meiosis Flashcards
homologous chromosomes
Body cells of adult organisms have 2 sets of homologous (matching) chromosomes – 1 set from female parent and 1 set from male parent
Cell Division –Meiosis
– the process in which the number of chromosomes in the original cell is reduced by HALF through the separation of homologous chromosomes
Meiosis occurs in sex organs (gonads) only
Males (XY) – sex organs are the testes in humans (spermatogenesis)
Females (XX) – sex organs are the ovaries in humans (oogenesis)
Meiosis also occurs in the sex organs of other animals, plants, fungi, etc…
meiosis details
The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are produced.
DIPLOID (2n) HAPLOID (n)
Meiosis is SEXUAL reproduction.
TWO divisions (MEIOSIS I and MEIOSIS II).
meiosis of males
Males – meiosis produces 4 sperm
meiosis of females
Females – meiosis produces 1 (viable) egg
The other 3 cells are called polar bodies – they give up their cytoplasm to nourish the 1 good egg.
how many chromosomes do gametes have
Gametes have ½ the number of chromosomes as somatic (body) cells. We call this the haploid number. Haploid means “1 set” and is written as “N”.
when does meiosis occur in humans
Males beginning at puberty
Females before birth – all eggs are produced before birth and at puberty eggs mature
fertilisation
process by which an egg and sperm unite
zygote
fertilised egg
embryo
organism in early stage of development
unique event in meiosis
Homologous (matching) chromosomes pair up before 1st cell division
Homologous chromosomes:
-look alike
-code for same traits
-receive one from each parent
During 1st division, homologous chromosomes exchange genes during process called “crossing over”
These homologous chromosomes separate during 2nd division of meiosis – so chromosomes in gametes are different from each other due to crossing over
Crossing over increases genetic variation and is the reason why siblings look different
what does crossing over causes
genetic variation
interphase I
Similar to mitosis interphase.
CHROMOSOMES (DNA) replicate in the S phase
Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES.
CENTRIOLE pairs also replicate
prophase I
Longest and most complex phase (90%).
Chromosomes condense.
Synapsis occurs - Homologous chromosomes come togetherto form a tetrad.
Tetrad is two chromosomesor four chromatids (sister and non-sister chromatids).
what are homologous chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes:
a. First 22 pairs of autosomes
b. Last pair of sex chromosomes
where does crossing over occur
Crossing over may occur between non-sister chromatids at sites called chiasmata.
what is chiasmata
Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.)
metaphase I
Shortest phase
Tetrads align on the equator.
Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION
anaphase I
Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.
meiosis II
No Interphase II or very short
No DNA Replication
Remember: Meiosis II is similar to mitosis
prophase II
Same as Prophase in mitosis
Nucleus & nucleolus disappear
Chromosomes condense
Spindle forms
metaphase II
Same as Metaphase in mitosis
Anaphase II
Same as Anaphase in mitosis
SISTER CHROMATIDS separate/ move apart
Telophase II
Same as Telophase in mitosis.
Nuclei and Nucleoli reform, spindle disappears
CYTOKINESIS occurs.
Remember: FOUR HAPLOID DAUGHTER cells are produced.
Called GAMETES (eggs and sperm)
variation
Also known as GENETIC RECOMBINATION
Important to population as the raw material for NATURAL SELECTION.
All organisms are NOT alike
Strongest “most fit” survive to reproduce & pass on traits
