Meiosis and Sexual Reproduction

Most eukaryotic species reproduce sexually at some point in their life cyclesGametes (both sperm and egg cell) of opposite sexes unite to form a zygote – starting a new generation of offspringGametes are formed from a special kind of cell division called

MeiosisMeiosis

Meiosis is one key factor that generates genetic diversity in a species population and produces the particular genetic mixes that contribute to evolution

For sexual reproduction,Meiosis has to occur first!

Notice the condition of each chromosome!

Meiosis reduces the number of chromosomes

• Early on it became clear that gamete formation must reduce the number of chromosomes

• Or else, with each successive generation, chromosome number would double!

• Gametes (sex cells) revealed half the chromo-somes of somatic (body) cells

• The fusion of gametes to form a zygote is called fertilization or syngamy

The life cycles of higher life forms show a pattern of both diploid and haploid

stages

a. Some Algae and fungi – reveal a Zygotic life cycle

b. Animals – reveal a Gametic Life cycle

c. Plants and some algae – reveal a Sporic Life Cycle

Humans exhibit what type of Life Cycle?

“Germ line” cells are specialized cells which may undergo Meiosis to produce gametes

Features of Meiosis

• Meiosis exhibits 2 separate division processes:– Meiosis IMeiosis I = Reduction division; number of chromosomes

divides in half– Meiosis IIMeiosis II = Equational division; double-stranded chromosomes

separate (as in mitosis)

• Homologous chromosomes (homologues) pair during meiosis I in a process called synapsis

• During synapsis, homologues exchange pieces of chromosomes in a process called crossing overcrossing over

Synapsisand

Crossing over

*Synapsis and crossing over occurs during Prophase of Meiosis I (Prophase I)

Alignment of chromosomes differs between Meiosis I and Mitosis

The process of Meiosis I

The process of Meiosis II

Meiosis vs Mitosis

• For either cell division process, chromosomes must replicate to form DS chromosomes

• In meiosis:» Homologous chromosomes come together (synapsis)

» Crossing over occurs

» Meiosis I divides number of chromosomes in half

» 2N, DS 1N, DS

» Meiosis II separates each of the DS chromosomes in each cell

» 1N, DS 1N, SS

» Due to crossing over between homologues, the resultant cells are not identical, genetically