Cell Division and ReproductionAsexual and Sexual

reproduction

By : D. Reis

Asexual Reproduction

A single organism produces offspring with identical genetic information

Sexual Reproduction

Haploid (n=23) sperm cell fertilizes a haploid (n=23) egg cell to produce a dipolid (2n=46) zygote.

Sexual Reproduction Cells of the human body have

46 chromosomesWhen fertilization occurs a

sperm and egg cell combine their chromosomes to make a zygote with 46 chromosomes.

How is this possible?

MeiosisThe division

of cells to produce specialized sperm and egg cells

46

23

23 23 23 23

23

Phases of Meiosis

Assume DNA replication occurs prior to meiosis I

Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

Homologous Chromosomes

Paired chromosomes similar in shape, size, gene arrangement and gene information.

Each somatic cell contains 23 pairs of homologus chromosomes, or 46 in total

Stages of Meiosis I

PROPHASE I – Nuclear membrane dissolves Centrioles move to opposite

poles Formation of spindle fibers

Stages of Meiosis I

PROPHASE I- Homologus chromosomes pair up in a process called synapsis and crossing over occurs

Crossing Over/ Genetic Recombination

The exchange of genetic material between two homologus chromosomes leads to genetic variability.

Tetrad – a pair of homologus chromosomes composed of 4 chromatids.

Stages of Meiosis I

METAPHASE I-

Homologus chromosomes line up at the equatorial plate.

Stages of Meiosis I

ANAPHASE I- Spindle fibers shorten, the homologus chromosome pairs move toward opposite poles. THE SISTER CHROMATIDS DO NOT PULL APART AS THEY DO IN MITOSIS.

23 duplicated chromosomes move to opposite poles.

TELOPHASE I- Two new genetically different haploid daughter cells are formed each containing 23 double stranded chromosomes

Formation of nuclear membrane

Stages of Meiosis II

PROPHASE II – Nuclear membrane dissolves, spindle fibers form and centrioles move to opposite poles.

METAPHASE II – Spindle fibers attach to the chromosomes and the chromosomes (2 sister chromatids) align along the equatorial plate.

Stages of Meiosis II

ANAPHASE II – Sister chromatids separate and move to opposite poles.

TELOPHASE II – Chromatids arrive at each pole and cell division begins.

Meiosis1. Homologus

chromosomes: 23 paternal and 23 maternal

2. Chromosomes are replicated and attached at the centromere

Meiosis3. Meiosis I

reduction stage- Separation of homologus chromosomes

4. Meiosis II- separation of sister chromatids

Results of Meiosis4 gametes/reproductive cells

which contain 23 chromosomes Haploid-each cell contains half

the number of chromosomes from the parent cell

All 4 cells are genetically different

Results of Meiosis

Results of MeiosisOogenesis- the

production of egg cells in females

Spermatogenesis- the production of sperm cells in males

Mitosis and Meiosis Compared

Mitosis and Meiosis Compared

Mitosis2 daughter cells are producedDaughter cells are diploid Daughter cells are the same as

parent cells

Meiosis4 daughter cells are producedDaughter cells are haploidDaughter cells are genetically

different from parent cells