Meiosis

VocabularySOMATIC:

Any cell except a sperm or eggAUTOSOME

Any chromosome other than a sex chromosome (X or Y)

GAMETE An egg or sperm

Mitosis allows asexual reproduction

Meiosis allows sexual reproduction.

Parents are diploid

Homologous chromosomes are separated into single sets.

Humans have 23 pairs of chromosomes.

One pair (the sex chromosomes) are not identical in males.

MALE

FEMALE

Sex ratios: 106 boys: 100 girls (at birth)

Karyotypes are pictures of chromosomes.

Some (More) Vocabulary

Alleles

Eye Color

Eye Color

Brown, green

Meiosis separates chromosome pairs to form

haploid gametes.Meiosis I: Homologous pairs are separated (as sister chromatids) into 2

cells

Meiosis II: Similar to mitosis: sister chromatids are separated = 4 haploid

cells.

MEIOSIS I

Interphase I• Interphase is very

similar to interphase in mitosis

MEIOSIS I

Prophase I• Double stranded

chromosomes and spindle fibers appear;

• Nuclear membrane and nucleolus fade (synapsis aligns homologous chromosomes)

MEIOSIS I

Metaphase I• Chromosome pairs

(chromatids) line up• Spindle fibers attach to

centromeres and centrioles

MEIOSIS I

Anaphase I• Chromatids separate

from matching pair (independent assortment occurs here)

MEIOSIS I

Telophase I• Cytoplasm divides

and 2 cells form• Each cell still has a

diploid (2n) number

MEIOSIS II

Prophase II

Metaphase II

Anaphase II

Telophase/cytokinesis

II

SUMMARY OF MEIOSIS II

Meiosis II follows the same steps as mitosis

Key differences:◦In interphase II, there is no replication of genetic material

◦When the sister chromatids separate during anaphase II, they are now haploid (1n)

◦After telophase II, you have 4 haploid (1n) cells with genetic variation (they are no longer identical)

Genetic VariationIndependent Assortment: Homologous pairs can separate

in different parental combinations.

2n possible gametes created

Possible Gametes

In humans:223 = 8 million possible gametes

This is without crossing over, which increases the number of distinct chromatids (exact number of chiasma determines how many different chromatids will exist)

Crossing Over: During synapsis (Prophase I) portions of non-sister chromatids are exchanged.

Genetic Variation

Random Fertilization: Each sperm will fertilize and egg randomly

Genetic Variation

8 million possible gametes for each parent:

8,000,0002 =

64 trillion possible zygotes

Without Crossing over. With it, there are even more!

These are a pair of homologous chromosomes with genes A - E.

These homologous chromosomes are made of 2 sister chromatids each.

These are called TETRADS

The tetrads line up side by side (synapsis) in Prophase I

Segments cross over (form chiasma) and the pieces are traded.

Synaptonemal Complex! CROSSING OVER

The sister chromatids end up with different copies of the genes.

Notice that genes that are located close together will not separate during crossing over as much as a pair of genes that are further apart.

Frequency of Crossing OverMeasuring the

frequency of crossing over tells you the distance between two genes (in map units)

What do you think?

With all of the variation produced in meiosis, how can parents have IDENTICAL TWINS?

How does the production of sperm differ from production of egg?

Errors in Meiosis

Errors in meiosis are passed down to offspring (because they occur in sperm or eggs)

These errors can create sperm and eggs that have the wrong number or types of chromosomes

Types of errors

Deletion: part of a chromosome is deleted (removed)

Duplication: part of a chromosome is repeated

Translocation: part of a chromosome is moved to another chromosome

Inversion: part of a chromosome is removed, then flipped around and reattached

Inversion

Non-disjunctionFailure of

chromosomes to separate properly in meiosis

Meiosis I: homologues don’t separate

Meiosis II: Sister chromatids don’t separate

Types of errors

Monosomy: one parent does not give a copy of one chromosome◦There is only 1 total in the child

45 instead of 46 total chromosomesThe only monosomy that we see in

humans (that is nonfatal) is Turner syndrome

Turner syndromeFemales inherit only one X chromosome

(XO), instead of XX

1/2500 girls98% of fetuses with Turner Syndrome

miscarry

Short stature, broad chest, webbed neck

Turner syndrome

TrisomiesWhen one parent

gives two copies of a chromosome◦The child has 3

total of that chromosome (47 instead of 46 total chromosomes)

Trisomy 21

Down’s syndrome3 copies of chromosome 21Impairment of cognitive ability (avg. IQ 50,

vs. 100 in rest of population)Almond-shaped eyesHealth problems: heart failure, GERD, ear

infections, sleep apnea1/733 births

◦More common with older mothers, but 80% are born to women under 35

Trisomy 18 Edwards

syndrome◦2nd most common

trisomy95% miscarryOf those born,

50% die within 2 months; 5-10% survive first year

Apnea, heart and kidney defects

1/3000 live births

May be a partial trisomy (less severe)

Trisomy 13

Patau syndrome◦Mental

impairment◦Polydactyly

(extra fingers or toes)

◦Microcephaly (small brain)

◦85% do not survive 1 year

May be a partial trisomy (less severe)

Trisomies

Trisomy 16◦Most common trisomy, but results in

miscarriageTrisomy 9

◦Mental retardation, deformities of skull, nervous system

Trisomies may be partial (due to translocation) or mosaic

Mosaicism

Mosaicism: During very early development, non-disjunction occurs in mitosis and as a result some percentage of an individual’s cells (but not all) have the trisomy/monosomy

46

4646

46

47

47 47

Trisomies in sex chromosomes

XXX: No difference between this and a normal woman (XX)◦(X inactivation -> Barr bodies)

XXY: Klinefelter◦Small testicles, reduced fertility◦Most common sex trisomy (1/500 males)

XYY: Not named because normally asymptomatic (normal phenotype)

Trisomies in sex chromosomes

Many other possibilities, many don’t have any symptoms

Meiosis Interphase I

Meiosis Early prophase I

Meiosis Mid-prophase I

Meiosis Late prophase I

Meiosis Metaphase I

Meiosis Anaphase I

Meiosis Telophase I

Meiosis Cytokinesis complete

Meiosis I complete; cytokinesis by cell plate formation.

Meiosis Prophase II

Meosis Metaphase II

Meiosis Anaphase II

Meiosis Telophase II and cytokinesis beginning.

Meiosis Cytokinesis complete.