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Bio 100 - Cellular Reproduction
Cellular Reproduction and Genetics among Eukaryotes
Overview
The perpetuation of living things (reproduction) requires cell division - the splitting of
one cell into two cells
The events that take place inside the cell between one division and the next are
collectively called the cell cycle
And the net results of these events is cellular reproduction
The Nucleus: Information for Directing Cell Growth
The nucleus contains the genetic information needed to govern what a cell looks like and
how it performs
The definitive answer to the question (that the nucleus governed cell growth) came from
some simple, yet elegant experiments conducted by the German biologist Joachim
Hammerling using the algae Acetabularia
Chromosomes
Stained structures containing the hereditary information of the cell
DNA exists as long threads called chromatin, that undergo compact folding and coiling
at the time of cell division to form structures called chromosomes
All along the chromosomes are discrete portions of the DNA, that represent heredity units
that specify a organisms traits = genes
The two members of each pair of chromosome are called homologous chromosomes
Homologous chromosomes are maternal and paternal copies of the same chromosome
As part of its preparation for cell division, a cell copies its entire genome by duplicating
each of its chromosomes.
After replication, each chromosome consists of 2 sister chromatids
A specialized region of the chromosome, called the centromere, holds the 2 chromatids
together
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Bio 100 - Cellular Reproduction
Mitosis versus Meiosis
Among multicellular organisms, mitosis is the basis for growth through repeated
divisions of the body's cells - called somatic cells
Mitosis maintains the number of chromosomes in all the somatic cells of an individual
In contrast, meiosis occurs only in the germs cells and is involved in the formation of the
gametes
Meiosis yields daughter cells that have half as many chromosomes as the parent cell;
these cells are said to be haploid (N cell; where N is the number of chromosomes)
The Cell Cycle of Mitosis
Mitosis represents just a small part of the entire cell cycle.
The cell spends about 95% of its time in that portion of the cell cycle called interphase
It is during interphase that the cell grows, duplicates its DNA, and copies it cellular
components in preparation for cell division
Overview of the cell cycle:
A. Interphase
i) G1 phase
ii) S phase
iii) G2 phase
B. M Phase
Mitosis - the division of the nucleus (genetic material) of the cell - nuclear division
i) Prophase
ii) Metaphase
iii) Anaphase
iv) Telophase
Mitosis is usually followed by the splitting of the cytoplasm of the cell to form 2
daughter cells - cytokinesis
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Bio 100 - Cellular Reproduction
Mitotic Spindle Apparatus
Just prior to nuclear division, the original system of microtubules supporting the cell and
organelles disassemble into subunits, and then reassemble into a new system of
microtubules or the spindle apparatus
Each spindle apparatus consists of a pair of barrel-shaped centrioles and 2 radial arrays
of microtubules:
1. Asters, a small array of microtubules radiating toward the poles
2. Spindle fibers, long array of microtubules that attach to chromosomes toward
the center and are responsible for chromosome movement during the different
stages of mitosis
Cytokinesis ("cell movement")
In both plant and animal cells, new cell membranes form near the midline (= metaphasal
plate) once occupied by the chromosomes during metaphase, thus separating the nuclei
into 2 cells
a. Animals
Animal cells divide from the outside-in as a circle of micofilaments called a
contractile ring, pinches each cell in two
The constriction of the ring creates a dent or furrow in the cell surface
This furrow deepens and eventually squeezes the cell in two
b. Plants
Plant cells retain their shape throughout the entire cell cycle, and divide from the
inside-out
During telophase, vesicles filled with cell wall and cell membrane precursors pinch off
from the Golgi apparatus and collect in the cell's center
The many vesicles gradually fuse, forming a central partition, or cell plate, made of cell
membrane and cell wall material
This central partition divides the cell into 2 daughter cells
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Bio 100 - Cellular Reproduction
Regulating the Cell Cycle
The starting and stopping of cell division is often regulated by some kind of factor
external to the cell
It is apparent that most cells of plants and animals will not divide unless they are in
contact with a solid surface - “anchorage dependence”
May prevent cells from dividing inappropriately when separated from their usual
surroundings
Cells often stop dividing in response to contact with other cells - density-dependent
(contact) inhibition
Many cells begin to divide in response to chemical signals called growth factors
Cancer
Cancer cells don't stop growing after contact with other cells
Cancer cells crawl over other cells, invade healthy tissues, and multiply into abnormal
masses called tumors
Causes Believed to be related to changes in a cell's DNA that can alter how receptor proteins on
the cell surface recognize other cells
Other cancerous cells make their own growth factors in order to keep dividing
Some seemingly do not require anchorage dependence
Malignant tumors consist of cancerous cells; these tumors metastasize
Benign tumors do not metastasize; they remain in the site of origin
Cancers are named according to the tissue or organ of origin:
- Carcinomas – originate in the external or internal coverings of the body
- Sarcomas – arise in supportive tissues (e.g., bone and muscle) of the body
- Leukemias are cancers of the blood vascular system, and lymphomas are
cancers of the lymph nodes
Two treatments for cancer include: radiation and chemotherapy
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Bio 100 - Cellular Reproduction
1. Asexual Reproduction
New but genetically identical offspring grow directly from a few cells of a single parent
The group of cells undergo mitotic division and often remain attached to the parent for a
period of time
There are many different types of asexual reproduction:
a. Fission - the splitting off of new individuals from existing ones.
b. Budding - in which a new individual grows out from the body of the original.
c. Fragmentation - a breaking of the body into several pieces, each of which develops
into a new organism.
d. Regeneration - the replacements of parts of an organism that are lost due to injury.
2. Sexual Reproduction
In this form of reproduction, parents generate specialized cells called gametes which
eventually fuse together to zygotes
Sperm cells and eggs cells come from a special group of cells called the germ cells
The gametes of sexual reproduction are formed by a special kind of cell division called
meiosis, a process that reduces the number of chromosomes that a cell possess in half
If a cell has just one set of chromosomes it is said to be haploid
Meiosis - The production of Haploid gametes
Interphase I
Meiosis I: prophase I, metaphase I, anaphase I, telophase I
Cytokinesis
Meiosis II: prophase II, metaphase II, anaphase II, telophase II
Cytokinesis
Meiosis resembles mitosis in that it has the same general phases
But there is a significant difference in these 2 processes
In meiosis, a cell undergoes 2 consecutive divisions: Meiosis I and Meiosis II, with each
part is preceded by a period of interphase, producing 4 daughter cells result from the
divisions rather than just 2 in mitosis
But, note that there is only a single duplication of chromosomes
Thus, the 4 daughter cells produced have half as many chromosomes as the starting cell
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Bio 100 - Cellular Reproduction
Genetic Variation Arises during Meiosis
While mitosis leads to the production of identical daughter cells, the cells produced by
meiosis can only be regarded as similar, not identical
Because during meiosis, paternal and maternal chromosomes are reshuffled - genetic
recombination - and new chromosome combinations occur
Independent assortment
The assortment of parental chromosomes is completely random during meiosis I
And this random distribution process is called independent assortment
Chromosomal crossing over
This is a process in which parts of the homologous maternal and paternal chromosomes
are exchanged
The sites of crossing-over appear as X-shaped regions called chiasma
During Meiosis I the homologous chromosomes are paired
Summary: Mitosis, Meiosis, and Sexual Reproduction
Asexual reproduction has several advantages:
- only requires on individual to produce offspring
- it requires very little energy expenditure
- many offspring can be produced in a short period of time
Theoretically this method of reproduction is most advantageous in stable, favorable
environments because it perpetuates the exact same genetic combination over and over.
Sexual reproduction can also be advantageous:
- inheriting genes from the 2 parents and genetic recombination (independent
assortment and crossing over) results in an increase in the genetic variability
of offspring
- genetic variability is particularly advantageous in unstable or unpredictable
environments.
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