Transcript Cell Division (2)

Cell Division (2)
Meiosis: two-stage cell division in which the
chromosome number of the parent cell is
reduced by half. Meiosis is the process by which
gametes are formed.
Gametes: sex cells that have a haploid (n)
chromosome number
Meiosis Terminology
Haploid: refers to the number of chromosomes in a
gamete (n). A gamete is a sex cell.
Diploid: refers to twice the number of
chromosomes in a gamete (2n). Every cell of the
body, with the exception of sex cells, contains a
diploid chromosome number.
In humans:
Haploid Chromosome # = 23
Diploid Chromosome # = 46
Meiosis
• Offspring carry genetic information from each of
the parents
• Each of the 23 chromosomes you receive from
your father is matched by 23 chromosomes from
your mother.
Homologous chromosomes: paired
chromosomes similar in shape, size,
gene arrangement, and gene
information
Meiosis Terminology
• During fertilization, a haploid (n =
23) sperm cell unites with a haploid
(n = 23) egg cell to produce a
diploid (2n = 46) zygote.
Zygote: a cell resulting from the union
of a male and female sex cell, until
it divides and then is called an
embryo
Mitosis vs Meiosis
Meiosis
Interphase
DNA replication occurs before meiosis
Stages of Meiosis
Overall: 2 nuclear divisions that produce 4 haploid cells
Meiosis I  Reduction Division – diploid reduced to haploid
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II  Separation of two chromatids
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I
Meiosis II
Meiosis I: Prophase I
• Homologous chromosomes come together
• Each pair is made up of 4 chromatids
Tetrad: a pair of homologous chromosomes, each
with two chromatids
Synapsis: the pairing of homologous
chromosomes
Meiosis I: Prophase I
Crossing over: the exchange of genetic material
between two homologous chromosomes
 recombination shuffles the allele content
between homologous chromosomes (increasing
genetic diversity)
Meiosis I: Metaphase I
• Attached spindle fibers line up the
homologous pairs at the equatorial plate
Meiosis I: Anaphase I & Telophase I
• Homologous chromosomes are moved to
opposite ends
• One member of each homologous pair will be
found in each daughter cell
Meiosis II
• Pairs of chromatids separate and move to opposite poles
There is NO replication of chromosomes before meiosis II
each cell has half the # of chromosomes as the parent cell
Meiosis II: Prophase II
• The nuclear membrane dissolves, the
centrioles move to opposite poles, and the
spindle fibres begin to form
Meiosis II: Metaphase II
• The chromosomes attach themselves to the
spindle fibres and line up at the equatorial
plate
Meiosis II: Anaphase II
• The sister chromatids are separated and move
to opposite poles
• The nuclear membrane begins to re-form
around the single chromosomes
Meiosis II: Telophase II
• The cytoplasm separates.
• The chromosome number has been reduced by half.
Cytokinesis
• Four haploid daughter cells are formed.
Worksheet 1
Worksheet 2
Formation of Sex Cells in Humans
• The formation of gametes in animals is called
gametogenesis
• In humans, gametes are produced in the
testes (males) and in the ovaries (female)
Spermatogenesis (Males)
• A primary spermatocyte (diploid sex cell)
undergoes meiosis I and II and forms 4
spermatids (haploid), which will develop into
sperm
• Males can produce one billion sperm cells
every day!
Oogenesis (Females)
• A primary oocyte (diploid sex cell) undergoes meiosis I, but the
division of the cytoplasm is unequal, resulting in the formation of a
polar body
• The larger cell (ootid) and the polar body both go through meiosis II
• However, the polar bodies die and the nutrients are absorbed by the
body
• Only one egg (ovum) is produced by meiosis
• Women have about 300 000 oocytes, of which only 400 – 500 will be
released during the reproductive years
Worksheet 3-Homework
Worksheet 4
Sex Chromosomes
• sex chromosomes: the pair of chromosomes that have a role
in the sex of an individual
• autosomes: the chromosomes not involved in sex
determination
• Although they are physically different, the X and the Y
chromosomes are able to synapse for part of their length and
function as homologous chromosomes during meiosis.
Sex Determination in Humans
• In humans, xx is female,
xy is male.
• Sex determination
depends on the sex
chromosome inherited
from the father, who is
xy.
Sex Determination in Non-Humans
• In birds, the mother
determines the sex of the
child.
– WZ is female, ZZ is male
• In ants, bees, and wasps,
sex is determined by the
number of chromosome
sets an individual has.
– males are haploid and
females are diploid
• In turtles, sex is determined
by the environment,
particularly the
temperature at which the
eggs are incubated.
– HOT=female, COLD=male
Amniocentesis
• In Amniocentesis, fetal cells are
obtained from 2 tablespoons of
amniotic fluid
• A 3 to 4 inch needle is pushed
through the abdomen and into
the amniotic fluid.
• Do 3 to 4 months after
pregnancy.
• This procedure is done without
an anaesthetic.
• The cells are then cultured and a
karyotype or other testing can be
done.
• This test can be used to detect
genetic disorders such as downs
syndrome (trisomy 21).
Chorionic Villus Sampling (CVS)
• CVS- cells are obtained from
the chorion of the placenta
(tissue from the placenta)
using a syringe.
• A needle is inserted through
the abdomen or through
the vagina and cervix, using
the ultrasound for guidance.
• Do after 10 or 12 weeks
after pregnancy.
• The cells are then cultured
and a karyotype or other
testing can be done.
• This test can be used to
detect genetic disorders
such as downs syndrome
(trisomy 21).
Karyotype Chart
• karyotype chart: a picture of chromosomes
arranged in homologous pairs
• monosomy: the condition where there is a single
chromosome in place of a homologous pair
• trisomy: the condition where there are three
homologous chromosomes in place of a
homologous pair
Abnormal Meiosis: Nondisjunction
• Nondisjunction occurs when two homologous chromosomes move to the
same pole during meiosis. The result is that one of the daughter cells will be
missing one chromosome while the other will retain an extra chromosome.
• -can occur in any cell during mitosis BUT effects are most devastating during
the formation of sex cells in meiosis.
• Can cause Down Syndrome.
• Nondisjunction means “doesn’t break apart”.
Down Syndrome
Down syndrome: a trisomic disorder in which a zygote receives
three homologous chromosomes for chromosome pair
number 21
-regardless of race: a round, full face; enlarged and creased
tongue; short height; and a large forehead.
-1 in 1000 babies is born with Down syndrome
Down Syndrome
• increased risk as mother gets older
Turner Syndrome (X)
Turner syndrome: a monosomic disorder in
which a female has a single X chromosome
-the individual has 45 chromosomes
-The only condition where humans can survive
without one of a pair of chromosomes.
-1 in every 5000 female babies
-In 98% of fertilized eggs, the egg is
spontaneously aborted.
Turner Syndrome
• short (average is 4 feet
and 8 inches)
• web of skin between neck
and shoulders
• sterile
• breasts and other
secondary sex
characteristics develop
incompletely
• normal intelligence but
individuals do have
learning difficulties
Klinefelter Syndrome (XXY)
Klinefelter syndrome: a trisomic disorder in
which a male carries an XXY condition
-child appears to be a male at birth
-however, as he enters sexual maturity, he
begins producing high levels of female sex
hormones.
Males with Klinefelter syndrome are sterile.
-1 in 1000 males have genotype XXY
Klinefelter Syndrome (XXY)
• Small testes
• Female features including
breast development
• Long limbs. Slightly taller
than average. (about 6
feet)
• Language impairments,
speak later.
• Might have more X
chromosomes: XXXY or
XXXXXY. These individuals
have mental retardation.
Super Males (XYY)
•
•
•
•
occurs in 1 in 1000 males
very tall (about 6 feet 2 inches)
moderate to severe acne
average to slightly lower than average
intelligence
• most men who are XYY don’t even know it!
Metafemales (XXX)
• occurs in 1 in 1000 females
• sterile
• no other physical or mental problems
Review
Worksheet 5
Karyotype Lab Part 1
Hand in your lab at the end of class.