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Chapter 3

55-211 Chapter Notes - Chapter 3: Oocyte, Antonie Van Leeuwenhoek, Corpus Luteum

Biological Sciences
Course Code
BIOL 2111

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Chapter 3 The Chromosome Theory of Inheritance
3.1 Chromosomes: The Carriers of Genes
Genes reside in the nucleus
The nature of the specific link between sex and reproduction remained a mystery until
Anton Van Leeuwenhoek, one of the earliest and most astute of microscopists,
discovered in 1667 that semen contains spermatozoa
Genes Reside on Chromosomes
In the 1880s, a newly discovered combination of organic
and inorganic dyes revealed the existence of the long,
brightly staining threadlike bodies, within the nucleus that
we call chromosomes
In embryonic cells, the chromosome threads spilt,
lengthwise in 2 just before cell division
Fertilization: The Union of Haploid Gametes to Produce Diploid Zygotes
Gametes and other cells that carry only a single set of chromosomes are called haploid
Zygotes and other cells carrying 2 matching sets are diploid
Zygotes and other cells carrying 2 matching sets are diploid
Microscopic studies suggest that the nuclei of egg and sperm contribute equally to the
offspring by providing a single set of n chromosomes. The zygote formed by the union of
haploid gametes is diploid (2n)
Species Variations in the Number and Shape of Chromosomes
Scientists analyze the chromosomal makeup of a cell when the chromosomes are most
visible, at a specific moment in the cell cycle of growth and division, just before the
nucleus divides (metaphase)
Cells in metaphase can be fixed and stained with one of several and accentuate the
Chromosomes that match in size, shape, and banding are called homologous
2 homologues of each pair contain the same set of genes, although for some there may
be different alleles
Modern methods of DNA analysis can reveal differences between the maternally and
paternally derived chromosomes of a homologous pair
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Sex Chromosomes
Water S. Sutton a young American graduate student at Columbia in 1st decade of 20th
century was one of the earliest cytologists to realize that particular chromosomes carry
the info for determining sex
If sex is an inherited trait determined by a pair of sex chromosomes that separate to
different cells during gamete formation, then XX and YY cross would account for both
the mutual exclusion of genders and the near 1:1 ratio of males to females
Species Variations in Sex Determination
Klinefelter males are typically tall and thin and sterile, and are sometimes retarded (XXY)
(XO) Turner females are usually sterile, lack secondary sexual characteristics (pubic hair)
short, webbed neck and shoulders
Gender having 2 difference sex chromosomes is termed the heterogametic sex because
it gives rise to 2 different types of gametes
3.2 Mitosis: Cell Division That Preserves Chromosome Number
Each convoluted thread of chromatin is composed mainly of DNA and protein
One or 2 darker areas would be called nucleoli, and they play a key role in the
manufacturing of ribosomes
The chromatin condenses into discrete threads, and then each chromosome compacts
even further together at the centromere that can be identified in karyotype analysis
Each rod in a dup is called a chromatid
During Interphase, Cells Grow and Replicate Their Chromosomes
Interphase consists of 3 parts: gap 1 (G1), synthesis (S), and gap 2 (G2)
G1 lasts from the birth of a new cell to the onset of chromosome replication
Synthesis is the time when the cell duplicates its genetic material by synthesizing DNA
G2 is the interval between chromosome duplication and the beginning of mitosis
Regulatory Checkpoints Ensure Correct Chromosome Separation
In higher organisms, a cells decision to divide depends
on both intrinsic and extrinsic factors
Breakdown of the mitotic machinery can produce
division mistakes that have crucial consequences
3.3 Meiosis: Cell Divisions That Have Chromosome Number
Mitotically dividing and G0 arrsted cells whose
descendants continue to make up the vast majority of
each organisms tissues throughout the life time of the
Germ cells arise later in plants during floral development
instead of during embryogenesis
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Germ cells become incorporated in the reproductive organsovaries and testes in
Union of haploid gametes at fertilization yields diploid offspring that carry the combined
genetic heritage of 2 parents
In Meiosis: The Chromosomes Replicate Once but the Nucleus Divides Twice
2 successive nuclear divisions
With each round, the cell passes through a prophase, metaphase, anaphase, and
telophase, followed by cytokinesis
Chromosomes duplicate at the start of meiosis I, but they do not duplicate in meiosis II,
which explains why the gametes contain half the number of chromosomes found in
other body cells
Mistakes in Meiosis Produce Defective Gametes
Segregational errors during either meiotic division can lead to aberrations such as
trisomies, in next generation
Most autosomal trisomies, are lethal in utero (exception is trisomy 21)
Some hybrid animals carry non homologous chromosomes that can never pair up and
segregate properly
Meiosis Contributes to Genetic Diversity
The wider the assortment of different gene combinations among members of a species
the greater the chance that at least some individuals will carry combinations of alleles
that allow survival in a changing environment
Because only chance governs which paternal or maternal homologues migrate to the 2
poles during the first meiotic division , different gametes carry a different mix of
maternal and paternal chromosomes
Reshuffling of genetic information through crossing over during prophase 1 ensures an
even greater amount of genetic diversities
3.4 Gametogenesis
Gamete formation gives rise to haploid gametes marked not only by the events of
meiosis per se but also by cellular events that precede and follow meiosis
Oogenesis in Humans Produces One Ovum from Each Primary Oocyte
Oogenesis begins when diploid germ cells in the ovary multiply rapidly by mitosis and
produce a large number of primary oocytes, which undergo meiosis
Meiosis 1 results in the forma5tion of 2 daughter cells that differ in size (asymmetric)
first polar body
Meiosis 11 undergoes another asymmetrical division to produce a large haploid ovum
and a small haploid 2nd polar body
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