meiosis Flashcards
Interphase
Before meiosis I interphase occurs. DNA replication and growth of the cell
S phase – DNA replication
Growth phase (G1 + G2) – new proteins and organelles are made
Meiosis 1
results in the production of two diploid daughter cells.
Crossing over occurs in meiosis I during prophase I.
The homologous chromosomes are separated from each other and assorted into two diploid daughter cells.
prophase 1
DNA condenses and becomes visible as chromosomes
each chromosome consists of two sister chromatids joined together by a centromere
chromosomes are arranged side by side in homologous pairs
A pair of homologous chromosomes is called a bivalent
crossing over of non-sister chromatids may occur - chiasma
In this stage centrioles migrate to opposite poles and the spindle is formed
The nuclear envelope breaks down and the nucleolus disintegrates
metaphase 1
bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
anaphase 1
homologous pairs of chromosomes are separated
The centromeres do not divide
telophase 1
The chromosomes arrive at opposite poles
Spindle fibres start to break down
Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
meiosis ii
the two diploid daughter cells divide produce a total of four haploid daughter cells
each with a single copy of every chromosome (only 23 chromosomes total per cell).
prophase II
If the nuclear envelope was re-made after telophase I, it will break down again.
The nucleolus disintegrates chromosomes condense and spindles are made.
metaphase II
chromosmes line up at equator
bind to SF by centromere
The chromatids of each chromosome are independently assorted
anaphase II
SF pull the chromatids to opposite ends of the cell. The centromere divides.
telephase II
each cell, two nuclear envelopes will develop to form two haploid nuclei.
The two cell divide by cytokinesis to produce four haploid daughter cells.
what happens in the different stages of meiosis
During meiosis I, homologous chromosomes are separated
during meiosis II, the chromatids (each half of a duplicated chromosome) are separated.
crossing over and importance
genetic material is exchanged between homologous chromosomes. formation of new combinations of genetic material
creating genetic diversity in the daughter cells.
day 1 menstrual cycle
oestrogen and progestrone levels low
uterus lining sheds
unfertilised egg passes out
Fsh produced by pituary gland increase
stimulates growth of several follicles in ovary - each follicle contains ovum
follicles secrete oestrogen- once ostrogen conc increases it suprresses growth of any more follicles
day 14 menstural cycle
lh and fsh level increases rapidly
lh stimualtes follicle to produce enzymes causing follicle to rupture and release ovum
ovum travels to Fallopian tube
Day 15
Luteal
Ovum travels along fallopian tube
Dead follicle secrets O and P
Causing lining of uterus to thicken and fill with nutrients
LH FSH level decrease
Day 28
Egg not fertilised copus luteum shrinks
Stops secreting O and P
O and P levels fall
