GENETIC DIVERSITY AS A RESULT OF MUTATIONS OR MEIOSIS

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  • GENE MUTATION: any change to one or more nucleotide bases, or a change in the sequence of the bases, in DNA
    • they can arise spontaneously during DNA replication (interphase)
    • involves base deletion / substitution
  • MUTATION: any change to the quantity of the base sequence of the DNA of an organism
  • GENE MUTATIONS
    BASE DELETION
    • when a nucleotide (base) is lost from the normal DNA sequence
    • one deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base, this is FRAME SHIFT
  • GENE MUTATIONS
    BASE SUBSTITUTION:
    • when a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base is known as a substitution
    • the polypeptide produced will differ in a single amino acid
    • the significance of this difference will depend upon the precise role of the original amino acid
  • CONSEQUENCES OF DELETION MUTATION
    Order of DNA bases in a gene determines the order of amino acids in a particular protein
    1. One nucleotide / base removed from DNA sequence
    2. causes frame shift where all triplets in a sequence are read differently bc each has been shifted to left by 1 base
    3. changes sequence of codons on mRNA after point of mutation
    4. changes sequence of amino acids in primary structure of polypeptide
    5. changes position of hydrogen/ionic/disulphide bonds in tertiary structure
    6. changes tertiary structure of protein + leaving it potentially unstable to carry out its function
  • CONSEQUENCES OF SUBSTITUTION OF BASES
    1. Nucleotide / base in DNA replaced with another nucleotide / base
    2. this changes one triplet
    3. severity of this depends on where the mutation occured:
    • non-sense
    • mis-sense
    • silent
  • CONSEQUENCES OF SUBSTITUTION OF BASES
    • MUTATION: non-sense
    • SEVERITY: severe
    • WHAT HAPPENS: the change in base sequence codes for a stop codon
    • REASONS: the polypeptide chain is cut short, this means it wont fold into the correct secondary or tertiary structure and will not be able to carry out its specific function
  • CONSEQUENCES OF SUBSTITUTION OF BASES
    • MUTATION: mis-sense
    • SEVERITY: varied severity
    • WHAT HAPPENS: the change in base sequence codes for a new amino acid
    • REASONS: if it’s important in forming bonds that determine the tertiary structure of the final protein, then the replacement amino acid may not form the same bonds. The protein may then be a different shape and therefore not function properly. Eg. If the protein is an enzyme, its active site may no longer fit the substrate and will no longer catalyse the reaction
  • CONSEQUENCES OF SUBSTITUTION OF BASES
    • MUTATION: silent
    • SEVERITY: not severe
    • WHAT HAPPENS:
    1. change in the base sequence but the same amino acid is coded for
    2. or it occurred in an intron
    • REASONS:
    1. This is because the genetic code is degenerate and 6 triplets can code for the same amino acid
    2. introns are removed during splicing in protein synthesis
  • MUTAGENIC AGENTS: increase the rate of gene mutation (above the rate of naturally occurring mutations)
    • high energy radiation
    • ultraviolet light
    • alpha particles
  • DIPLOID AND HAPLOID CELLS
    DIPLOID CELLS
    • diploid number of chromosomes (2n)
    • in humans 2n = 46
    • Each cell has 2 of each chromosome (a homologous pair) one set provided by each parent
    HAPLOID CELLS
    • haploid number of chromosomes (n)
    • in humans n = 23
    • each cell contains one copy of each chromosome in a homologous pair
  • DIPLOID CELLS
    • diploid number of chromosomes (2n)
    • in humans 2n = 46
    • Each cell has 2 of each chromosome (a homologous pair) one set provided by each parent
  • HAPLOID CELLS
    • haploid number of chromosomes (n)
    • in humans n = 23
    • each cell contains one copy of each chromosome in a homologous pair
  • GAMETES: sex cells

    • they contain a haploid number (n) of chromosomes
    -this is one copy of each chromosome in a homologous pair
    -the haploid number for humans is 23
    • gametes are the sperm cells in males and egg cells in females
    • they’re all genetically different from each other and the parent cell
    • in most animals meiosis occurs in the formation of gametes
  • MEIOSIS
    • meiosis forms gametes
    • it takes place in the reproductive organs
    • cells that divide by meiosis start as diploid and end as haploid
    • two divisions
    -1st - homologous pairs separate
    -2nd - chromatids separate
    • the chromosome number halves
  • MEIOSIS: 1
    1, INTERPHASE
    • DNA unravels and replicates to form 2 copies of each chromosome
    • DNA condenses into double armed chromosomes - each with 2 sister chromatids joined at centromere
    2. MEIOSIS I (first division, homologous pairs separate)
    • chromosomes arrange themselves into homologous pairs (one mum, one dad)
    • crossing over occurs in prophase 1
    • Homologous chromosomes pairs line up at middle of the cell double file
    • independent segregation in metaphase 1 (random if mum or dads is on top)
    • pairs of chromosomes separate
    • this halves no. of chromosomes (23 chromosomes, 46 chromatids)
  • MEIOSIS: 2

    3. MEIOSIS II (second division, chromatids separate)
    • chromosomes made up of 2 chromatid (not homologous as only one parent) line up in the middle in single file
    • the pairs of sister chromatids separate and pulled by spindle fibres to poles (centromere divided)
    4. END RESULT
    • 4 haploid gametes are produced, each genetically different from each other. Each has 23 chromosomes, 23 chromatids, as all have centromere
  • GENETIC VARIATION AS A RESULT OF MEIOSIS
    1. Crossing over and recombination
    2. independent segregation
    3. random fertilisation
  • GENETIC VARIATION AS A RESULT OF MEIOSIS
    1, CROSSING OVER AND RECOMBINATION
    1. in prophase 1, the chromosomes in a homologous pair come together forming BIVALENTS
    2. they become twisted round each other (crossing over) at CHIASMATA
    3. during twisting, tensions are created and portions of the chromatid break off
    4. equivalent portions of homologous chromosomes exchanged
    5. recombination occurs where the broken portions rejoin with chromatids of its homologous partner
    6. in this way new genetic combinations of maternal and paternal alleles are produced
  • GENETIC VARIATION AS A RSULT OF MEIOSIS
    2. INDEPENDENT SEGREGATION
    1. In a homologous pair, one chromosome is from each parent
    2. when they align in metaphase 1 they are in double file (two rows) with random orientation meaning its random which parent chromosome is on each side
    3. in meiosis 1 the homologous pairs separate so one chromosome from each pair ends up in the daughter cell
    4. it’s completely random which chromosome ends up in which daughter cell
  • GENETIC VARIATION AS A RESULT OF MEIOSIS
    3. RANDOM FERTILISATION
    1. When gametes (haploid, formed by meiosis) fuse together they form a zygote
    2. all gametes are genetically different from each other so when they fuse together, they increase genetic variation
  • IMPORTANCE OF MEIOSIS
    1. The two divisions creates haploid gametes (half number of chromosomes)
    2. in sexual reproduction, fusion of male and female gametes occurs. During this fertilisation, the diploid number is restored forming a zygote. Half the chromosomes are from the mother and half from the father
    3. this maintains chromosome number between generations
    4. independent segregation and crossing over creates genetic variation which is important in evolution
  • OUTCOMES OF MITOSIS
    PURPOSE:
    • USED FOR: cell repair and growth
    OUTCOMES:
    • CHROMOSOMES NUMBER IN DAUGHTER CELLS: diploid - 46
    • NUMBER OF DIVISIONS: 1
    • DAUGHTER CELL NUMBER: 2
    • GENETICALLY IDENTICAL: yes
    • GENETIC VARIATION: no
    DURING THE PROCESS:
    • INDEPENDENT SEGREGATION? No
    • CROSSING OVER? No
    SIMILARITIES MEIOSIS AND MITOSIS
    • SAME IN BOTH? Daughter cells produced, DNA replicated in interphase, same PMAT basic steps, starts with a single parent cell
  • OUTCOMES OF MITOSIS
    PURPOSE:
    • USED FOR: gamete formation for sexual reproduction
    OUTCOMES:
    • CHROMOSOMES NUMBER IN DAUGHTER CELLS: haploid - 23
    • NUMBER OF DIVISIONS: 2
    • DAUGHTER CELL NUMBER: 4
    • GENETICALLY IDENTICAL: no
    • GENETIC VARIATION: yes
    DURING THE PROCESS:
    • INDEPENDENT SEGREGATION? Yes - metaphase 1
    • CROSSING OVER? Yes - prophase 1
    SIMILARITIES MEIOSIS AND MITOSIS
    • SAME IN BOTH? Daughter cells produced, DNA replicated in interphase, same PMAT basic steps, starts with a single parent cell
  • CHROMOSOME MUTATIONS - NON-DISJUNCTION
    • CHROMOSOME MUTATION: changes in the structure or number of whole chromosomes by non-disjunction
    • NON-DISJUNCTION: where chromosomes fail to separate in meiosis 1 or chromatids fail to separate in meiosis II
  • CONSEQUENCES OF CHROMOSOME NON-DISJUNCTION
    1, CHANGES IN WHOLE SETS OF CHROMOSOMES(polyploidy)
    • POLYPLOIDY: when organisms have 3 or more sets of chromosomes rather than the usual 2
  • CONSEQUENCES OF CHROMOSOME NON-DISJUNCTION
    1, Changes in whole sets of chromosomes (polyploidy)
    1. Individual homologous pairs of chromosomes fail to separate during meiosis 1 or chromatids fail to separate in meiosis II
    2. doesn’t create haploid gametes - creates diploid, triploid or tetraploid
    3. upon fertilisation, a mutated gamete fuses with a normal gamete, same chromosomes group together so zygote has more than 2 copies of every chromosome
    4. occurs mostly in plants as in humans the zygote wouldn’t develop
  • CONSEQUENCES OF CHROMOSOME NON-DISJUNCTION
    2. CHANGES IN THE NUMBER OF INDIVIDUAL CHROMOSOMES (ANEUPLOIDY)
    • when an organism has an extra copy of just one individual chromosome
    1. Individual homologous pairs of chromosomes fail to separate during meiosis I or chromatids fail to separate in meiosis II
    2. this results in one gamete having one more copy of a chromosome (n+1)
    3. the other gamete doesn’t possess copy of this chromosome (n-1)
    4. upon fertilisation, resultant zygote will have one more (2n+1) or one fewer (2n-1) chromosomes in all body cells
    5. this leads to genetic diseases eg. Down syndrome
  • SYNDROMES CAUSED BY TRISOMES IN NON-SEX CHROMOSOMES (NON-DISJUNCTION)

    SYNDROME: Downs
    TRISOMY OF WHICH CHROMOSOME: 21
  • SYNDROMES CAUSED BY TRISOMES IN NON-SEX CHROMOSOMES (NON-DISJUNCTION)

    SYNDROME: Edwards
    TRISOMY OF WHICH CHROMOSOME: 18
  • SYNDROMES CAUSED BY TRISOMES IN NON-SEX CHROMOSOMES (NON-DISJUNCTION)

    SYNDROME: Patou
    TRISOMY OF WHICH CHROMOSOME: 13
  • HOW MANY CHROMOSOMES IN DOWN SYNDROME?
    21
  • CALCULATING NUMBER OF CHROMOSOME COMBOS
    1. Calculate number of possible different combinations of chromosomes in daughter cells following meiosis (assuming no crossing over): 2n^n
    2. calculate number of different combinations of chromosomes following the random fertilisation of two gametes: (2n^n)2^2
    3. Diploid Number / 2 = number of homologous pairs (n)
    4. then put n into formulas
    (n = the number of pairs of homologous chromosomes)
  • COMPLETE DIAGRAMS SHOWING CHROMOSOME CONTENT AFTER MEIOSIS I AND II, GIVEN CHROMOSOME CONTENT OF PARENT CELL
    If given number of chromosomes in parent cell, remember it halves in meiosis I
  • MEIOSIS IN UNFAMILIAR LIFE CYCLES
    • remember in any organism, meiosis is needed for sexual reproduction because it produces daughter cells (usually gametes) with half the number of chromosomes of the parent cell
    • where the cell goes from 2n —> n, meiosis has occurred