meiosis

Question 1

homologous chromosomes

Answer 1

Body cells of adult organisms have 2 sets of homologous (matching) chromosomes – 1 set from female parent and 1 set from male parent

Question 2

Cell Division –Meiosis

Answer 2

– 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…

Question 3

meiosis details

Answer 3

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).

Question 4

meiosis of males

Answer 4

Males – meiosis produces 4 sperm

Question 5

meiosis of females

Answer 5

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.

Question 6

how many chromosomes do gametes have

Answer 6

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”.

Question 7

when does meiosis occur in humans

Answer 7

Males beginning at puberty
Females before birth – all eggs are produced before birth and at puberty eggs mature

Question 8

fertilisation

Answer 8

process by which an egg and sperm unite

Question 9

zygote

Answer 9

fertilised egg

Question 10

embryo

Answer 10

organism in early stage of development

Question 11

unique event in meiosis

Answer 11

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

Question 12

what does crossing over causes

Answer 12

genetic variation

Question 13

interphase I

Answer 13

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

Question 14

prophase I

Answer 14

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).

Question 15

what are homologous chromosomes

Answer 15

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

Question 16

where does crossing over occur

Answer 16

Crossing over may occur between non-sister chromatids at sites called chiasmata.

Question 17

what is chiasmata

Answer 17

Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.)

Question 18

metaphase I

Answer 18

Shortest phase

Tetrads align on the equator.

Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION

Question 19

anaphase I

Answer 19

Homologous chromosomes separate and move towards the poles.

Sister chromatids remain attached at their centromeres.

Question 20

meiosis II

Answer 20

No Interphase II or very short
No DNA Replication
Remember: Meiosis II is similar to mitosis

Question 21

prophase II

Answer 21

Same as Prophase in mitosis
Nucleus & nucleolus disappear
Chromosomes condense
Spindle forms

Question 22

metaphase II

Answer 22

Same as Metaphase in mitosis

Question 23

Anaphase II

Answer 23

Same as Anaphase in mitosis
SISTER CHROMATIDS separate/ move apart

Question 24

Telophase II

Answer 24

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)

Question 25

variation

Answer 25

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