( Mitosis and Meiosis, Phenotype and Phenotype ( Session 6

( Mitosis and Meiosis, Phenotype and Phenotype ( Session 6, Lecture 11 )
Chromosomes: are the thread-like structures in the cell nucleus that carry genetic information.
? Human somatic cells: are diploid and carry 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes, XX or XY) (46 chromosomes), collectively composed of 6x109 bp.
? Human gametes (sperms and eggs): are haploid and contain only one set of 23 chromosomes, collectively composed of 3x109 bp
Cell Division
In a diploid cell, the chromosomes occur in pairs. The 2 members of each pair are called homologous chromosomes or homologues
? Homologous chromosomes: Look the same , Control the same traits , and May code for different forms of each trait
? Non-homologous chromosomes: Look different and Control different traits.
?Sex chromosomes: Are distinct from each other in their characteristics ,are represented as X and Y Determine the sex of the individual, XX being female, XY being male.
Diploid (2n)- A cell possessing two copies of each chromosome (human body cells).
Haploid (1n) - A cell possessing a single copy of each chromosome (human sex cells).

Cell Cycle Phases:
Phases of cell cycle
1- Interphase
G1 - primary growth
S - genome replicated
G2 - secondary growth
2- M – phase (mitotic phase)
3- Cytokinesis
The durations of the phases
of a cell cycle
1- Interphase (23 hr)
2- M phases (1 h)
Mitosis:
Although a continuous process, is conventionally divided into five stages: prophase,prometaphase,metaphase,anaphase and telophase.

Meiosis:
Meiosis= spermatogenesis and oogenesis
The main consequences of meiosis are that:
it maintains a constant number of chromosomes from generation to generation
generates genetic diversity, which is achieved in two ways: independent assortment of chromosomes (during meiosis I) and crossing-over.