Download File

Levels of
Organization
in DNA
• Double helix
• Chromatin
with
nucleosome
packaging via
histone
proteins
• Chromosome
Outcome of Mitosis
• Result: two DIPLOID cells that are identical to one
another
• Diploid: describes a cell or nucleus that contains two
complete sets of paired chromosomes, one from each
parent
• Haploid: describes a cell or nucleus that contains only
one set of unpaired chromosomes
•
Animation for Review:
https://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cyto
kinesis.html
Centromere
• Homologous chromosomes: pair of chromosomes with
the same general structure, and sequence of genes ;
one chromosome comes from mom, the other from
dad
• Sister chromatids: either of the two identical copies of
a replicated chromosome ; one half of the duplicated
chromosome
• Locus/loci: the specific location or position of a gene
on a chromosome (Ex: gene for eye color vs. enzyme A
vs. cytochrome C)
How do chromosomes differ? Alleles
• Allele: specific form or variation of a gene ; you inherit one from
mom, and one from dad ; source of variation in traits
• Alleles differ from other alleles by one to few bases (nucleotides)
• Example 1: alleles for blue eyes vs. brown eyes for the gene of eye
color
• Example 2: alleles for A blood-type vs alleles for B blood type for
gene of blood type
Meiosis
• Meiosis: process where a single diploid cell divides
to produce four cells containing half the original
amount of DNA (haploid)
• Produces gametes (eggs and sperm) – also called
“sex cells”
• Only occurs in sex organs (testes or ovaries)
•
•
How Meiosis Works:
http://highered.mheducation.com/sites/0072495855/student_view0/chapter3/animation__how_meiosis_works.html
Stages of Meiosis:
http://highered.mheducation.com/sites/0072495855/student_view0/chapter28/animation__stages_of_meiosis.html
How does meiosis contribute to the variation in
traits?
• Crossing Over during Prophase I:
– Parts of non-sister chromatids cross over and exchange genetic
material
– Produces chromatids with a new combination of alleles
(recombinants)
– Can occur along any part or length of the chromosome
• Independent (or Random) Assortment during Metaphase
I:
– Chromosomes line up randomly
– The inheritance of one gene/trait is independent of the
inheritance of another trait (this means that an allele of one
chromosome has an equal chance of being paired with any allel
on another chromosome) **ONLY FOR NON-LINKED TRAITS
– 2^23 combinations in humans
What is synapsis?
• Synapsis: Pairing of homologous chromosomes
What is a tetrad?
• Tetrad: two pairs of
homologous chromosomes
next to each other
• “Homologous chromosomes in a tetrad
cross over each other, resulting in sister
chromatids with a different set of alleles.”
Variation due to Crossing Over
• Chiasma/Chiasmata: specific location where alleles cross
over
• Recombinant (HL Only): allele combinations that were not
present in the original parents ; result from crossing over
Recombinant Chromosomes After Crossing Over
Variation Due to Independent
Assortment
Outcome of Meiosis
• Gametes (sperm or egg cells) that are unique in
their genetic composition, and haploid nuclei
Why is meiosis considered a “reduction
division”?
• Meiosis consists of two divisions. DNA is replicated
before the first division, resulting in two diploid
cells. DNA is not replicated before the second
division, which causes resulting daughter cells to
have haploid nuclei.
Meiosis in Testes vs. Ovaries
Spermatogenesis
Oogenesis
Formation of Gametes in Testes vs.
Ovaries
Stages of Gamete Development
Gamete Formation in Ovaries
(Oogenesis)
Gamete Formation in Testes
(Spermatogenesis)
Overview of Spermatogenesis
Overview of Oogenesis
11.4.2 Outline the processes involved in spermatogenesis within the
testes, including mitosis, cell growth, the two divisions of meiosis and
cell differentiation
• Spermatogenesis describes the production of spermatozoa
(sperm) in the seminiferous tubules of the testes
• The first stage of sperm production requires the division of
germline epithelium by mitosis
• These cells (spermatogonia) then undergo a period of
growth
• This is followed by two meiotic divisions that result in four
haploid daughter cells
• These haploid cells then differentiate to form sperm cells
• The developing sperm cells are nourished throughout by
the Sertoli cells
11.4.5 Outline the processes involved in oogenesis within the ovary,
including mitosis, cell growth, the two divisions of meiosis, the unequal
division of cytoplasm and the degeneration of polar body
• Oogenesis describes the production of female gametes (ova) within the
ovary
• The process begins during foetal development, when a large number of
cells (oogonia) are formed by mitosis before undergoing a period of growth
• These cells begin meiosis but are arrested in prophase I until puberty
• At puberty, some follicles continue to develop each month is response to
FSH secretion
• These follicles complete the first meiotic division to form two cells of
unequal size
• The cell with less cytoplasm is a polar body (which degenerates), while the
larger cell forms a secondary oocyte
• The secondary oocyte begins the second meiotic division but is arrested in
prophase II (until fertilisation)
• It is released from the ovary (ruptured follicle develops into corpus luteum)
and, if fertilisation occurs, will complete meiosis
• The second meiotic division will produce an ovum and a second polar body