Cell Reproduction
Chromosomes—composed of chromatin that has become
tightly
wrapped around globules of proteins called histones. Each
chromosome consists of two individual parts called
chromatids. The
point at which each chromatid is attached is called a
centromere.
In all sexually reproducing organisms, chromosomes occur
in
homologous pairs. Each chromosome of a pair has the
same size
and shape as its homologue. A cell that contains both
chromosomes
of a homologous pair is said to be diploid. A cell that
contains only
one chromosome from each homologous pair is said to be
haploid.
Cell reproduction—occurs when the parent cell divides
producing
two new daughter cells. Types of cell reproduction include:
1. Binary fission—two identical cells produced from the
splitting of the original cell in two equal halves. Most cells
reproduce in this manner.
2. Budding—one larger cell and one smaller cell produced
from the splitting of the original cell into two cells of
different sizes. A unicellular fungus called yeast
reproduces in this manner.
Cell cycle—includes the non-reproducing stage in which
cells spend
most of their life, called interphase, and the reproducing
stage in which
new cells are formed called mitosis. During interphase, cells
are
using energy to carry out the basic cell functions. New cell
parts such
as mitochondria and ribosomes are being produced early in
interphase. About midway through interphase, each
chromosome and
the DNA it contains replicates.
Mitosis—division of the nucleus during the reproductive
portion of the
cell cycle. Occurs in somatic cells throughout the body.
Mitotic phases:
1. Prophase
a. nucleoli disappear
b. nuclear membrane disintegrates
c. coiling and shortening of chromosomes give nucleus
a grainy appearance
d. centrioles break apart and begin to migrate toward
opposite poles, spindle begins to form
e. during late prophase, chromosomes appear as two
joined sister chromatids joined at the
centromere
2. Metaphase
a. microtubules of the spindle capture the
chromosomes by the centromere
b. chromosomes move to the equator of the spindle
and become aligned
3. Anaphase
a. centromeres split and sister chromatids begin
to move apart
b. chromosomes migrate to opposite poles of the cell
4. Telophase
a. cell cleavage furrow appears in animal cells as
plasma membrane is pulled together by microtubules
b. cell plate begins to appear in plants
c. telophase ends with the reverse of the events that
began prophase
Control of the cell cycle:
1. An estimated 25 million cell divisions occur per second in
the
human body
2. Some cells divide at a faster rate than others, some never
reproduce once they mature (nerve cells)
3. Cell-to-cell contact may be one of the mechanisms that
control the rate of cell reproduction
Meiosis—cell division that occurs in gonads (reproductive
organs)
resulting in haploid (1/2 the normal chromosome number)
gametes
(sex cells).
Characteristics of meiosis:
1. Occurs only in gonads
a. Males—testes
b. Females—ovaries
2. Produces cells called gametes that have half the normal
chromosome number for that species. Haploid is the term
used to describe the chromosome number in this type of
cell.
3. A two stage cell division.
a. Meiosis one—involves the formation of tetrads
and is the reduction division, that is, the division
that makes the resulting daughter cells haploid.
Chromosomes do not break apart at the
centromeres during the first meiotic division as
they do in mitosis. Crossing over may occur early
in this stage of meiosis.
b. Meiosis two—involves the separation of the
chromatids and formation of the actual daughtercells
or gametes.
4. Types of meiosis include:
a. Spermatogenesis—production of the male
Gametes in humans, called sperm. Four full-sized
daughter cells produced per mother cell
(primary spermatocyte) in this type of meiosis.
b. Oogenesis—production of the female gametes in
humans, ova (eggs). Produces only one full-sized
daughter cell per mother cell (primary
oocyte) in this type of meiosis.
5. The union of a haploid sperm and a haploid ovum
produces a diploid (containing the normal number of
chromosomes for a species) zygote which develops into
a new individual of that species.