Meiosis Notes

1.   Why do we look like our parents? Each parent donates genes to their offspring via sexual reproduction. The genes combine to give different but similar looking offspring.

2.   Homologous chromosomes

a)  Humans have 46 chromosomes consisting of 23 homologous pairs. Each parent donates one chromosome to each of the 23 homologous pairs. I.e., half of an individual’s chromosomes come from the female parent while half come from the male parent.

b)  Homologous chromosomes are the same length and carry the same genes in the same location. Those genes could be different versions. E.g., imagine the homologous chromosomes carry the eye color gene but one produces blue eyes while the other produces brown.

c)  The exception is the sex chromosomes. For these, females have a homologous pair (XX) while males do not (Xy).

d)  The other chromosomes are called autosomes.

3.   Two types of cells in general

a)  Somatic – diploid (2n) body cells. Contain a complete set of chromosomes.

b)  Reproductive cells – haploid (n) sex cells. These cells are called gametes and contain only half the number of chromosomes. If one somatic cell is fertilized by another, the resulting zygote would contain twice the number of chromosomes. I.e., the chromosome number would double each generation. For this reason, the chromosome number must be reduced during the production of gametes. This way, one haploid gamete is fertilized by another and the resulting zygote is diploid.

c)  The zygote carries a complete set of chromosomes. Half from the the female parent and half from the male.

d)  Every cell in the resulting organism is diploid and arises by mitotic division of the original zygote.

4.   The main stages of meiosis are the same as those in mitosis (Fig. 23.2 p. 551N).

a)  Meiosis happens in two stages: Meiosis I and Meiosis II. The two cell divisions result in 4 haploid daughter cells.

b)  During Meiosis I (the reduction division) homologous chromosomes are separated.

c)  In Meiosis II, sister chromatids are separated.

d)  After the chromosomes are replicated, sister chromatids remain attached at the centromere. Also, homologous pairs (each consisting of two sister chromatids) remain close together. The four sister chromatids are called a tetrad and the process is called synapsis.

e)  During synapsis, the arms of chromosomes in a homologous pair become intertwined. Pieces of the homologous chromosomes break off and switch places. This phenomenon is called crossing over (Fig. 23.4 p. 552N). This increases genetic diversity which results from sexual reproduction

5.   Events contributing to Genetic Diversity.

a)  Independent Assortment

i)   The orientation of homologous chromosomes on one side of the metaphase plate or the other in Meiosis I is random.

ii)  The number of possible orientations is 2ⁿ, where n is the haploid number. For humans, the number is 2²³ = 8.4 million

b)  Random fertilization

i)   Any of a male’s 8.4 million sperm can fertilize any of a woman’s 8.4 million eggs. The total number of combinations is over 70 trillion.

c)  Crossing over

i)   When crossing over is considered, the number of combinations is nearly infinite.

6.   Genetic diversity contributes to evolutionary change. If an offspring inherits a combination of genes that gives it a survival advantage, it is better able to survive and pass on its genes. This means the chance that the combination is passed on increases. As a result, there is an accumulation of favorable characteristics.