Mitosis Vs Meiosis

MITOSIS• Somatic cells• 2N (2 chromatids) /4C>2N/2C

MEIOSIS• Germ cells• 2N (4 chromatids-tetrad)/4C> after 2 divisions =N/C

N=haploid # of chromosomes. C=DNA content.

MITOSIS-SOMATIC CELLS• These basic events of mitosis

include , nuclear membrane break down chromosome condensation, formation of the mitotic spindle, and attachment of chromosomes to the spindle microtubules. Sister chromatids then separate from each other and move to opposite poles of the spindle, followed by the formation of daughter nuclei and nuclear membrane and cytoplasm divides.

• MPF-mitosis promoting factor (a protein kinase (Cdc2/cyclin B). Triggers APF

• APF: anaphase promoting factor a ubiquitin ligase degrades cyclin B promotes-cytokinesis

Mitosis and Meiosis • Chromosomes separate in

• Meiosis IAnd

• Chromatids divide in• Meiosis II

Meiosis In Females

• Meiosis-Prenatal (3mts)• 7 to 9 mts: diplotene-

dictyotene• Nuclear memb/nucleolus-

resting stage• 99% oocytes degenerate-

puberty onset• First half cycle-Pit.LH Stimulates-meiosis-1st polar

body; If fertilized (fallopian tube) 2nd polar

• Nucl mem M & P-Pronucleus fuse----first cleavage

IN FEMALES • In females, meiosis occurs in precursor cells known as oogonia. Each oogonia that initiates meiosis will divide twice to form a single oocyte and three polar bodies. However, before these divisions occur, these cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the Dictyate stage and lacks the assistance of centrosomes.

•

Diakinesis Dictoytene

3monthAND 2 pronuclei join zygote is formed

MALE AND FEMALE MEIOSIS PRODUCT BY #’s.

Swimmers vs Keepers• Of the 7 million potential oocytes

that form during the 5th month of pregnancy.

• 2 million at birth, vast majority will be eliminated prior to being ovulated and these are resorbed by the body.Typical female # of meitic products:

• 45 yrs x 12 cycles/yr = 540 eggs.

• A 74 hour cycle occurs with several hundred million sperm cells being produced daily with no similar selection process involving polar bodies or

daughter cells.• Meiosis in males is a lifetime

endeavor and while sperm production decreases after reaching a peak in the mid 20's and the percentage of sperm that swim erratically increases with age, a healthy human male will continue manufacturing sperm from puberty until death.

Meiosis male• After pubertyonly.• Human males produce

200,000,000 sperm per day. • In males, meiosis occurs in

precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males occurs during spermatogenesis.

• Reabsorbed and cycled.

Age Ovum and Meiosis

• The tracing (b) identifies these components, and the smooth or wavy lines suggest, respectively, an intact or degenerating spindle apparatus (the ages of the women are indicated). The chromosomes are well organized at the metaphase plate at the equator of the cells in the younger women (the 22-year-old’s oocyte, on the upper left, is viewed on a tilt). In contrast, the 44-year-old woman’s oocyte has one chromosme, at the top, dislocated from the metaphase plate, and the disposition of the other chromosmes at the equator is not as regular as in the younger women. (the color photographs are from Battaglia, D.E., et al. Influence of maternal age on mitotic spindle assembly in oocytes from naturally cycling women. Hum. Reprod. 1996, 11, 2217-2222)

MEIOSIS OOCYTES FROM YOUNGER AND OLDER WOMEN

• Figure illustrating what may be the physical basis of the maternal age effect. The microtubules of the spindle stain green, and the chromosomes stain orange.

Mosaicism- Post Zygotic

• Mosaics: Individuals with two or more genetically different cell populations are referred mosaics.

Mostly seen as sex chromosomal aberrations, but also occurs

in autosomal chromosomes.

1. Post-zygotic-mitotic nondisjunction.

2. Anaphase lag: one chromosome is lost during anaphase movement.

• Severity based on when it occurred.

Background slides for meiosis

Will explain in class…

MEIOSIS-2 DivisionsDiv 2Div 1

Div 1

Meiosis I: Division 1Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to as a reductional division In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromatids, N) per daughter cell from the first division.

Prophase I (Leptotene, Zygotene, Pachytene , Diplotene (dictyotene), DiakinesisDuring prophase I: DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma)LeptoteneThe first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads. During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.ZygoteneThe zygotene stage, also known as zygonema, from Greek words meaning "paired threads", occurs as the chromosomes approximately line up with each other into homologous chromosome pairs. This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place.

Pachytene stage, also known as pachynema, from Greek words meaning "thick threads” contains the following chromosomal crossover. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. Sex chromosomes in PAR1 and 2 exchange. Exchange takes place at sites where recombination nodules (the chiasmata) have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the processDiplotene (ARRESTED HERE IN FEMALE FETUS)During the diplotene stage, also known as diplonema, from Greek words meaning "two threads", chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed in Anaphase I.In human fetal oogenesis all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty. In males, only spermatogonia ( meiosis has NOT started) exist until meiosis begins at puberty.DiakinesisChromosomes condense further during the diakinesisstage, from Greek words meaning "moving through".] This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. Meiosis division II same as Mitosis Division.