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