BSC 215 Lecture Notes - Lecture 4: Intron, Arginine, Start Codon
8 Jun 2018
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Cell Division
Cell division consists of two phases— nuclear division followed by cytokinesis.Nuclear
division divides the genetic material in the nucleus, while cytokinesis divides the
cytoplasm. There are two kinds of nuclear division—mitosis and meiosis. Mitosis divides
the nucleus so that both daughter cells are genetically identical. In contrast, meiosis is
a reduction division, producing daughter cells that contain half the genetic information
of the parent cell.
The first step in either mitosis or meiosis begins with the condensation of the
genetic material, chromatin, into tightly coiled bodies,
the chromosomes. Each chromosome is made of two identical halves called
sister chromatids, which are joined at the centromere. Each chromatid
consists of a single, tightly coiled molecule of DNA. Somatic cells (all body
cells except eggs and sperm) are diploid cells because each cell contains two
copies of every chromosome. A pair of such chromosomes is called a
homologous pair. In a homologous pair of chromosomes, one homologue
originates from the maternal parent, the other from the paternal parent. In
humans there are 46 chromosomes (23 homologous pairs). In males there
are only 22 homologous pairs (autosomes) and one nonhomologous pair—
the sex chromosomes of X and Y.
When a cell is not dividing, the chromatin is enclosed within a clearly defined
nuclear envelope, one or more nucleoli are visible within the nucleus, and
two centrosomes (each containing two centrioles) lie adjacent to one
another outside the nuclear envelope. These features are characteristic
of interphase, the nondividing but metabolically active period of the cell cycle
(Figure 1). When cell division begins, these features change, as described in
the following sections.
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Mitosis
There are four phases in mitosis (adjective, mitotic): prophase, metaphase,
anaphase, and telophase:
• During prophase, the nucleoli disappear, the chromatin condenses into
chromosomes, the nuclear envelope breaks down, and the mitotic
spindle is assembled. The development of the mitotic spindle begins as
the centrosomes move apart to opposite ends (poles) of the nucleus.
As they move apart, microtubules develop from each centrosome,
increasing in length by the addition of tubulin units. Microtubules from
each centrosome connect to specialized regions in the centromere
called kinetochores. Microtubules tug on the kinetochores, moving the
chromosomes back and forth toward one pole, then the other. Within
the spindle, there are also microtubules that overlap at the center of
the spindle and do not attach to the chromosomes.
• Metaphase begins when the chromosomes are distributed across the
metaphase plate, a plane lying between the two poles of the spindle.
Metaphase ends when the microtubules, still attached to the
kinetochores, pull each chromosome apart into two chromatids. Each
chromatid is complete with a centromere and kinetochores. Once
separated from its sister chromatid, each chromatid is called a
chromosome. (To count the number of chromosomes at any one time,
count the number of centromeres.)
• Anaphase begins after the chromosomes are separated into individual
chromatids. During anaphase, the microtubules connected to the
chromatids (now chromosomes) shorten, effectively pulling the
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chromosomes to opposite poles. Overlapping microtubules, originating
from opposite centrosomes but not attached to chromosomes, interact
to push the poles farther apart. At the end of anaphase, each pole has
a complete set of chromosomes, the same number of chromosomes as
the original cell. (Since it consists of only one chromatid, each
chromosome contains only a single copy of the DNA molecule.)
• Telophase concludes the nuclear division. During this phase, a nuclear
envelope develops around each pole, forming two nuclei. The
chromosomes within each of these nuclei disperse into chromatin, and
the nuclei reappear. Simultaneously, cytokinesis occurs, dividing the
cytoplasm into two cells. Microfilaments form a ring inside the plasma
membrane between the two newly forming nuclei. As the
microfilaments shorten, they act like purse strings to pull the plasma
membrane into the center, dividing the cell into two daughter cells.
The groove that forms as the purse strings are tightened is called
a cleavage furrow.
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Document Summary
Cell division consists of two phases nuclear division followed by cytokinesis. nuclear division divides the genetic material in the nucleus, while cytokinesis divides the cytoplasm. There are two kinds of nuclear division mitosis and meiosis. Mitosis divides the nucleus so that both daughter cells are genetically identical. In contrast, meiosis is a reduction division, producing daughter cells that contain half the genetic information of the parent cell. The first step in either mitosis or meiosis begins with the condensation of the genetic material, chromatin, into tightly coiled bodies, the chromosomes. Each chromosome is made of two identical halves called sister chromatids, which are joined at the centromere. Each chromatid consists of a single, tightly coiled molecule of dna. Somatic cells (all body cells except eggs and sperm) are diploid cells because each cell contains two copies of every chromosome. A pair of such chromosomes is called a homologous pair.
