This is a summary of the whole Chromosomes and Meiosis chapter in the Life Sciences Mind Action Series Grade 12 IEB textbook.
The work is summarised in detail, and then very brief summaries are also included at the end.
Diagrams, images and a definitions column is included in order to help y...
Are long, thread-like structures that form part of the chromatin network in the nuclei of cells. Made up of a
strand of DNA wound around proteins.
In somatic (body) cells of diploid organism:
→ Number of chromosomes is the same
→ Diploid (two sets of chromosomes): mother (maternal chromosome) and father (paternal
chromosome)
→ Paternal chromosome & maternal chromosome = homologous pair
→ DNA of each chromosome replicates to from two identical threads or chromatids joined by a
centromere.
Meiosis
DEFINITIONS:
Is cell division in the reproductive organs to produce gametes (sex cells)
DIPLOID – two sets of
chromosomes in a cell
→ Number of chromosomes is reduced from two sets (2n) to one set (n)
→ Gametes/spores formed are called haploid (one set of chromosomes)
HAPLOID – one set of
chromosomes in a cell
Male haploid gamete & female haploid gamete fuse during fertilization and form
a diploid zygote.
SPERMATOGENESIS – formation of sperm cells in testis
OOGENESIS – formation of egg cells / ova in the ovaries
MICROSPORANGIA – pollen sacs in the male anthers
MEGASPORANGIA – ovules in the female ovaries
Importance:
1. Form gametes
2. Chromosome number is kept constant from generation to generation
3. Creates variation through crossing over and random arrangement of chromosomes
, Summary by: Kaylin Teneal
PROCESS OF MEIOSIS
Prophase
Metaphase
Anaphase
Telophase
Meiosis 1 results in two cells
Meiosis 2 results in four cells
MEIOSIS 1 – a reduction division
Prophase 1:
1. Nuclear membrane disappears
2. Chromatin network unwinds and forms chromosomes in homologous pairs
3. Crossing-over takes place (results in the exchange of genetic material)
4. Centrosome splits into two centrioles
Metaphase 1: PROPHASE 1
1. Spindle threads form between centrioles
2. Chromosomes move to the equator in homologous pairs and are attached to
spindle threads
Anaphase 1: METAPHASE 1
1. Spindle threads contract
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