genetic diversity: mutations and meiosis

• • A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
  • Mutations occur continuously
  • As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for
  • Most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed
  • This is because the genetic code is degenerate 
• insertion of nucleotides

  mutation when nucleotide has a new base randomly inserted into DNA sequence

  • changes original amino acid, creates different triplet of bases
  • frameshift
  • may change amino acid sequence produced from gene and may affect the ability of polypeptides function
• deletion of nucleotide

  mutation where a nucleotide ( and so a base) is randomy deleted from DNA sequence

  • changes the amino acid that would have been coded
  • frameshift
  • ay change amino acid sequence drastically and affect polypeptide to function
• substitution of nucleotides

  mutation when base in DNA sequence randomly swapped for a different base

  • may not affect the polypeptide sequence and no frameshift in sequence since is substituting

  three forms of substitutions

  • silent mutations: does not alter amino acid sequence (since degenerate codes)
  • missense mutations: mutation alter single amino acid in polypeptide chain
  • nonsense mutations: mutation creates premature stop codon, so polypeptide chain is incomplete and can affect final structure of protein and function
• effect of gene mutations on polypeptides

  • Most mutations do not alter the polypeptide or only alter it slightly so that its appearance or function is not changed
  • However, a small number of mutations code for a significantly altered polypeptide with a different shape
  • This may affect the ability of the protein to perform its function. For example:
  • If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
  • A structural protein (like collagen) may lose its strength if its shape changes
• Mutagenic agents

  • There are natural mechanisms that take place within cells to ensure the accuracy of DNA replication
  • These mechanisms involve proofreading and repairing damaged DNA
  • When the mutation rate of a cell rises to above a normal (usually low) rate then these mechanisms have become ineffective
  • Mutagenic agents are environmental factors that increase the mutation rate of cells
  • Examples include:
  • High-energy radiation such as UV light
  • Ionising radiation such as X rays
  • Toxic chemicals such as peroxides
• • Non-disjunction occurs when chromosomes fail to separate during meiosis
  • This occurs spontaneously
• • The gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome
  • These gametes will have a different number of chromosomes compared to the normal haploid number
  • If the abnormal gametes take part in fertilization, then a chromosome mutation occurs as the diploid cell will have the incorrect number of chromosomes
  • Chromosome mutations involve a change in the number of chromosomes
  • an example of chromosome mutation is down syndrome
• meiosis

  • Meiosis produces daughter cells that are genetically different from each other and to the parent cell
• Crossing over


  The process whereby a chromatid breaks during meiosis and rejoins to the chromatid of its homologous chromosome so that its alleles are exchanged
• • Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
  • It produces gametes in plants and animals that are used in sexual reproduction
  • It has many similarities to mitosis however it has two divisions: meiosis I and meiosis II
  • Within each division there are the following stages: prophase, metaphase, anaphase and telophase
• • Independent assortment
  • The alleles of two (or more) different genes get sorted into gametes independently of one another
  • The allele a gamete received for one gene does not influence the allele received for another gene
  • This is because homologous chromosomes line up in random orientations at the middle of the cell at metaphase as they prepare to separate, meaning that the same parent cell can produce different combinations of chromosomes in the daughter cells
• Prophase I

  1. DNA condenses and becomes visible as chromosomes

  2. DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere

  3. The chromosomes are arranged side by side in homologous pairs

  4. A pair of homologous chromosomes is called a bivalent

  5. Crossing over of non-sister chromatids may occur at the chiasma

  6. Centrioles migrate to opposite poles and the spindle is formed

  7. The nuclear envelope breaks down and the nucleolus disintegrates
• Metaphase I

  • The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
• Anaphase I

  • The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
  • The centromeres do not divide
• Telophase I

  • The chromosomes arrive at opposite poles
  • Spindle fibres start to break down
  • Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
  • Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
• Cytokinesis

  The division of the cytoplasm
• Cytokinesis in animal cells

  1. Cell surface membrane pinches inwards creating a cleavage furrow

  2. Cleavage furrow contracts, dividing the cytoplasm in half
• Cytokinesis in plant cells

  1. Vesicles from Golgi apparatus gather along the equator of the spindle

  2. Vesicles merge to form new cell surface membrane

  3. Vesicles secrete calcium pectate to form middle lamella

  4. Layers of cellulose laid upon middle lamella to form primary and secondary cell walls
• Cytokinesis in meiosis I

  • End product is two haploid cells
  • Cells are haploid as they contain half the number of centromeres
• Cell organelles also get distributed between the two developing cells during cytokinesis
• There is no interphase between meiosis I and meiosis II so the DNA is not replicated
• Meiosis II

  • The second division of meiosis
  • Almost identical to the stages of mitosis
• Prophase II

  1. Nuclear envelope breaks down

  2. Chromosomes condense

  3. Spindle forms at a right angle to the old one
• Metaphase II


  Chromosomes line up in a single file along the equator of the spindle
• Anaphase II

  1. Centromeres divide

  2. Individual chromatids are pulled to opposite poles

  3. Creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell
• Telophase II


  Nuclear membranes form around each group of chromosomes
• Cytokinesis

  1. Cytoplasm divides as new cell surface membranes are formed

  2. Creates four haploid cells

  The cells still contain the same number of centromeres as they did at the start of meiosis I but they now only have half the number of chromosomes (previously chromatids)
• • Having genetically different offspring can be advantageous for natural selection
  • Meiosis has several mechanisms that increase the genetic diversity of gametes produced
  • Both crossing over and independent assortment (random orientation) result in different combinations of alleles in gametes
• Crossing over


  The process by which non-sister chromatids exchange alleles
• Crossing over

  1. During meiosis I homologous chromosomes pair up and are in very close proximity to each other

  2. The non-sister chromatids can cross over and get entangled

  3. These crossing points are called chiasmata

  4. The entanglement places stress on the DNA molecules

  5. A section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
• Crossing over can result in a new combination of alleles on the two chromosomes
• There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
• Crossing over is more likely to occur further down the chromosome away from the centromere
• Independent assortment

  The production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
• Independent assortment

  • Increases genetic variation between gametes
• Independent assortment

  1. In prophase I homologous chromosomes pair up

  2. In metaphase I they are pulled towards the equator of the spindle

  3. Each pair can be arranged with either chromosome on top, this is completely random

  4. The orientation of one homologous pair is independent / unaffected by the orientation of any other pair

  5. The homologous chromosomes are then separated and pulled apart to different poles
• the formula to calculate the number of combinations of chromosomes after the random fertilisation of two gametes is (2n)2

  • n is the haploid (1)
• How optical microscopes work

  • Light is directed through the thin layer of biological material that is supported on a glass slide
  • This light is focused through several lenses so that an image is visible through the eyepiece
  • The magnifying power of the microscope can be increased by rotating the higher power objective lens into place
• Key components of an optical microscope

  • Eyepiece lens
  • Objective lenses
  • Stage
  • Light source
  • Coarse and fine focus