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Chapter-6 Cell Cycle and Division
Cell Division
Cells reproduce by cell division, in which a parent cell normally gives rise to two daughter cells
Each daughter cell receives a complete set of hereditary information (DNA) from the parent cell and about half its cytoplasm
The hereditary information DNA is usually identical with that of the parent cell
The cell division of eukaryotic cells by which organisms grow or increase in number is called mitotic cell division
After cell division, the daughter cells may differentiate, becoming specialized for specific functions
The repeating pattern of divide, grow, and differentiate, then divide again is called the cell cycle
Most multicellular organisms have three categories of cells
1. stem cells
2. Other cells capable of dividing
3. Permanently differentiated cells
1.Stem cells :
- have two important characteristics: self-renewal, and the ability to differentiate into a variety of cell types
-Stem cells self-renew because they retain the ability to divide, perhaps for the entire life of the organism
-Some stem cells in early embryos can produce any of the specialized cell types of the entire body
2. Other cells capable of dividing -Some cells other than stem cells are capable of continuing to divide, but typically differentiate into only one or two different cell types -Dividing liver cells, for example, can only become more liver cells 3. Permanently differentiated cells
-Permanently differentiated cells differentiate and never divide again -For example, most heart and brain cells cannot divide
CELL CYCLE
Both prokaryotic and eukaryotic cells have cell cycles that include growth, metabolic activity, DNA replication, and cell division
However, they have major structural and functional differences
Eukaryotic chromosome
Eukaryotic chromosomes are separated from the cytoplasm by a membrane-bound nucleus
Eukaryotic cells always have multiple chromosomes
Eukaryotic chromosomes are longer and have more DNA than prokaryotic chromosomes (human chromosomes are 10 to 80 times longer and have 10 to 50 times more DNA)
Genes
Genes are segments of the DNA of a chromosome
Genes are sequences of DNA from hundreds to thousands of nucleotides long
Each gene occupies a specific place, or locus (plural, loci) on the chromosome
Two important parts of chromosome
Two telomeres
One centromere
It temporarily holds two daughter DNA double helices together after DNA replication
It is the attachment site for microtubules that move the chromosomes during cell division
Homologous
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Chromosomes that contain the same genes are called homologous chromosomes, or homologues
Cells with pairs of homologous chromosomes are called diploid, which means “double”
Cells with half the number of chromosomes are called haploid
Human Chromosomes
A typical human cell has 23 pairs of chromosomes, for a total of 46
Twenty-two out of 23 pairs are called autosomes
The twenty-third pair are called sex chromosomes and are different in the male and the female
The female has two X chromosomes that usually look similar
The male has an X and a Y chromosome that appear very different
However, in a male, the X and Y chromosomes behave as a pair during meiotic cell division
Eukaryotic Cell Cycle
The eukaryotic cell cycle consists of interphase and cell division
Interphase is a time for acquisition of nutrients, growth, and chromosome duplication
During cell division, one copy of every chromosome and half of the cytoplasm and organelles are parceled out into the two daughter cells
Most eukaryotic cells spend the majority of their time in interphase
Interphase is divided into three phases
G1 (growth phase 1):
--cell acquires nutrients
--It grows in size
--It specializes or differentiates
--It decides whether to divide
S (synthesis phase) is characterized by DNA synthesis, during which every chromosome is replicated
G2 (growth phase 2) includes completion of cell growth, protein synthesis for division and preparation for division of the cell into daughter cells
Eukaryotic Cell Cycle
Types of Cell division
There are two types of cell division in eukaryotic cells
Mitotic cell division (mitosis)
Meiotic cell division (meiosis)/reduction division
Over view of mitosis
Prior to cell division, the DNA is replicated
At the end of DNA replication, a duplicated chromosome consists of two identical DNA double helices, called sister chromatids, which are attached to each other at the centromere
During mitotic cell division, the two sister chromatids separate, each becoming an independent chromosome that is delivered to one of the two daughter cells
Mitotic cell division involves two steps
During mitosis (nuclear division), the nucleus of the cell and the chromosomes divide
Each daughter nucleus receives one copy of each of the replicated chromosomes of the parent cell
During cytokinesis (cytoplasmic division), the cytoplasm is divided roughly equally between the two daughter cells, and one daughter nucleus enters each of the daughter cells
Mitosis consists of four phases followed by cytokinesis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Three major events occur in prophase
1. Duplicated chromosomes condense and the nucleolus begins to disappear
2. Spindle microtubules form from centrioles and move toward the nucleus, at the same time nuclear envelop disintegrates, releasing the duplicated chromosome.
3.Chromosomes are captured by the spindle fiber. Each chromatid is attached to a microtubule from opposite pole
PROPHASE
Metaphase
microtubule from one pole that is attached to a chromatid’s centromere complex lengthens or shortens, as necessary, to draw the chromosome to the cell’s equator, in a line perpendicular to the spindle….creating a metaphase plate
During mitotic anaphase, daughter chromosomes (formerly sister chromatids) are drawn to opposite poles
Sister chromatids separate during anaphase into daughter chromosomes
chromatids are pulled apart along the microtubules and toward opposite poles
Clusters of chromosomes that gather at each pole contain one copy of every chromosome
Mitotic stages of animal cell
Telophase is the end stage of mitotic cell division
The spindle microtubules disintegrate
A nuclear membrane forms around each group of chromosomes at the pole
Chromosomes unwind (decondense) and revert to their extended state
The nucleoli (which disappeared in prophase) reappear
Cytokinesis in animal cells
Microfilaments attached to the plasma membrane form a ring around the equator of a cell
The ring contracts and constricts the cell’s equator
Eventually, contraction of the ring pinches off the membrane, forming two daughter cells, each with a nucleus identical with the other
Following cytokinesis, animal cells enter G1 of interphase, thus completing the cell cycle
Cytokinesis in plant cells
Stiff plant cell walls prevent the “pinching off” of cytokinesis seen in animal cells, which only have a plasma membrane
Instead, carbohydrate-filled vesicles assemble along the cell’s equator, between the daughter nuclei
The vesicles fuse into a continuous flattened sac, surrounded by plasma membrane and filled with sticky carbohydrates
This is called a cell plate
The plasma membranes of the plate fuse with the plasma membrane of the cell, forming two cells, with the carbohydrate in between becoming part of the cell wall
As in animals, plant cells enter G1 of interphase following cytokinesis, thus completing the cell cycle
Cytokinesis in plant cells
Meiosis
Meiosis separates homologous chromosomes, producing haploid daughter nuclei
Meiosis is a specialized cell division process that produces haploid gametes
Each gamete receives one member of each pair of homologous chromosomes
Meiosis consists of one round of DNA replication, followed by two rounds of nuclear divisions
One round of DNA replication produces two chromatids in each duplicated chromosome
Because diploid cells have pairs of homologous chromosomes, with two chromatids per homologue, a single round of DNA replication creates four chromatids for each type of chromosome
The first nuclear division, meiosis I, separates the pairs of homologues, with each daughter nucleus receiving one. Each daughter nucleus is haploid, even though each homologue it receives had two chromatids
The second nuclear division, meiosis II, separates the chromatids and parcels one chromatid into each of two more daughter nuclei
At the end of meiosis, there are four haploid daughter nuclei, each with one copy of each homologous chromosome
Meiotic cell division normally produces four haploid cells from a single diploid parent cell
Meiosis Is a Reduction Division That Halves the Number of Chromosomes
Importance of Meiosis
Fusion of gametes keeps the chromosome number constant between generations
Meiosis reduces the chromosome number by half, producing haploid (n) gametes (eggs and sperm)
Fusion of the gametes (fertilization) combines the two haploid chromosome sets to produce a diploid (2n) zygote
If halving of the chromosome number did not occur in gametes, sexual reproduction would double the chromosome number in each new generation, leading to inviability
Meiotic Cell Division Is Essential for Sexual Reproduction
Meiotic Cell Division in an Animal Cell
Meiotic Cell Division in an Animal Cell
The life cycles of all eukaryotic organisms have a common overall pattern
Two haploid cells from different parental organisms fuse during the process of fertilization, creating a diploid cell with new gene combinations
Meiotic cell division occurs, re-creating haploid cells
Mitotic cell division results in the growth of multicellular bodies, or in asexual reproduction
The Human Life Cycle
