Chromosomes and Meiosis Unit
Cell Types
1) Somatic Cells (aka: body cells)
- are diploid (2n)
- DNA not passed on
2) Gametes (aka: sex cells)
- ex: ova (egg) and spermatozoa (sperm)
- are haploid (1n)
- DNA passed on
Chromosomes
Each species has a characteristic number of chromosomes
Types
1) Autosomes (#1-22)
2) Sex Chromosomes (#23)
- female = XX
male = XY
Humans have 46 chromosomes (23 pairs)
- 44 autosomes (22 pairs) and 2 sex chromosomes (1 pair)
- a diploid # (2n) of 46 and a haploid # (1n) of 23
Karyotype = picture of an individual’s chromosomes
Homologous Chromosomes vs. Sister Chromatids
Homologous Chromosomes = two separate chromosomes, one from mom and one from dad
- very similar in size and carry the same genes, but they are not copies of one another
In contrast, each half of a duplicated chromosome is called a chromatid
- together, the two chromatids are called sister chromatids
- thus, sister chromatids refers to the duplicated chromosome that remain attached (by the
centromere)
Homologous chromosomes split in Meiosis I; sister chromatids split in Meiosis II
Meiosis
= a form of nuclear division that divides a diploid cell into a haploid cell
- makes egg and sperm
- 2 rounds of division (Meiosis I and Meiosis II)
Meiosis I – divides homologous chromosomes
Meiosis II – divides sister chromatids
The overall process produces haploid cells.
Gametogenesis
1) spermatogenesis = sperm production (4 haploid cells)
- even distribution
2) oogenesis = egg production (1 haploid cell)
- uneven distribution (polar bodies created)
Comparing Mitosis and Meiosis
Mitosis
- Produces genetically identical cells
- Results in diploid cells
- Takes place throughout an organism’s life
- Involved in asexual reproduction
Meiosis
- Produces genetically unique cells
- Results in haploid cells
- Takes place only at certain time in life cycle
- Involved in sexual reproduction