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

http://wps.prenhall.com/wps/media/objects/487/498728/FG09_03LFR.JPG

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

HOMEWORKVocab

Meiosis, reduction division, diploid, haploid, sister chromatids, centromere, condensation, crossing over, homologous chromosomes, random orientation, Down syndrome, non-disjunction

Other Summarize Nature of ScienceChallenge yourself 6-10 Exercises 6-10