Sexual Reproduction and Meiosis

• Gametes


  Sex cells (the sperm and egg in humans)
• Gametes


  • They are haploid which means they contain half the number of chromosomes as the rest of the cells which make up our body
  • When two gametes fuse during sexual reproduction, the fertilised egg (called a zygote) contains the full number of chromosomes i.e. it is diploid
  • In humans, the diploid number of chromosomes is 46 (23 pairs), which means that gametes contain just 23 chromosomes
• Sexual reproduction


  1. Nucleus of the sperm cell fuses with the nucleus of the egg cell

  2. This fusion of nuclei is called fertilisation
• Adaptations of gametes

  Egg cells are specialised for fertilisation in the following ways:

  • Cytoplasm contains nutrients for growth of the developing embryo.
  • There is an outer layer called the zona pellucida - this changes after fertilisation so that no further sperm can penetrate.
  • Follicle cells outside the zona pellucida form a protective coating around the egg cell. 

  
• Sperm cells are also specialised to carry out fertilisation:

  • Overall streamlined shape for faster swimming.
  • Tail (flagellum) enables the sperm to swim.
  • Contain lots of mitochondria to provide ATP for movement.
  • The head of the sperm contains the acrosome - this is filled with digestive enzymes which break down the egg’s zona pellucida and allow the sperm to reach the egg cell membrane.

  
• The Acrosome Reaction

  When a sperm reaches the egg cell, it binds to its outer layer (the zona pellucida) through attachment to sperm-binding proteins. Once bound, it releases the digestive enzymes that are contained in the acrosome. These enzymes digest a tunnel through the zona pellucida so that the sperm can reach the plasma membrane of the egg cell. The plasma membranes of the sperm cell and the egg cell fuse and the sperm releases its nucleus into the egg cell cytoplasm. The nuclei of the sperm and egg fuse - fertilisation has taken place and a zygote is formed.
• The Cortical Reaction


  1. Fusion of sperm cell and egg cell membranes

  2. Release of calcium ions

  3. Stimulation of vesicles containing cortical granules to move and fuse with egg cell membrane

  4. Release of cortical granules into zona pellucida

  5. Breakdown of zona pellucida

  6. Removal of remaining sperm-binding proteins

  7. Production of new thick, impenetrable outer layer
• Cortical reaction process


  Prevents multiple sperm cells from fertilising the egg, which would result in the zygote having an abnormal number of chromosomes
• Meiosis


  The type of cell division which produces gametes for sexual reproduction
• Meiosis


  • Unlike mitosis, the daughter cells are genetically different from the parent cell and contain just half the number of chromosomes (i.e. they are haploid)
  • When two haploid gametes join during fertilisation, a diploid cell called a zygote is formed
  • Meiosis involves two rounds of cell division which are referred to as meiosis I and meiosis II
• Meiosis


  1. Interphase: DNA replicates so there are now two identical copies of each chromosome (referred to as chromatids)

  2. Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I

  3. Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II
• Prophase I


  • Chromatids condense and arrange themselves into homologous pairs (called bivalents)
  • Crossing over occurs
  • Nuclear envelope disintegrates and spindle fibres form
• Metaphase I


  • Homologous chromosomes line up along the equator and attach to the spindle fibre by their centromeres
• Anaphase I


  • Homologous chromosomes are separated
• Telophase I


  • Chromosomes reach opposite poles of the cell
  • Nuclear envelope reforms around the chromosomes
  • Cytokinesis results in the formation of two daughter cells
• Prophase II


  • Chromosomes condense, nuclear envelope disintegrates and spindle fibres form
• Metaphase II


  • Chromosomes attach to the spindle fibre by their centromeres
• Anaphase II


  • Sister chromatids are separated
• Telophase II


  • Chromatids reach opposite poles of the cell
  • Nuclear envelope reforms
  • Cytokinesis takes places
  • Four genetically unique daughter cells are produced
• Meiosis


  Increases genetic variation
• From an evolutionary point of view, it is important that organisms produce offspring that show as much genetic variation as possible
• If a mother duck gave birth to a group of ducklings that were all had very similar genes


  These ducklings will all be equally vulnerable to the same diseases and other threats to their survival
• Meiosis increases genetic variation in two ways


  • Crossing over
  • Independent assortment
• Crossing over


  1. Occurs during prophase I of meiosis

  2. Homologous chromosomes move towards each other and exchange genetic material

  3. A chromatid from the maternal chromosome becomes twisted around the paternal chromosome and they connect through a structure called the chiasmata

  4. Pieces of chromosomes are exchanged and the chromatids separate, forming chromosomes with different combinations of alleles
• Independent assortment


  1. Depending on the order in which chromosomes line up along the equator of the cell during metaphase, different combinations of chromosomes will end up in each gamete

  2. The way in which the chromosomes align themselves on the spindle fibre is completely random, resulting in a huge number of possibilities of chromosomal combinations in the gametes
• Locus


  The position of a gene on a chromosome
• Linked genes


  If the loci of two different genes are on the same chromosome, they are likely to be inherited together
• Crossing over


  1. Separates linked genes during meiosis

  2. Chiasmata would have to form between the two genes
• The closer the loci of the two genes


  The less likely crossing over is to happen and the higher the probability that the genes will be inherited together
• Any offspring will probably express both phenotypes together than either phenotype separately
• Gene linkage in rats


  • Rats with black fur and red eyes
  • Rats with brown fur and white eyes
  • Not many rats with black fur + white eyes (or brown fur + red eyes)
• To get combinations of phenotypes like black fur + white eyes (or brown fur + red eyes), crossing over must have occurred between the homologous chromosomes to cause the alleles to end up on the same chromosome and be inherited together
• Sex Linkage


  Genes which are located on one of the sex chromosomes (X or Y) are said to be sex-linked and their expression will depend on whether the offspring is male (XY) or female (XX)
• The Y chromosome is much smaller than the X chromosome, so most alleles are carried on the X chromosome (they are X-linked)
• Men


  Only have one X chromosome which means that they will only inherit one allele for these genes, compared to women who will inherit two
• Women


  If they inherit a recessive allele (which causes disease) for a gene found on the X-chromosome, they will just be a carrier, since they have another X chromosome with the dominant, functioning allele
• Men


  If they inherit a recessive allele (which causes disease) for a gene found on the X-chromosome, they will have the disease
• Women


  For them to have X-linked diseases, they must inherit two disease alleles (they will have a homozygous recessive phenotype)
• Sex linked disorders


  • haemophilia
  • red-green colour blindness