Meiosis: Sex Cell Formation

Bio 100

Tri-County Technical College

Pendleton, S. C.

Mitosis vs. Meiosis• Mitosis

– resulting cells have same chromosome number– one division cycle– resulting cells are same as parent cells

• Meiosis– resulting cells -- 1/2 the chromosome number– two division cycles– resulting cells are gametes

Some key terms

• Somatic cells are all the body’s cell except sex cells

• Sex cells produce gametes

• Karotype is photo taken of chromosomes during late prophase, enlarged, cut out and homologous chromosomes are paired for comparison

One long, light chromosome; one long, dark chromosome; one short, light chromosome, one short, dark chromosome. The two long chromosomes are similar and the two short ones are similar.

Prophase of mitosis and prophase I of meiosis look the same.

In metaphase of mitosis, the chromosomes line up single file. In meiosis, the similar chromosomes line up opposite each other.

Anaphase of mitosis involves separating the chromatids each of which becomes a chromosome. In Anaphase I of meiosis similar chromosomes are separated but the chromatids remain attached.

The cells of late telophase of mitosis each have 4 chromosomes. Mitosis stops here. Meiosis goes into a second division. The cells in Prophase II have 2 chromosomes composed each of two chromatids.

During metaphase II, the chromosomes line up single file like they line up during mitosis.

During anaphase II, the chromatids are separated just like they are in the metaphase of mitosis.

The result of meiosis is 4 cells that have 1/2 the original chromosome number.

Plain and Simple

• Primary purpose of meiosis I is separation of the homologous chromosomes

• Primary purpose of meiosis II is separation of sister chromatids

• Diploid (2N) cell contains ALL of the homologous pairs of chromosomes

• Haploid (N) cell contains only ONE of each of the homologous pairs of chromosomes

A Few General Points

• Reduction division actually occurs in Anaphase I.

• The cells resulting from meiosis are sperm cells in the male and an ovum in the female.

• The way the chromosomes line up at metaphase I is related to Independent Assortment.

Crossing Over

• During metaphase I while similar chromosomes (homologous chromosomes) are lined up, they may exchange pieces.

• The resulting gametes contain new combinations of genetic information.

• Crossing over is a source of variation.

Segregation

• A concept of genetics.

• Genes separate from one another during the formation of gametes.

• Occurs because of the Anaphase I and II divisions of Meiosis.

Independent Assortment

• A concept of genetics.

• Genes end up in gametes independent of each other during gamete formation.

• In metaphase I of meiosis we show the long light on the left, the long dark on the right, the short dark on the left, the short light on the right.

Let me answer this one, Doc

• Sexual reproduction increases genetic variability in the offspring

• Crossing over in Prophase I• Independent Assortment at Metaphase I• Segregation during Anaphase II• Random fertilization (300+ million and just

matter of chance which one (if any) will fertilize egg

• Mate choice? (Darwin was really right!!)

Nondisjunction

• Rare event

• Chromosomes don’t separate correctly during Anaphase 1

• One of the cells at the end of Meiosis I has an extra chromosome and the other has one less

Trisomy

• Results when one of the gametes with an extra chromosome participates in fertilization.

• Three of one of the kinds of chromosomes instead of two which is normal.

Down’s Syndrome

• Nondisjunction of the 21st chromosome

• Trisomy of the 21st chromosome

• Symptoms of Down’s Syndrome– mongoloid facial features– reduced mental ability– susceptibility to infections

Frequency of Nondisjunction

• Increases with age of the mother

• Increases very rapidly after age 37

• Fetal cells can be checked for chromosome number

– amniocentesis

– Genetic Center in Greenwood, S. C.

Inheritance of Sex

• There are two chromosomes that determine sex.

• Autosomes– 44 chromosomes that do not determine sex.

• Sex chromosomes– males - XY– females - XX

Who determines the sex of a baby?

• Males do that!• Females always produce eggs with an X-

chromosome• 50% of all sperm have the X-chromosome and 50%

have the Y-chromosome• If a X-containing sperm fertilizes the egg the

offspring will be female• If a Y-containing sperm fertilizes the egg the

offspring will be male.

Once more..with feeling

• Several other situations affect the genetic material (karotype helpful sometimes)

• Deletion = some genetic material missing• Duplication = missing from one homolog and

attached to the other homolog• Translocation = missing from one chromosome

and attached to a chromosome other than its homolog

• Inversion = detached and then inserted back but in reverse order

More key terms and concepts

• Gametes are sex cells—we are familiar with sperm and egg but could also be + cell or a – cell

• Fertilization is term applied to union of sperm and egg (or + and – cell)

• Zygote is single cell produced by the union of sperm and egg (union of two sex cells)

Other nondisjunction errors

• Turner syndrome (X0)

• Metafemale (XXX)

• Klinefelter syndrome (XXY)

• XYY