2.12 Flashcards
Gametes are made by
meiosis
Meiosis is a
2-stage division in which a diploid cell divides to produce 4 haploid gametes
ORDER OF MEIOSIS:
INTERPHASE PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II
Interphase:
immediately before meiosis
DNA replicates so that the cell now contains 4 rather than 2 copies of each chromosome
Prophase I (4 stages):
- Chromosomes shorten and fatten and come together in their homologous pairs to form a BIVALENT
- Chromatids wrap around one another and attach at points called CHIASMATA
- Chromosomes break at these points and swap similar sections of chromatids with one another in a process called CROSSING OVER
- Nucleolus disappears and the nuclear envelope disintegrates
Metaphase I (2 stages):
- Centromeres attach to the spindle
- Bivalents arrange themselves randomly on the equator of the cell with each of a pair of homologous chromosomes facing opposite poles
Anaphase I:
-One of each pair of homologous chromosomes is pulled by spindle fibres to opposite poles
Telophase I:
-A nuclear envelope re-forms around the chromosomes at each pole (in most animal cells)
(in most plant cells there is no telophase I, cell directly goes into metaphaseII)
Prophase II (4 stages):
- Where the nuclear envelope has re-formed it breaks down again
- Nucleolus disappears
- Chromosomes shorten and thicken
- Spindle re-forms
Metaphase II:
-Chromosomes arrange themselves on the equator of each cell
Anaphase II (2 stages):
- Centromeres divide
- Chromatids are pulled to opposite poles by the spindle fibres
Telophase II (3 stages):
- The tetrad of 4 cells is formed
- Each cell has the haploid number of chromosomes
- Crossing over during prophase I has produced genetic variety
Bivalents are:
The pair of homologous chromosomes that pair and form chiasmata in the first division of meiosis
Chiasmata are:
The places at which the chromatids of homologous chromosomes wrap around each other and exchange pieces
The first division of meiosis involves the homologous chromosomes pairing together to form a
bivalent
Chromatids of homologous chromosomes wrap around each other at places called
chiasmata
At chiasmata, chromatids break and equivalent pieces of chromatids are exchanged. This process is called
crossing over
The second division of meiosis is like mitosis. In this stage the chromatids
separate from each other
Meiosis brings about variation in 3 ways:
- Crossing over during prophase I produces new combinations of alleles
- Independent assortment of homologous chromosomes - in M1 pairs of homologous chromosomes line up on the spindle randomly, each pair separates independently of any other pair, assortment occurs in anaphase 1
- Independent assortment of chromatids in anaphase 2 again occurs randomly, so each pair of chromatids separates independently of any other pair
Another source of variation occurs at
fertilisation
(Fertilisation) each gamete produced by an individual is genetically different from any other. There are usually many different male gametes, and it is purely random which one of these will fertilise the oocyte. As a result, the genetic make-up of every new individual is
unique
