E - Bio @ Horton
Unit – Cell Reproduction
Notes – Meiosis
AP Biology
Sexual Reproduction
A. Halving the Chromosome Number
1.
2.
3.
4.
5.
Meiosis is nuclear division reducing chromosome number from diploid (2n) to haploid (n)
number. The process can be called reduction division.
Haploid (n) number is half the diploid number of chromosomes.
Requires gamete formation and then fusion of gametes to form a zygote.
A zygote always has a full or diploid (2n) number of chromosomes.
If gametes contained same number of chromosomes as body cells, doubling would soon
fill cells.
B. Homologous Pairs of Chromosomes
1. In a diploid body cell, chromosomes occur as pairs.
a. Each set of chromosomes is a homologous pair; each member is a
homologous chromosome or homologue.
b. Homologues look alike; they have same length and centromere position; have
similar banding pattern when stained.
c. A location on one homologue contains the same types of gene which occurs at
the same locus on other homologue.
2. Chromosomes duplicate just before nuclear division.
a. Duplication produces two identical parts called sister chromatids, held together at
centromere.
b. Non-sister chromatids do not share the same centromere.
c. Duplication allows each new sex cell to have a copy of the genetic material.
3. One member of each homologous pair is inherited from either male or female parent; one
member of each homologous pair is placed in each sperm or egg.
C. Overview of Meiosis
1. Meiosis involves two nuclear divisions and produces four haploid daughter cells.
2. Each daughter cell has half the number of chromosomes in the diploid parent nucleus.
3. Meiosis I is the nuclear division at the first meiotic division.
a. Prior to meiosis I, DNA replication occurs and each chromosome has two sister
chromatids.
b. During meiosis I, homologous chromosomes come together and line up (cause
unknown) in synapsis.
c. During synapsis, the two sets of paired chromosomes lay alongside each other
as bivalents or a tetrad.
d. Crossing over is an exchange of homologous segments between non-sister
chromatids of bivalent during meiosis I; results in genetic recombinations.
e. After crossing over occurs, sister chromatids of a chromosome are no longer
identical.
4. Meiosis I
a. No replication of DNA is needed between meiosis I and II because chromosomes
were already doubled.
b. During meiosis II, centromeres divide; daughter chromosomes derived as sister
chromatids separate.
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c. Chromosomes in the four daughter cells have only one chromatid.
d. Counting the number of centromeres verifies that parent cells were diploid, each
daughter cell is haploid.
e. In the animal life cycle, daughter cells become gametes that fuse during
fertilization; this restores the diploid number in body cells.
D. Genetic Recombination
1. This is the exchange of genetic material between homologous partners during tetrad.
2. Due to genetic recombination, offspring have a different combination of genes than their
parents.
3. Without recombination, asexual organisms must rely on mutations to generate variation
among offspring; this is sufficient because they have great numbers of offspring.
E. Crossing-Over Introduces Variation
1. Crossing-over results in exchange of genetic material between non-sister chromatids.
2. At synapsis, homologous chromosomes are held in position. This occurs in Prophase I.
3. As the synapsis complex breaks down at the beginning of anaphase I, homologues are
temporarily held together by chiasmata, regions where the non-sister chromatids are
attached due to crossing-over.
4. The homologues separate and are distributed to separate cells.
5. Due to crossing-over, daughter chromosomes derived from sister chromatids are no
longer identical.
F. Independent Assortment of Homologous Chromosomes Produces Variation
1. This occurs due to the independent lin-up of homologous pairs during Metaphase I.
3
2. Independent assortment in a cell with only three pairs of chromosomes is 2 or eight
combinations.
23
3. In humans with 23 pairs of chromosomes, the combinations possible are 2 or
8,388,608.
G. Fertilization Also Promotes Variation
1. Meiosis increases variation due to the fact that any sperm can fertilize the egg.
2. When gametes fuse at fertilization, chromosomes donated by parents combine.
23 2
3. Chromosomally different zygotes from same parents are (2 ) or 70,368,744,000,000
combinations possible without crossing over.
23 2
4. If crossing over occurs once, then (4 ) = 4,951,760,200,000,000,000,000,000,000
combinations of genetically different zygotes are possible for one couple.
Meiosis Has a Number of Phases
1. Both meiosis I and meiosis II have four phases: prophase, methaphase, anaphase and
telophase.
2. Horton Biology Meiosis Simulation will provide further info on this topic.
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A. Prophase I
1. Nuclear division is about to occur: nucleolus disappears; nuclear envelope fragments;
centrosomes migrate away from each other; and spindle fibers assemble.
2. Homologous chromosomes undergo synapsis forming bivalents; crossing over may occur
at this time in which case sister chromatids are no longer identical.
3. Chromatin condenses and chromosomes become microscopically visible.
B. Metaphase I
1. During prometaphase I, bivalents held together by chiasmata have moved toward the
metaphase plate.
2. In metaphase I, there is a fully formed spindle and alignment of the bivalents at the
metaphase plate.
3. Kinetochoares are regions just outside centromeres; they attach to spindle fibers call
kinetochore spindle fibers.
4. Bivalents independently align themselves at the metaphase plate of the spindle.
5. Maternal and paternal homologues of each bivalent may be oriented toward either pole.
C. Anaphase I
1. The homologues of each bivalent separate and move toward opposite poles.
2. Each chromosome still has two chromatids.
D. Telophase I
1. Only occurs in some species.
2. When it occurs, the nuclear envelope feforms and nucleoli reappear.
E. Interkinesis
1. This period between meiosis I and meiosis II is similar to interphase of mitosis.
2. However, no DNA replication occurs.
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F. Meiosis II
1. During metaphase II, the haploid number of chromosomes align at metaphase plate.
2. During anaphase II, centromeres divide and daughter chromosomes move toward the
poles.
3. At the end of telophase II and cytokinesis, there are four haploid cells.
4. Due to crossing-over, each gamete can contain chromosomes with different types of
genes.
5. In animals, the haploid cells mature and develop into gametes.
6. In plants, the daughter cells become spores and divide to produce a haploid adult
generation.
7. In some fungi and alge, a zygote results from gamete fusion and immediately undergoes
meiosis; therefore, the adult is always haploid.
G. How Meiosis I Differs from Mitosis
1. DNA is replicated only once before both mitosis and meiosis; in mitosis there is only one
nuclear division; in meiosis there are two nuclear divisions.
2. During prophase I of meiosis, homologous chromosomes pair and undergo crossingover; this does not occur during mitosis.
3. During metaphase I of meiosis, paired homologous chromosomes align at the metaphase
plate; in mitosis individual chromosomes align.
4. During anaphase I in meiosis, homologous chromosomes with centromeres intact
spearate and move to opposite poles; in mitosis at this stage, sister chromatids separate
and move to poles.
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H. How Meiosis II Differs from Mitosis
1. Events of meiosis II are same stages as in mitosis.
2. However, the nuclei contain the haploid number of chromosomes in meiosis.
3. Mitosis produces two daughter cells; meiosis produces four daughter cells.
Gametogenesis
1. Life cycle refers to all reproductive events between one generation and next.
2. In animals, the adult is always diploid [Instructors note: some bees, etc. have haploid
male adults].
3. In animals, it occurs during production of gametes; adult is diploid and gametes are
haploid.
4. Mosses are haploid most of their cycle; oak trees are diploid most of their cycle.
5. In fungi and some algae, organisms you see is haploid and produces haploid gametes.
6. In males, meiosis is part of spermatogenesis, the production of sperm, and occurs in the
testes.
7. In females, meiosis is part of oogenesis, the production of eggs cells, and occurs in the
ovaries.
8. After birth, mitotic cell division is involved in growth and tissue regeneration.
A. Spermatogenesis and Oogenesis in Humans
1. Spermatogenesis
a. In the testes of males, primary spermatocytes with 46 chromosomes divide
meiotically to form two secondary spermatocyes, each with 23 duplicated
chromosomes.
b. Secondary spermatocyes divide to produce four spermatids, also with 23
daughter chromosomes.
c. Spermatids then differenitiate into sperm (spermatozoa).
d. Meiotic cell division in males always results in four cells that become sperm.
2. Oogenesis
a. In the ovaries of human females, primary oocytes with 46 cchromosomes divide
meiotically to form two cells, each with 23 duplicated chromosomes.
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b. One of the cells, a secondary oocyte,, receives most cytoplasm; the other cell,
a polar body, disintegrates or divides again.
c. A secondary oocyte begins meiosis II and then stops at metaphase II.
d. At ovulation, the secondary oocyte leaves the ovary and enters an oviduct where
it may meet a sperm.
e. If a sperm enters secondary oocyte, oocyte is activated to continue meiosis II to
completion; result is a mature egg and another polar body, each with 23
daughter chromosomes.
f. Polar bodies serve to discard unnecessary chromosomes and retain most of the
cytoplasm in the egg.
g. The cytoplasm serve as a source of nutrients for the developing embryo.
Chromosomal Mutations
A. Mutations
1. Changes in chromosomes or genes that pass to offspring if they occur in gametes.
2. Mutations increase the amount of variation among offspring.
3. Chromosomal mutations include changes in chromosome number and structure.
B. Changes in Chromosome Number
1. Monosomy occurs when and individual has only one of a particular type of chromosome.
2. Trisomy occurs when and individual has three of a particular type of chromosome.
3. Nondisjunction is the failure of chromosomes to separate; it is more common during
meiosis I than meiosis II; it can occur in mitosis.
4. Monosomy and trisomy occur in plants and animals; in autosomes of animals, it is
generally lethal.
5. Nonlethal human monosomies and trisomies include the following:
a. Turner syndrome: monosomy where the individual has single X chromosome.
b. Down syndrome is most common trisomy among humans; it involves
chromosome 21.
6. Polyploidy: offspring end up with more than two complete sets of chromosomes.
a. Terms indicate how many sets of chromosomes are present (triploids [3n],
tetraploids [4n], etc.).
b. Polyploidy does not increase variation in animals; judging from trisomies, it would
be lethal.
c. Polyploidy is a major evolutionary mechanism in plants; probably involved in 47%
of flowering plants including major crops.
d. Hybridization in plants can result in doubled number of chromosomes; an even
number of chromosomes can undergo synapsis during meiosis; successful
polyploidy results in a new species.
C. Changes in Chromosomal Structure
1. Environmental factors including radiation, chemicals, and viruses, can cause
chromosomes to break; if the broken ends do not rejoin in the same pattern, this causes
a change in chromosomal structure.
2. Inversion: a segment that has become separated from the chromosome is reinserted at
the same place but in reverse; the position and sequence of genes are altered.
3. Translocation: a chromosomal segment is removed from one chromosome and inserted
into another, nonhomologous chromosome. Translocation heterozygotes usually have
reduced fertility due to production of abnormal gametes.
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4. A deletion is a type of mutation in which an end of a chromosome breaks off or when two
simultaneous breaks lead to the loss of a segment.
a. Even if only one member of pair of chromosomes is affected, a deletion can
cause abnormalities.
b. Cri du chat syndrome is deletion in which an individual has a small head, is
mentally retarded, has facial abnormalities, and abnormal glottis and larynx
resulting in a cry resembling that of a cat.
5. A duplications is a doubling of a chromosomal segment.
a. A broken segment from one chromosome can simply attach to its homologue.
b. Unequal crossing-over may occur.
D. Identification of Chromosomal Defects
A. Karyotypes
1. Human somatic (body) cells have 22 pairs of autosomes, one pair of sex chromosomes;
total of 46.
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2. Karyotypes shows chromosomes paired according to size, shape, and appearance in
metaphase.
a. To view chromosomes, cells are treated and photographed just prior to dividing.
b. Chromosomes are then sorted and arranged by homologous pairs, often by
computer imaging.
c. Members of a pair have the same size, shape, and banding pattern.
d. Chromosomes in a karyotype are aligned from largest to smallest.
e. A karyotype can be used to diagnose chromosomal abnormalities.
3. Sex chromosomes of a normal male are X and Y; a normal female has two X
chromosomes.
4. All chromosomes besides X and Y are autosomes.
5. To view chromosomes of an unborn child, cells must be sampled from an embryo.
a. Chorionic villi sampling removes a small tissue sample from the embryo side of
the placenta.
b. Amniocentesis uses a long needle to extract fetal cells floating in the amniotic
fluid.
Some Conditions Identified
A. Down Syndrome
1. Down syndrome is most common autosomal trisomy, involves chromosome 21.
2. Most often, Down syndrome results in three copies of chromosome 21 due to
nondisjunction during gametogenesis.
a. In 23% of cases, the sperm had the extra chromosome 21.
b. In 5% of cases, there is translocation with chromosome 21 attached to
chromosome 14; this translocation could have occurred generations earlier and
is not age-related.
3. Chances of a woman having Down syndrome child increase with age.
4. Chorionic villi sampling testing or amniocentesis and karyotyping detects a Down
syndrome child.
5. Down syndrome child has tendency for leukemia, cataracts, faster aging, and mental
retardation.
6. Gart gene, located on bottom third of chromosome 21, leads to high level of purines and
is associated with mental retardation; future research may lead to suppression of this
gene.
B. Abnormal Sex Chromosomal Inheritance
1. Turner (XO) syndrome females have only one sex chromosome, and X.
a. Turner females are short, have broad chest and webbed neck.
b. Ovaries of Turner females never become functional; therefore, do not undergo
puberty.
2. Klinefelter syndrome males have one Y chromosome and two or more X chromosomes.
a. Affected individuals are sterile males; testes and prostate are underdeveloped.
b. Individuals have large hands and feet and long arms and legs.
3. Triplo-X females have three or more X chromosomes.
a. There is no increased femininity; most lack any physical abnormalities.
b. There is an increased risk of having triplo-X daughters of XXY sons.
c. May experience menstrual irregularities, including early onset of menopause.
4. XYY males with Jacob syndrome have two Y chromosomes instead of one.
a. Results from nondisjunction during meiosis II.
b. Usually taller than average; suffer from persistent acne; tend to have lower
intelligence.
c. Earlier claims that XYY individuals were likely to be aggressive are not correct.
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C. Fragile X Syndrome
1. X chromosome is nearly broken; most often found in males.
2. As children: hyperactive or autistic; delayed speech; account for part of higher proportion
of males in institutions for mentally retarted.
3. As adults: large testes, unusually protruding ears.
4. Occurs in females, but symptoms are less severe.
5. Passes from symptomless male carrier to grandson.
6. Traced to excessive repeats of base triple CGG (cytosine, guanine, guanine); up to 230
copies compared to normal 6-to-50 copies.
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Images: Campbell, Neil and Reece, Jane. Biology (6 ed.) San Francisco: Benjamin Cummings
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Modified Notes: Mader, Sylvia. Biology (7 ed) New York: McGraw Hill Publishing
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