Oct 30 Chapter 8 (continued)

Meiosis and crossing over (continued)

 

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)

 

Number of chromosomes in a haploid cell is abbreviated as n.  Number of chromosomes in a diploid cell is 2n. 

humans                       2n = 46

n = 23

Human life cycle (Fig 8.13)

 

      Fertilized egg (zygote), 2n

↓ mitosis and cell differentiation

mature adult, 2n

      ↓ meiosis

sperm or egg cells, n

                  ↓ fertilization

fertilized egg (zygote), 2n

                  ↓

     

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

A)                 Interphase – chromosome duplication (same as mitosis)

B)                 Meiosis I – separation of homologous chromosome

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

 

4)     Meiosis I

A)                 Prophase I

Same as mitosis

-          Chromatin coils up, nuclear envelop disappears and two centrioles migrates toward opposite directions.  Chromosomes move toward the center.  Spindle microtubules form.

 

Unique to meiosis

-          Homologous chromosomes, each composed of sister chromatids, come together to form tetrad.

-          Crossing over of homologous chromosomes.

 

B)                 Metaphase I

-          Tetrads are aligned at the center.

-          Spindle microtubules are attached to one of the homologous chromosomes.  Microtubules from the other pole are attached to the other homologous chromosomes.

 

C)                 Anaphase I

-          Homologous chromosomes migrate to opposite poles with help of spindle mictrotubules.

-          Sister chromatids remain attached to each other.

 

D)                 Telophase I and cytoinesis

-          Separation of homologous chromosomes complete, making two haploid cells.

-          Cytokinesis separates two haploid cells.

-          Chromosomes may or may not uncoil.

 

5)     Meiosis II

-          No chromosome duplication between meiosis I and II.

-          Sister chromatids separate to make daughter chromosomes.  This process is essentially same as mitosis.

 

The result of meiosis I and II is four haploid cells.

 

6)     Mitosis vs. meiosis (Fig 8.15)

Similar in that chromosomes replicate only once during interphase.

 

Mitosis      - involves one cell division. 

                  - separates sister chromatids.

                  - generates two daughter cells genetically identical to the parent cell

 

Meiosis

-          involves two cell divisions

-          separates homologous chromosomes during meiosis I

-          separates sister chromatids during meiosis II

-          generates four haploid cells (gametes)

-          shares a half of genetic materials with the parent cell.

 

7)     Meiosis and genetic variation

- If a somatic cell contains only one set of homologous chromosomes (A1 and A2), gametes contain either A1 or A2.

 

- If a somatic cell contains two sets of homologous chromosomes (A1, A2, B1, B2), there are ____ possible combinations of chromosomes in gametes.

 

 

 

 

 

 

- If a somatic cell contains three sets of homologous chromosomes (A1, A2, B1, B2, C1, C2), there are ___ possible combinations of chromosomes in gametes.

 

 

 

 

 

 

- If a somatic cell contains four sets of homologous chromosomes, there are ____ possible combinations of chromosomes in gametes.

 

 

 

 

 

 

 

- A human somatic cell contains 23 sets of chromosomes.  The number of possible combinations of chromosomes in gametes is _________!!

 

 

 

 

 

 

 

 

 

- There are _______________ possible combinations of chromosomes in offspring of a single couple!!!

 

 

 

8)     Crossing over

- The exchange of segments between of homologous chromosomes.

- Occurs during synapsis (prophase I).

- Results in genetic recombination.