Mitosis versus Meiosis
Mitosis
- 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
2n 46 chrom
2n (46) 2n
(46)
n (23) n
(23)
2n 46 chrom
n 23
n
23 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
Spermatogen
esis
• 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.
Spermatogen
esis
Sketch Spermatogeneis showing meiotic divisions
1. Spermatogoni um
2. 1°
Spermatocyte 3. 2°
Spermatocyte 4. Spermatid
5. Sperm
46
23 23
23 23 23 23
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) 1
stpolar body 4. a) Ootid
b) polar bodies 5. Ovum
46
23 23 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 PARENT CELL MEIOSIS
(before chromosome replication) Site of
crossing over MEIOSIS I PROPHASE I Tetrad formed by synapsis of homologous chromosomes PROPHASE
Duplicated chromosome
(two sister chromatids)
METAPHASE
Chromosome
replication Chromosome
replication 2n = 4
ANAPHASE TELOPHASE
Chromosomes align at the metaphase plate
Tetrads align at the metaphase plate
METAPHASE I
ANAPHASE I TELOPHASE I Sister chromatids
separate during anaphase
Homologous chromosomes separate during anaphase I;
sister
chromatids remain together No further
chromosomal replication; sister chromatids
separate during anaphase II
2n 2n
Daughter cells of mitosis
Daughter cells of meiosis II
MEIOSIS II Daughter
cells of meiosis I
Haploid n = 2
n n n n
Mistakes 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 red And white blood cells
Fluid
2
Hypotonic solution
3
Fixative
White Blood cells
Stain
4 5
Centromere
Sister chromatids
Pair of homologous chromosomes
Non-disjunctions
A number of disorders are caused by non- disjunctions
1. Down’s Syndrome 2. Edward’s Syndrome 3. Patau’s Syndrome 4. Turner’s Syndrome
5. Kleinfelter’s Syndrome
6. Other severe abnormalities
Non-disjunctions
The 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
Syndrome
Trisomy 21
Down’s
Syndrome Trisomy 21
Cause:
- Non-disjunction of chromosome #21 - Individual has 47
chromosomes Karyotype:
- has 3 chromosome #21 Symptoms:
- mentally & physically
delayed, large forehead, large space between eyes
What does this karyotype
tell us?
Patau’s Syndrome (trisomy)
Cause:
- non-disjunction of chromosome
# 13
Karyotype:
- has 3 chromosome #13 Symptoms:
- 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 #18 Karyotype:
-has 3 of chromosome #18 Symptoms:
-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 chromosomes
Karyotype:
- 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 females Karyotype:
- females have a single X chromosome (XO) Symptoms:
- sterile females, short stature,
underdeveloped gonadal structures
- Affects 1/5000 female births
Non-disjunction in Males
XY XX
XY X X
XXY XO
O
Klinefelter Turner
Non-disjunction in females
XY XX
Y XX O
XXY XO
X
Klinefelter Turner