Sexual Reproduction

1 st ….HOMEWORK! Why do you think that more money is spent on treatment than on prevention? Who benefits from this? What lifestyle changes could we encourage in attempt to reduce the incidence of cancer?

It has been suggested that people who put themselves at risk of cancer (by certain lifestyle choices) should have to pay more for health insurance and life insurance. Strongly AgreeStrongly Disagree AgreeDisagree

By the end of this lesson you will…  Know the difference b/w haploid & diploid cells  Understand the process of meiosis  Understand how genetic variation occurs  Learn some new terminology: ◦ Gamete ◦ Tetrad ◦ Homologous chromosomes ◦ Single vs double chromosomes ◦ Chiasmata

Why is Meiosis important?  Genetic Variation!  Tongue Roller?  Dimples?  Straight / Bent Thumb?  Hair on midsection of finger?

Chromosome – DNA - Gene

 Our ________ cells have 46 chromosomes (diploid) Biological mother & Father  Human _______ cells have 23 chromosomes (haploid) Somatic Sex

 Sex cells divide to produce gametes (sperm or egg).  Gametes have half the # of chromosomes.  Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis

 The fusion of a sperm and egg to form a zygote.  A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

eye color locus eye color locus hair color locus hair color locus

Synthesis Phase

 Cell division that reduces the chromosome number by one-half.  four phases: a.prophase I b.metaphase I c.anaphase I d.telophase I

 Longest and most complex phase (90%).  Chromosomes condense.  Synapsis occurs: homologous chromosomes come together to form a tetrad.  Tetrad is two chromosomes or four chromatids

Homologous chromosomes sister chromatids Tetrad

 Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.  Genetic material is exchanged  Chiasmata are the sites of crossing over.

nonsister chromatids chiasmata: site of crossing over Tetrad

 Homologous chromosomes attach to the spindle fibres and line up on the equator metaphase plate

 Homologous chromosomes separate and move towards the poles.  Sister chromatids remain attached at their centromeres.

 Cytoplasm divides forming 2 cells  Nuclear membrane forms  Each cell has half the number of chromosomes (reduced from 2n to n)

 Remember: Meiosis II is similar to mitosis  There is NO DNA replication  Prophase II – nuclear membrane dissolves and spindle fibres form

 Chromosomes line up on the equator (with 2 chromatids) metaphase plate Metaphase II

 Chromatids move to opposite poles

 Nuclear membrane forms  4 haploid daughter cells are produced

n=23 sperm haploid (n) meiosis II n=23 meiosis I 2n=46 sex cell diploid (2n)

 Your genes are: ◦ Eye color (you pick the colors) ◦ ?  Go through the process of meiosis  Describe the 4 traits of your gametes  Have two of your gametes “get together”  Go through the process of mitosis with your zygote  Identify the traits of your two cells!