Spring BSCI103, 2012
Cellular Basis for Reproduction and Inheritance, Chapter 8
I.
Relationship between cell division and reproduction
a. Genes are specific portions of chromosomes, DNA, that carry out functions, these genes are
the heritable bits of information that are passed on to new cells and to offspring
b. Asexual reproduction offspring are identical copies of parents
1. Many animals and plants reproduce all by themselves aphids, orchids this is called
asexual reproduction
2. Offspring have the same number and kinds of DNA as parents they are clones =
identical genetic copies
c. Sexual reproduction offspring look somewhat like parents
1. Gametes, special sex cells, are formed by a process called meiosis, these cells contain
half the number of chromosomes found in the regular or somatic cells. Gametes are
brought together during fertilization
2. The product of fertilization - a new cell which has pair of chromosomes and pairs of
genes on each chromosome, one from the mother, one from the father
3. These pairs of genes may differ somewhat in their molecular form and function these
different forms of genes are called alleles.
4. Through sexual reproduction new combinations of alleles come together and this results
in the diversity of forms and behaviors that we see in peoples traits
5. This variation is the foundation for evolutionary change
d. Cell division, cells arise from existing cells = cellular reproduction has two main roles
1. Building a complete organism - from a fertilized egg (1 cell) becomes complete
organism (millions of cells)
2. Allowing for sexual and asexual reproduction
II. Prokaryotes (bacteria and archaea) and binary fission
II a. Prokaryotes are simple cells with single chromosome
1. DNA and attached proteins
2. DNA duplicates ( = replicates)
3. Plasma membrane and cell wall divide to form new cell
III. Eukaryotic cell cycle and mitosis
a. Eukaryotic chromosomes
1. DNA + attached proteins
2. Usually stretched out
3. During cell division the DNA is duplicated and duplicated strands are joined. These
duplicated strands are called = sister chromatids of the chromosome
3. Held together at the centromere = point of attachment for microtubules
4. The sister chromatids eventually separate to form two new cells with complete sets of
chromosomes
b. Cell cycle two major phases
1. Mitosis or M phase is one of the two parts or phases of the cell cycle, it is the shorter
phase during M phase the nucleus divides followed by cytoplasmic division in G1
2. Interphase is the longer phase and has three parts
1 a. Gap 1 or G1 phase here cell growth takes place
2 b. Synthesis or S phase here DNA replicates sister chromatids form.
c. Gap 2 or G2 phase here the cell prepares for division
c. Stages of mitosis or M phase
1. Prophase chromosomes condense, microtubules form, centrosomes containing
(centrioles) move to opposite sides (poles) of cell
2. Metaphase chromosomes line up along equator of cell, microtubules attach to
centromeres at kinetochore (motor proteins)
3. Anaphase sister chromatids break apart, move along microtubules to opposite poles
4. Telophase chromosomes de-condense, new nuclear envelope forms, cytokinesis
takes place
d.
Division of the cytoplasm -In plants a cell plate forms to divide the old cell into two
new ones, In animals the cytoplasm pinches in two, also called cleavage
III.
Factors affecting cell division
a. anchorage dependence to divide cells must be in contact with a surface = anchored
b. density-dependent inhibition cell division slows down as cell population becomes denser
c. growth factors - inhibition results when growth factors, growth stimulating proteins, are no
longer produced by neighboring cells
IV. Cell-cycle control system and growth factors
a. The cell cycle is directed by a control system of proteins that trigger and coordinate the major
events
b. The phases in the cell cycle have regulated by check points in the G1, G2, and M, stages
c. The cell cycle stops unless these checkpoints are overridden by go ahead signals, these
signals are growth factors = proteins
IV.
Cell-cycle out of control and cancer
a.
Cancer cells do not have a properly functioning cell-cycle control system
b.
They divide excessively and form a mass called a tumor
c.
Regulated check points are bypassed without growth factors
d.
Density- dependent regulation does not work
e.
Cancer cells make their own growth factors and do not respond to inhibitory signals
f.
Benign tumors are an abnormal mass of normal cells
g.
Malignant tumor are abnormal masses of cancerous cells
h.
Cancer cells may spread beyond their original site - this is called metastasis
i.
Cancers are named by the type of tissue they originate in
1. carcinomas external or internal coverings of the body such as linings of skin or
intestine
2. sarcomas tissues that support the body such as muscle or bone
3. leukemias lymphomas and blood forming and related tissues such as bone marrow,
spleen, lymph nodes
j.
Therapy
1. Radiation disrupts cell division thereby differentially killing cancer cells
2. Chemotherapy disrupts cell division. Eg. vinblastin prevents spindle fibers from
forming, taxol freezes the spindle fiber both compounds were discovered in plants
V.
Meiosis and Crossing over
a. Chromosome number
3 1. Each species has a unique number of chromosomes: Humans = 46, Gorillas = 48.
4 2. Two sets of chromosomes: half (23) from mother, half (23) from father
3. Typical body cells called somatic cells are diploid (2n) meaning both sets of
chromosomes are present - the diploid number for humans is 46.
5 4. Some cells, the gametes or egg and sperm cells, are only haploid (1n) for humans eggs
and sperm are haploid (23).
6 5. The process of meiosis produces gametes - each daughter cell will receive the of a
full set of chromosomes and will be haploid
7 6. Intercourse allows one haploid sperm to unite with one haploid egg to create a diploid
cell called the zygote = fertilized egg
8 7. Mitotic divisions then ensure that all somatic cells have 46 chromosomes
b. Meiosis halves the number of chromosomes from diploid to haploid
Meiosis I
1. Interphase chromosomes replicate and sister chromatids form
2. Prophase 1 maternal and paternal copies (homologous) of each chromosome pair =
form tetrads, condense, and swap genetic material = crossover the points of crossover
are called chiasma), nuclear envelope breaks apart
3. Metaphase 1 pair of homologous chromosomes line up at equator of cell and
microtubules attach to kinetochore of centromere
4. Anaphase 1 Homologous chromosome separate and move to opposite poles of cells
5. Telophase 1 Cytoplasm divides and two new cells each haploid form
Meiosis II
6. Prophase 2 New microtubules form and attach to centromeres
7. Metaphase 2 Chromosomes line up along equator of daughter cell
8.
9.
VI.
VII.
VIII.
IX.
Anaphase 2 Sister chromatids break apart and travel to opposite poles
Telophase 2 Nuclear envelopes reform, cytoplasm of new daughter cells divide and 4
haploid cells are formed; these are the gametes eggs or sperm cells in animals and spores
in plants.
Sources of variation in offspring independent orientation of chromosomes in meiosis and
random fertilization
1.
The orientation of homologous chromosomes at tetrad formation during meiosis is
random this allows for different combinations of chromosomes in the gametes
2.
There are 2n (n = is the haploid number) possible combinations of chromosomes in
gametes during meiosis
3.
Human females can produce about 223 = 8 million possible combinations of maternal and
paternal chromosomes in her gametes, males do the same, therefore, there can be about
64 trillion combinations of chromosomes in a zygote
4.
Random fertilization increases the number of possible combinations even more
Sources of variation in offspring crossing over
1.
Chromosomes have genes that may be in different forms called alleles
2.
During meiosis in prophase I, homologous chromosomes may exchange portions of DNA
that contain different alleles of genes
3.
These sites of exchange or crossing-over are called chiasma
4.
The new combinations of genes are called recombinant and the process is called genetic
recombination
5.
Genetic recombination is another important source of genetic variation
Alterations to chromosome numbers
1.
A karyotype is a display of an organisms chromosomes for humans 22 pairs of
autosomes and 1 pair of sex chromosomes
2.
An extra copy of autosome #21 in humans causes Downs syndrome also knows as
trisomy 21. The chance of Downs syndrome increases as women age.
3.
Sometimes chromosomes fail to separate correctly during meiosis I or meiosis II these
accidents are called nondisjunctions and result in gametes with extra or missing
chromosomes this results in zygotes and offspring with extra or too few chromosomes.
4.
Nondisjunction of autosomes often results in death of the embryo
5.
Nondisjunction of sex chromosomes may not be lethal but can result in abnormalities
such as; Klinefelter syndrome = XXY = male, sterile (1/2000), normal male XYY =
male, taller than average (1/2000), metafemale = XXX female, limited fertility (1/1000),
Turner syndrome XO = female, sterile, short (1/000).
Alterations to chromosome structure
1. Deletion fragment lost
2. Duplication fragment from one chromosome attaches to homologous chromosome
3. Inversion fragment from one chromosome reattached to the same chromosome but in
reverse direction cri du chat is and inversion
4. Translocation fragment from one chromosome reattaches to non-homologous chromosome
these may be reciprocal chronic myelogenous leukemia
5. These alterations may or may not be harmful and result in serious illness
6. Chromosome alterations in somatic cells may lead to cancer