

• Chromatids separate/ move apart

Outcome

• 4 genetically varied daughter cells (normally haploid)

Draw a diagram to show chromosome content of cells during meiosis

Why is the number of chromosomes halved during meiosis?

Homologous chromosomes are separated during the first division of meiosis

How does crossing over create genetic variation?

• Homologous pairs of chromosomes associate/ form a bivalent (pair of homologous chromosomes)

• Chiasmata form (point of contact/ connection between non-sister chromatids)

• Alleles/ equal proportions of non-sister chromatids are exchanged between chromosomes

• Creating new combinations of maternal and paternal alleles on chromosomes

How does does independent segregation cause genetic variation?

• Homologous pairs of chromosomes randomly align at the equator, so it is random which chromosome from each pair goes into each daughter cell

• Creating different combination of maternal and paternal chromosomes/ alleles in daughter cells

Other than mutation and meiosis, explain how genetic variation within a species is increased

• Random fertilisation of gametes creates new allele combinations/ new maternal and paternal chromosome combinations

Which three processes increase genetic variation?

• Random fertilisation

• Crossing over

• Independent segregation

Explain the different outcomes of mitosis and meiosis

• Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells

  • As 1 division in mitosis, whereas 2 divisions in meiosis

• Mitosis maintains the chromosome number (eg. diploid → diploid or haploid → haploid) whereas meiosis halves the chromosome number (eg. diploid → haploid)

  • As homologous chromosomes separate in meiosis but not mitosis

• Mitosis produces genetically identical daughter cells, whereas meiosis produces genetically varied daughter cells

  • As crossing over and independent segregation happen in meiosis but not mitosis

When is the process of meiosis used and briefly explain it?

Sexual reproduction which creates gametes: sperm and eggs in animals

As a cell divides to form gametes:

• copies of the genetic information are made

• the cell divides twice to form four gametes, each with a single set of chromosomes

• gametes are haploid

• all gametes are genetically different

During fertilisation:

• two haploid gametes (e.g. one sperm and one egg) fuse to form one diploid cell, which divides by mitosis to form an organism

Explain the importance of meiosis

• Two nuclear divisions creates haploid gametes (so only half the number of chromosomes)

• So diploid number is restored at fertilisation when one haploid gamete fuses with another haploid gamete; chromosome number is maintained between generations

How can you recognise where meiosis and mitosis occur in a life cycle?

● Mitosis occurs between stages where chromosome number is maintained

• e.g. diploid (2n) → diploid (2n) OR haploid (n) → haploid (n)

● Meiosis occurs between stages where chromosome number halves

• e.g. diploid (2n) → haploid (n)

Describe how mutation in the number of chromosomes arises

• Spontaneously by chromosome non-disjunction during meiosis

• Homologous chromsomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis

• So some gametes have an extra copy of a particular chromosome (n+1) and others have none (n-1)

How can we calculate the number of possible combinations of chromosomes in daughter cells?

2ⁿ where n= number of pairs of homologous chromosomes (half the diploid number)