3.3 MEIOSIS
Understandings - One diploid nucleus divides by meiosis to produce 4 haploid nuclei.
- The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
- DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
- The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
- Orientation of pairs of homologous chromosomes prior to separation is random.
- Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
- Crossing over and random orientation promotes genetic variation.
- Fusion of gametes from different parents promotes genetic variation.
Applications/Skills A: Non-disjunction can cause Down syndrome and other chromosomal abnormalities.
A: Studies showing age of parents influences chances of non-disjunction.
A: Description of methods used to obtain cells for karyotype analysis, e.g. chorionic villus sampling and amniocentesis, and the associated risks.
S: Drawing diagrams to show the stages of meiosis resulting in the formation of 4 haploid cells.
Mitosis Meiosis
One division Two divisions
Diploid cells produced Haploid gametes produced
No crossing-over in prophase Crossing-over in prophase I
No chiasmata formation Chiasmata form
Homologous pairs do not associate and line up at the equator in
metaphase
Homologous pairs associate as bivalents and lined up at the equator
in metaphase I
Sister chromatids separate in anaphase
Homologous pairs separate in anaphase I
Sister chromatids separate in anaphase II
REDUCTION DIVISION Gametes are produced with half the number of chromosomes in order to preserve the correct number of chromosomes from generation to generation.
n=23egg
sperm n=23
2n=46zygote
CHROMOSOMES If an organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA). Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for humans is 46 (46 chromosomes per cell).
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.
22 pairs of autosomes 1 pair of sex chromosomes
eye color locus
eye color locus
hair color locus
hair color locus
Paternal Maternal
Sister chromatids
centromere
Homologous chromosomes (tetrad)
PHASES OF MEIOSIS Meiosis I- chromosome number reduced by half
Prophase IMetaphase IAnaphase ITelophase I
Meiosis II- sister chromatids are separated
Prophase IIMetaphase IIAnaphase IITelophase II
PROPHASE I• Chromosomes become visible as DNA becomes compact• Homologous chromosomes pair up• Crossing over occurs• Spindle fibers form Segments of
non-sister chromatids break off and reattach to the other chromosome (increased genetic variation).http://2.bp.blogspot.com/_PeYFXonbRb8/SyVdcPYiGjI/AAAAAAAAAKs/MUNXQharSrM/s320/
MEIOSE+CROSSING+OVER.jpg
METAPHASE I• Homologous chromosomes line up across the cell’s equator by random orientation.• Nuclear membrane disintegrates.
http://www.quia.com/files/quia/users/dschnepp/Meiosis/Metaphase-I
ANAPHASE I• Spindle fibers attach to chromosomes and pull them to opposite poles of the cell.
http://www.quia.com/files/quia/users/dschnepp/Meiosis/Anaphase-I
TELOPHASE I• Spindle fibers disintegrate • Chromosomes uncoil and new nuclear membranes form
•At the end of Meiosis I, cytokinesis occurs. Cells are now haploid.
http://www.quia.com/files/quia/users/dschnepp/Meiosis/Telophase-I
PROPHASE II• DNA condenses into visible chromosomes• Spindle fibers produced
http://www.narragansett.k12.ri.us/NHS/scienceweb/mrreis/AP%20Biology_files/AP%20Biology%20Midterm%20Review%20Sheet%20Diagrams_files/image067.gif
METAPHASE II• Nuclear membranes disintegrate.• Chromosomes line up at the equator (random orientation).• Spindle fibers attach to each sister chromatid at the centromere.•
http://www.narragansett.k12.ri.us/NHS/scienceweb/mrreis/AP%20Biology_files/AP%20Biology%20Midterm%20Review%20Sheet%20Diagrams_files/image067.gif
ANAPHASE II• Centromeres split. Sister chromatids become individual chromosomes.• Spindle fibers pull chromatids to opposite end of the cell.• Animal cells: cell membrane pinches off in the middle; plant cells: cell plate forms.
http://www.narragansett.k12.ri.us/NHS/scienceweb/mrreis/AP%20Biology_files/AP%20Biology%20Midterm%20Review%20Sheet%20Diagrams_files/image067.gif
TELOPHASE II• Chromosomes unwind.• Nuclear envelopes form.•Prepare for cytokinesis.
http://www.narragansett.k12.ri.us/NHS/scienceweb/mrreis/AP%20Biology_files/AP%20Biology%20Midterm%20Review%20Sheet%20Diagrams_files/image067.gif
http://www.narragansett.k12.ri.us/NHS/scienceweb/mrreis/AP%20Biology_files/AP%20Biology%20Midterm%20Review%20Sheet%20Diagrams_files/image067.gif
NON-DISJUNCTION• Chromosomes fail to separate properly during meiosis.• Can result in an abnormal amount of chromosomes in a gamete.• Following fertilization, offspring has either too many or not enough chromosomes.
NON-DISJUNCTION Down syndrome Chromosome 21 fails to separate during Anaphase I
Edward’s syndrome Extra chromosome 18
Non-disjunction of sex chromosomes Klinefelter- XXY Turner syndrome- X XYY male- XYY Trisomy X- XXX
http://img.gawkerassets.com/img/18u9adc8ywun5jpg/ku-xlarge.jpg