Transcript Chapter 8 Section 3 Meiosis

Chapter 8 Section 3 Meiosis
• Objectives: List and describe the phases of
meiosis.
• Compare the end products of mitosis with
those of meiosis.
• Explain crossing-over and how it contributes
to the products of unique individuals.
• Summarize the major characteristics of
spermatogenesis and oogenesis.
Cells undergoing meiosis have a duplicate set of chromosomes, just
as cells beginning mitosis do. Cells undergoing meiosis divide twice .
Diploid cells that divide meiotically become 4 haploid cells rather
than two diploid cells.
Prophase 1
Every chromosome lines up next to its homologue. The pairing of
homologous chromosome is called synapsis. Each pair of
homologous chromosomes is called a tetrad. During synapsis the
chromatids within a homologous pair twist around one another.
Portions of chromatids may break off and attach to adjacent
chromatids on the homologous chromosome – a process called
crossing over. Crossing over causes the exchange of genetic material
between maternal and paternal chromosmes.
Metaphase 1
Tetrads line up randomly along the midline of the dividing cell.
Anaphase 1
Each homologous chromosome move s to an opposite pole
of the dividing cell. Independent assortment results in genetic
variation.
Telophase 1
The chromosomes reach the opposite ends of the cell and
cytokinesis begins.
Meiosis II
The steps continue without an interphase and result in four new
cells. Each new cell contains half of the original cell’s number of
chromosomes.
Formation of Gametes
In animals, meiosis produces haploid reproductive cells, called
gametes. Meiosis occurs only in reproductive organs. In
humans, meiosis occurs in the testes and in the ovaries.
Spermatogenesis
It occurs in the testes. Four haploid cells called spermatids are
formed.
Oogenesis
Oogenesis is the production of matrue egg cells or ova. During
cytokinesis I and II of oogenesis, the cytoplasm of the original
cell is divided unequally between new cells. As a result, one
egg cell is produced by meiosis. The other three products of
meiosis, called polar bodies, degenerate.
Asexual reproduction- production of offspring from one parent.
Offspring are geneticall y identical to the parent.
Sexual reproduction – production of offspring through meiosis and
the union of sperm an egg. Offspring are genetically different from
their parents.