Oct 28 Chapter 8 (continued)

 

Cell cycle (continued)

 

3) Control of cell cycle -To divide or not to divide

- Decision to divide is influenced by environmental and developmental signals.

- Cells stop dividing when reaches a certain density (density-dependent inhibition). (Fig 8.8, p135)

- Most cells in mature adult do not normally divide.  Their cell cycles are arrested.

- Growth factors (protein signal molecules) can reinitiate the cell cycle.

 

- Cell cycle control system (Fig 8.9, 136; last year’s Nobel Prize in Medicine)

·        Proteins within the cell serve as “timer” to control the cell cycle.

·        Checkpoints determine whether to divide or not to divide.

·        Growth factors give a “go-ahead” signal at the checkpoints.

 

 

Meiosis and crossing over

 

1)     Human chromosomes

 

A human somatic cell contains 46 chromosomes, half (23) derived from the father and half (23) derived from the mother (Fig 8.19, p146).

·        Autosomes – Chromosomes found in both male and female.  A human somatic cell contains 22 types of chromosomes.  Each type consists of two homologous chromosomes.

·        Homologous chromosomes – A set of chromosomes that carry genes controlling same characteristics.  One derived from the father and one derived from the mother.

·        Sex chromosomes – Chromosomes that determines gender.  A human female contains two X chromosomes.  A human male contains one X chromosome and one Y chromosome.

 

2)     Human life cycle – somatic cells vs. gametes, haploid vs. diploid

 

Somatic cells     – Body cells.  Contain a whole set of chromosomes (diploid)

Gametes             - Sperm and egg cells.  Contain a half set of chromosome (haploid)

 

3)     Meiosis overview (p140, 141) – process of generating gamete cells

a.      Meiosis I – separation of homologous chromosome

b.      Meiosis II – separation of sister chromatids (same process as mitosis)