Mitosis versus MeiosisMitosis

- the division of 1 cell to produce 2 genetically identical daughter cells

- diploid (2n) = contains 2 copies of the genome

Meiosis- the division of 1

cell into 4 daughter cells (egg or sperm) containing ½ the genetic material

- haploid (n) = contains 1 copy of the genome

2n46 chrom

2n(46)

2n(46)

n(23)

n(23)

2n46 chrom

n23

n23 n

23 n

23

Development of male and female gametes

The Formation of sex cells during meiosis is referred to as gametogenesis

Sperm and egg production are different:

• In males 4 viable sperm are produced Spermatogenesis

• In females 3 of the cells produce are known as polar bodies and do not survive. Only one egg is formed

Oogenesis

Differences• The cyctoplasm of the

female oocyte does not divide equally; one of the daughter cells called an ootid, receives most of the cyctoplasm while the other cells called polar bodies (“nurse cells”) die and are reabsorbed to provide nutrients

• Sperms show equal division of cytoplasm; all 4 daughter cells become viable sperm

• Occurs in the seminiferous tubules where diploid spermatogonia, stem cells that are the precursors of sperm.

•Spermatogonia divide by mitosis to produce more spermatogonia or differentiate into spermatocytes

Spermatogenesis

• Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete

• Then the spermatids differentiate into sperm, losing most of their cytoplasm in the process.

Spermatogenesis

Sketch Spermatogeneis showing meiotic divisions

1. Spermatogonium

2. 1° Spermatocyte

3. 2° Spermatocyte

4. Spermatid

5. Sperm

46

2323

23 232323

1st Meiotic division

2nd Meiotic division

46

Oogenesis• Egg formation

takes place in the ovaries.

• the initial steps in egg production occur prior to birth.

• Diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes

Sketch Oogeneis showing meiotic divisions

1. Oogonium

2. Oocyte

3. a) 2° Oocyte b) 1st polar body

4. a) Ootidb) polar bodies

5. Ovum

46

2323

1st Meiotic division

2nd Meiotic division

46

• By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed.

• By the time she is born, 1–2 million of these remain. Each has begun the first steps of the first meiotic division (meiosis I) and then stopped.

• No further development occurs until years later when the female becomes sexually mature.

• Then the primary oocytes recommence their development, usually one at a time and once a month.

Unfertilized oocyte

• The primary oocyte grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.

Meiosis: Male and Female Differences

• In humans, 22 of the 23 are homologous; these are the autosomes

• Thomas Hunt Morgan discovered that having 2 rod shaped (X chromosome) indicated female and 1 rod with 1 hooked shaped chromosome (Y chromosome) identified males

• The X and Y are not homologous, they are the sex chromosomes

Meiosis and Variation• Unlike mitosis, meiosis does not produce

identical cells

• The cells produced only have half the number but the chromosomes therefore only ½ the genetic information

• What chromosomes end up in what cell all depend upon how the chromosomes line up in Metaphase I

Meiosis and Variation• If the two blue

chromosomes line up on the same side and the two red chromosomes line up on the same side

• Then the daughter cells will have either the genetic information from the red or blue chromosome

Meiosis and Variation• If 1 red and 1 blue

line up on the same side

• The daughter cells will have genetic information from both red and blue chromosomes

MITOSIS MEIOSISPARENT CELL

(before chromosome replication) Site ofcrossing over MEIOSIS I

PROPHASE ITetrad formedby synapsis of homologous chromosomes

PROPHASE

Duplicatedchromosome(two sister chromatids)

METAPHASE

Chromosomereplication

Chromosomereplication

2n = 4

ANAPHASETELOPHASE

Chromosomes align at the metaphase plate

Tetradsalign at themetaphase plate

METAPHASE I

ANAPHASE ITELOPHASE I

Sister chromatidsseparate duringanaphase

Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together

No further chromosomal replication; sister chromatids separate during anaphase II

2n 2n

Daughter cellsof mitosis

Daughter cells of meiosis II

MEIOSIS II

Daughtercells of

meiosis I

Haploidn = 2

n n n n

Abnormalities during Meiosis

The movement of the chromosomes in a dividing cell is so precise that only 1 in every 100,000 divisions will contain an error

Non-disjunction= an error during meiosis where sister

chromatids fail to come apart resulting in gametes that are missing or have extra chromosomes

Non-disjunction• One daughter cell

will be missing one of the chromosomes (22)

• Other daughter cell will contain an extra chromosome (24)

• In humans, non-disjunction produces gametes with 22 or 24 chromosomes

Non-disjunctions• When gamete with 24 chromosomes

joins a normal gamete with 23 chromosomes, the zygote will contain 47 (instead of 46). This zygote will have 3 chromosomes in place of the normal pair = trisomy

• When the gamete with 22 chromosomes joins a normal gamete with 23 chromosomes, the zygote has 45; this zygote will have 1 chromosome in place of the normal pair = monosomy

• The best way to study non-disjunctions is by looking at karyotypes

• Karyotypes are an inventory of an individuals chromosomes– A karyotype

usually shows 22 pairs of autosomes and one pair of sex chromosomes

KARYOTYPES

Preparation of a karyotype

Figure 8.19

Blood culture

1

Centrifuge

Packed redAnd white blood cells

Fluid

2

Hypotonic solution

3

Fixative

WhiteBloodcells

Stain

4 5

Centromere

Sisterchromatids

Pair of homologouschromosomes

Non-disjunctionsA number of disorders are caused by non-

disjunctions1. Down’s Syndrome2. Edward’s Syndrome3. Patau’s Syndrome4. Turner’s Syndrome5. Kleinfelter’s Syndrome6. Other severe abnormalities

Non-disjunctionsThe risk of

chromosomal abnormalities increases with maternal age because the egg cells are older

Women over the age of 35, who have children increase their chance exponentially

Karyotype for Down’s SyndromeTrisomy 21

Down’s SyndromeTrisomy 21Cause: - Non-disjunction of

chromosome #21- Individual has 47

chromosomesKaryotype: - has 3 chromosome #21Symptoms: - mentally & physically

delayed, large forehead, large space between eyes

What does this karyotype tell us?

Patau’s Syndrome (trisomy) Cause:

- non-disjunction of chromosome # 13Karyotype: - has 3 chromosome #13Symptoms: - child with multiple and severe abnormalities, and severe mental retardation

-  head very small, eyes absent or very small

- hairlip, cleft palate, usually malformations of internal organs

- most cases child dies soon after birth

What does this karyotype tell us?

Edward’s Syndrome: Trisomy 18

Cause: - Non-disjunction of chromosome #18Karyotype: -has 3 of chromosome #18Symptoms: -very small and weak, head flattened, hands short with very little development

- severe mental handicap with life expectancy of less than one year

What does this Karyotype tell us?

Klinefelter’s Syndrome (Trisomy 23)Cause: - Non-disjunction sex

chromosomesKaryotype: - 2 X chromosomes and a Y

chromosome (XXY)Symptoms: - Appears male at birth but

upon puberty releases high levels of female hormones

- Sterile males, breast development, slightly feminized physique

- Affects 1/1000 male births

What does this karyotype tell us?

Turner Syndrome (Monosomy)

Cause: - Non-disjunction of X chromosome in femalesKaryotype: - females have a single X chromosome (XO)Symptoms: - sterile females, short stature,

underdeveloped gonadal structures- Affects 1/5000 female births

Non-disjunction in MalesXY XX

XY X X

XXY XO

O

Klinefelter Turner

Non-disjunction in femalesXY XX

Y XX O

XXY XO

X

Klinefelter Turner

Other possible abnormalites caused by non-disjunction

-these infants do not live very long – a few days at the most; many are premature and stillborns-These photos may disturb you, you are not required to watch