Lecture 6 - Sexual Reproduction

• • Meiosis is the process by which sex cells or gametes are formed.
• • The daughter cells produced in meiosis will have half the number of chromosomes of the parent cell.
• • Human cells containing 46 chromosomes, this number is called the Diploid number and is referred to as “2n”
• • In meiosis these cells will produce a gamete of 23 chromosomes also known as “n” Haploid number.
• Meiosis

  • In females it will take place in the ovaries and testes for males
• • The two gametes formed in Meiosis will come together to form a zygote
  • This zygote will thus have a full set of chromosomes
• • You receive genetic information from both parents 

  I.e. 23 chromosomes from each.

  • These 2 sets of 23 will pair off into their homologous chromosomes which are similar in shape, size, and gene arrangement and deal with the same traits. 
• Stages of Meiosis

  • Meiosis involves two divisions to produce 4 haploid cells. Just like Mitosis DNA replication occurs before Meiosis

  1. Meiosis l ~ “Reduction Division” 
  2. Meiosis ll ~ separation of 2 chromatids (maintains haploid number)
• Meiosis 1 

  • Prophase 1
  • Replicated chromosomes condense and become visible
  • Homologous chromosomes come together in their pairs to form a tetrad
  • The tetrad chromosomes will twist around each other ~ this process is called synapse
  • The exchange of genetic material between two homologous chromosomes is called crossing over
• What is this?

  A) Meiosis 1

  
• Meiosis 1

  • Metaphase 1
  • Chromosomes attach to spindle fibres
  • Chromosomes line up at the equatorial plate
• Metaphase 1

  
• Meiosis 1

  • Anaphase 1
  • Homologous chromosomes separate and move to opposite poles 
• Anaphase 1

  
• Meiosis 1

  • Telophase 1
  • Membrane forms around each nucleus
  • Chromosomes in the two nuclei are not identical (unlike mitosis where they both have identical genetic information)
• Telophase 1

  
• Meiosis II

  • Prophase II
  • Nuclear membrane dissolves and spindle fibres form for chromosomes to attach (no nuclear membrane) 
• Meiosis II

  • Metaphase II 
  • Chromosomes line up at the equatorial plate
• Meiosis II

  • Anaphase II
  • Sister chromatids separate and move to opposite poles
• Meiosis II

  • Telophase II 
  • Cytoplasm separate (cytokinesis) leaving 4 haploid daughter cells
  • These cells may become gametes
• Meiosis is important…

  • It allows for sexual reproduction
  • It allows for a greater range of characteristics
  • It ensures the recombination of genes
• How do we get Variation?

  1. Independent Assortment: the chromosomes in the homologous pairs introduces variation since there are many different combinations of chromosomes that can end up in a given gamete.
  2. Chromosomal Crossover: during prophase 1 of meiosis 1 it introduces variation because variation of the inner two chromatids of a synapse homologous pair exchange segments. 
• MITOSIS

  Function of the process:

  • Ensure genetic continuity
  • Growth, repair, maintenance

  Location of Process:

  • Body (somatic) cells
  • Exception of reproductive organs

  # of Divisions:

  • 1

  # of DNA Duplications:

  • 1

  # of cells produced (at the end):

  • 2 identical daughter cells

  # of Chromosomes (at the end):

  • 2N (Diploid)
  • 46 Chromosomes
• MEIOSIS

  Function of the process:

  • Ensures genetic variability
  • Prevent doubling of chromosomes numbers between generations

  Location of Process:

  • Only in reproductive organs (designative cells)

  # of Divisions:

  • 2

  # of DNA Duplications:

  • 1

  # of cells produced (at the end):

  • Male: 4 functional sperm
  • Female: 1 Functional egg

  # of Chromosomes (at the end):

  • 1 N (Haploid)
  • 23 Chromosomes