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LAMC Bio 3 Lecture Notes: Chapter 8 - The Cellular Basis
of Reproduction & Inheritance
Reading Assignment: Chapter 8
Learning Outcomes:
By the end of today’s lecture, you should be able to:
- explain the difference between binary fission, mitosis and meiosis
- list and describe the steps of mitosis and meiosis
- list the processes by which cell division is regulated and explain what
happens to cells that don’t respond to these regulatory mechanisms
- explain the processes that lead to genetic variation during meiosis
- explain the chromosomal problems that can occur during meiosis
I. Definitions:
Chromosome: the structure that contains the genes of a cell; located in
the nucleus.
Chromatin: the material that chromosomes are composed of (a combination
of DNA and protein molecules)
Sister chromatids: results from chromosome duplication; identical copies
of the same chromosome. Sister chromatids are joined at the centromere.
Karyotype: ordered display of an individual’s chromosomes
Homologous chromosomes: pair of chromosomes that are the same size, shape
and code for the same genes; one copy comes from the mother and the other from
the father. Homologous chromosomes can carry different versions of genes.
Genes: nucleotide sequences of DNA. Different forms of a gene are called
alleles.
Locus (plural: loci): the location of the gene on the chromosome
A cell with two copies of a chromosome is diploid (2N). A cell with one
copy of a chromosome is haploid (N).
Gametes: the haploid egg and sperm cells.
Zygote: the fertilized, diploid egg that is formed when a haploid sperm
fuses with a haploid egg.
Cell division functions in reproduction, growth &
development and tissue renewal.
Binary fission (“dividing in half”): the method of cell division in prokaryotic
cells.
II. Mitosis:
Mitosis: the division of a single nucleus into two
genetically identical daughter nuclei.
i) interphase: the period in the cell cycle when the
chromosomes have duplicated into sister chromatids, although the cell is not
actually dividing yet.
ii) prophase: the chromosomes condense; the mitotic spindle
begins to form.
iii) prometaphase: the nuclear membrane starts to break
down; the mitotic spindles attach to the chromatids.
iv) metaphase: all the cell’s duplicated chromosomes are
lined up along the center of the cell.
v) anaphase: sister chromatids separate from each other and
arrive at the two ends of the cell.
vi) telophase: daughter nuclei form around the chromosomes
at either end of the cell.
Cytokinesis: the division of the cytoplasm to produce two separate daughter
cells; occurs during telophase.
One main difference between mitosis in plant and animal
cells is during cytokinesis. In animal cells, the cell membrane simply “pinches
together” to form two separate cells. In plant cells, this can’t happen because
the cell wall is rigid, so a new “cell plate” is formed between the two daughter
cells.
Anchorage, cell density, and chemical growth factors all
affect cell division.
Anchorage dependence: cells must be in contact with a solid surface in order to
divide.
Density-dependent inhibition: cells stop dividing when they touch each other.
Growth factor: a protein secreted by certain cells that stimulates other cells
to divide. When cells run out of the growth factor, they stop dividing.
Cancer cells do not respond normally to the cell cycle
control system; they don’t show density-dependent inhibition and can grow even
in the absence of growth factors.
III. Meiosis:
Meiosis: in a sexually reproducing organism, the division
of a single diploid nucleus into four haploid daughter nuclei; occurs in the
reproductive organs of the parents.
a) Meiosis I: homologous chromosomes separate
includes: interphase I, prophase I, metaphase I, anaphase I, telophase I and
cytokinesis
b) Meiosis II: sister chromatids separate
includes: interphase II, prophase II, metaphase II, anaphase II, telophase II
and cytokinesis
Processes that can lead to variation in the offspring:
i) independent assortment of chromosomes: during meiosis, each homologous pair
of chromosomes lines up along the cell independently of every other homologous
pair.
ii) random fertilization: a single sperm cell randomly fuses with an egg cell
iii) crossing over: an exchange of corresponding segments between two homologous
chromosomes.
Accidents during meiosis can alter chromosome number. E.g.
an extra copy of chromosome 21 causes Down syndrome. Abnormal numbers of sex
chromosomes do not usually affect survival.
Alterations of chromosome structure can cause birth defects
and cancer.
i) deletion: a fragment of a chromosome is lost
ii) duplication: a fragment from one chromosome joins to a sister chromatid or
homologous chromosome, there will be a duplicate copy of those genes.
iii) inversion: a fragment attaches to the original chromosome but in the
reverse direction
iv) translocation: attachment of a fragment to a non-homologous chromosome
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