Meiosis and Human Cells

Meiosis (Sex Cell Reproduction)

• Meiosis is a type of cell division that produces sex cells.
• The process of meiosis ensures offspring have the same diploid number as their parents.
• Two haploid sex cells combine to form a diploid zygote.
• Meiosis involves two divisions, Meiosis I and Meiosis II.
• Meiosis I involves crossing over, a process where homologous chromosomes switch genes. This process leads to genetic variation
• Meiosis produces four haploid sex cells.
• During Meiosis I, duplicated chromosomes condense, homologous chromosomes pair up, and crossing-over occurs.
• These paired chromosomes then separate.
• A second division, Meiosis II, separates the sister chromatids of each chromosome.

Human Cells

• Most cells in the human body have a nucleus except sex cells.
• Somatic cells have 46 chromosomes (23 pairs).
• Homologous chromosomes in each pair carry genes for the same trait, but not necessarily the same form of the gene.
• Example: Eye color genes are on homologous chromosomes; a person may have one chromosome with a dominant gene for brown eyes and another with a recessive gene for blue eyes.

Gametes (Sex Cells)

• Sex chromosomes (X and Y) are an important exception.
• A person with XX chromosomes is female and a person with XY is male.

Reproduction

• Reproduction is the process by which organisms produce others of their own kind.
• Two types of reproduction are asexual and sexual.

Asexual Reproduction

• A new organism is produced from one organism.
• The new organism has identical hereditary material to the parent.
• Methods include budding and regeneration.

Budding and Regeneration

• Budding: A new organism grows from the parent's body.
• Regeneration: Cell division is used to re-grow damaged or lost body parts.
• Example of organisms that can regenerate are sea stars and hydra.

Sexual Reproduction

• Two sex cells (egg and sperm) combine during fertilization.
• The resulting fertilized egg (zygote) has a unique combination of genetic material from both parents.

Diploid Cells

• Body cells are diploid.
• Diploid cells have pairs of similar chromosomes.
• The number of chromosomes is represented as 2n, where n represents the number of chromosomes in a single set, i.e., 23.

Haploid Cells

• Sex cells are haploid.
• Haploid cells do not have pairs of chromosomes (n)
• Example: sperm and eggs.
• Haploid sex cells contain half the number of chromosomes as body cells.
• Female eggs: 22 + X
• Male sperm: 22 + X or 22 + Y

Comparison: Mitosis vs. Meiosis

• Mitosis produces two identical diploid daughter cells.
• Meiosis produces four genetically diverse haploid daughter cells

Crossing Over

• Crossing over occurs between two non-sister chromatids.
• The process involves an exchange of genetic material between maternal and paternal chromosomes.
• Crossing over leads to genetic variation

Stages of Meiosis

• Two nuclear divisions occur during meiosis.

Check for Understanding

• Human somatic cells have 46 chromosomes.
• Human sex cells have 23 chromosomes.
• Haploid means half the number of chromosomes.
• A fruit fly has 8 chromosomes in its body cell. After mitosis, the daughter cells will have 8 chromosomes each.
• A fruit fly sex cell has 4 chromosomes.