Chromosomes are rod-shaped structures made of DNA and proteins. The DNA in eukaryotic cells wrap tightly around proteins called histones. Each half of a chromosome is called a chromatid. The two chromatids attach at the centromere. (Figure 1) While cells are not dividing, chromosomes are not as tightly coiled. Regions of DNA must be uncoiled for use inside the cell. This less tightly coiled DNA is called chromatin. Human and animal chromosomes are categorized as either sex chromosomes or autosomes. Sex Chromosomes are chromosomes that determine the gender of an organism and other traits as well. In humans, XX is a female, and XY is a male. All of the other chromosomes in an organism are autosomes. Each offspring organism gets one copy of an autosome from each parent. The two copies that are created are called homologous chromosomes, or homologues. Homologous chromosomes are the same size and shape. A karyotype (Figure 2) is a diagram in which the homologous chromosomes are layed out right next to each other. If something has two sets of chromosomes, it is said to be diploid, or 2n. Almost all human cells are diploid (except sex cells). If something only has 1 set of chromosomes, it is said to be haploid, or 1n. The human sex cells, egg and sperm, are haploid.
Figure 1
Figure 2
8.2 Cell Division (Asexual)
Prokaryotes
Prokaryotes have cell walls but no nuclei and membrane bound organelles. A prokaryote's single DNA molecule is shaped as a circular chromosome attached to the inner plasma membrane [imagine a rope attached to the inner wall of a tent]. Binary fission is the division of a prokaryotic cell into two offspring cells (Figure 3). First, the DNA within a cell is copied. The cell grows in size while a new cell membrane forms between the two DNA copies. Eventually, the cell membrane splits the one large cell into two identical cells.
Figure 3
Figure 4
Eukaryotes
In eukaryotic cell division, both the cytoplasm and the nucleus divide. Mitosis results in new cells with identical genetic material of the orginal cells. Asexual reproduction is the production of offspring from one parent. Meiosis occurs during the formation of gametes which are haploid reproductive cells. Through this process, the chromosome number is reduced to half the original. The cell cycle is the repeating set of events in the life of a cell. The time between divisions is called interphase, which is split into 3 phases. Cytokineses is the division of the cell's cytoplasm. During the first stage of interphase, G1, offspring cells grow to mature size. From here, cells go to either the G0 phase, or the S phase. The G0 phase is a way to exit the cell cycle. Many cells in the human body are in this stage, like nevous system cells that stop dividing at maturity and don't normally divide again. During the S phase, the cell's DNA is copied (synthesised).During the G2 phase, the cell prepares for cell division.
Checkpoints
There are a few "traffic lights" that trigger mitosis. The first is in between G1 and S. This checks if the cell has grown enough and whether or not it should go to S or G0. In the second checkpoint, in late G2 phase, DNA repair enzymes check the results of DNA replication. At the mitosis checkpoint, before cytokinesis, a cell is passed to enter th eG1 phase again. Cancer disrupts these checkpoints and make cell growth unpredictable and harmful.
Prophase
The first phase of mitosis, prophase begins with the shortening and tight coiling of DNA into rod-shaped chromosomes that can be seen with a light microscope. The nucleus and the nuclear membrane break down and disappear. Two centrosomes appear, each containing a pair of centrioles (plants do not have centrioles). The centromeres move to opposite poles of the cell. Spindle fibers, made of microtubules radiate from the centrosomes in preparation for metaphase. These are collectively called the mitotic spindle, which serves to equally divide the chromatids between the two offspring cells during cell division. Kinetochore fibers attach to the chromatids, while Polar fibers elongate the cell.
Metaphase
The kinetichore fibers move the chromosomes to the center of the cell.
Anaphase
During anaphase, the chromatids of each chromosome seperate at the centromeres and slowly move, centromere first, toward opposite poles of the dividing cells. Centromere breaks.
Telophase
After chromosomes reach opposite ends of the cell, spindle fibers disassemble, and the chromosomes become chromatin. A nuclear envelope forms around each set of chromosomes. Telophase is often grouped with Cytokinesis.
Cytokenesis
A cleavage furrow forms and pinches the cell into two individual cells. In plant cells, vesicles from the golgi apparatus join together to form a cell plate.
8.3 Meiosis
In animals, meiosis produces gametes, which are haploid. Meiosis is characterized by 2 stages of cell division.
Meiosis I
Prophase I
Just like in mitosis, spindle fibers appear, and the nuclear membrane disassembles.Each chromosome lines up next to its homologue. This pairing of chromosomes is called synapsis. Each pair is a tetrad and line up in lengthwise (figure 6) . During Synapsis, the chromatids undergo crossing-over. The genetic material between the chromatids crosses over and is recombined.
Metaphase I
During Metaphase I, the tetrads line up randomly along the midline of the cell, with spindle fibers attaching to the centromeres.
Anaphase I
During anaphase, each homologous chromosome moves to an opposite side of the cell. The random seperation of the homologous chromosomes is called independent assortment.
Telophase I and Cytokenisis I
The chromosomes reach opposite sides and the cells split. However, each cell still has 2 copies (2n), in the form of chromatids, of the original cell DNA. The new cells are genetically different from the mother cell and each other.
Meiosis II
Prophase II, Metaphase II, Telophase II and Cytokenisis II
Spindle fibers form, and the centromere breaks as the chromatids seperate and move towards opposite poles. Telophase II and Cytokenisis II
The cells split. At the end, now there are 4 cells. Organisms vary in timing and structures associated with gamete formation. Each male gamete is called sperm, and each of the 4 cells develop into a mature sperm in a process called spermatogenesis. In females, oogenesis, creating eggs, results in 4 cells, just like males, but only one of them becomes an egg. The other 3 become polar bodies that eventually degenerate. Sexual reproduction allows for evolution through the traits passed through crossing over.
8.1 Chromosomes
Chromosomes are rod-shaped structures made of DNA and proteins. The DNA in eukaryotic cells wrap tightly around proteins called histones. Each half of a chromosome is called a chromatid. The two chromatids attach at the centromere. (Figure 1) While cells are not dividing, chromosomes are not as tightly coiled. Regions of DNA must be uncoiled for use inside the cell. This less tightly coiled DNA is called chromatin. Human and animal chromosomes are categorized as either sex chromosomes or autosomes. Sex Chromosomes are chromosomes that determine the gender of an organism and other traits as well. In humans, XX is a female, and XY is a male. All of the other chromosomes in an organism are autosomes. Each offspring organism gets one copy of an autosome from each parent. The two copies that are created are called homologous chromosomes, or homologues. Homologous chromosomes are the same size and shape. A karyotype (Figure 2) is a diagram in which the homologous chromosomes are layed out right next to each other. If something has two sets of chromosomes, it is said to be diploid, or 2n. Almost all human cells are diploid (except sex cells). If something only has 1 set of chromosomes, it is said to be haploid, or 1n. The human sex cells, egg and sperm, are haploid.
8.2 Cell Division (Asexual)
Prokaryotes
Prokaryotes have cell walls but no nuclei and membrane bound organelles. A prokaryote's single DNA molecule is shaped as a circular chromosome attached to the inner plasma membrane [imagine a rope attached to the inner wall of a tent]. Binary fission is the division of a prokaryotic cell into two offspring cells (Figure 3). First, the DNA within a cell is copied. The cell grows in size while a new cell membrane forms between the two DNA copies. Eventually, the cell membrane splits the one large cell into two identical cells.
Eukaryotes
In eukaryotic cell division, both the cytoplasm and the nucleus divide. Mitosis results in new cells with identical genetic material of the orginal cells. Asexual reproduction is the production of offspring from one parent. Meiosis occurs during the formation of gametes which are haploid reproductive cells. Through this process, the chromosome number is reduced to half the original. The cell cycle is the repeating set of events in the life of a cell. The time between divisions is called interphase, which is split into 3 phases. Cytokineses is the division of the cell's cytoplasm. During the first stage of interphase, G1, offspring cells grow to mature size. From here, cells go to either the G0 phase, or the S phase. The G0 phase is a way to exit the cell cycle. Many cells in the human body are in this stage, like nevous system cells that stop dividing at maturity and don't normally divide again. During the S phase, the cell's DNA is copied (synthesised).During the G2 phase, the cell prepares for cell division.
Checkpoints
There are a few "traffic lights" that trigger mitosis. The first is in between G1 and S. This checks if the cell has grown enough and whether or not it should go to S or G0. In the second checkpoint, in late G2 phase, DNA repair enzymes check the results of DNA replication. At the mitosis checkpoint, before cytokinesis, a cell is passed to enter th eG1 phase again. Cancer disrupts these checkpoints and make cell growth unpredictable and harmful.
Prophase
The first phase of mitosis, prophase begins with the shortening and tight coiling of DNA into rod-shaped chromosomes that can be seen with a light microscope. The nucleus and the nuclear membrane break down and disappear. Two centrosomes appear, each containing a pair of centrioles (plants do not have centrioles). The centromeres move to opposite poles of the cell. Spindle fibers, made of microtubules radiate from the centrosomes in preparation for metaphase. These are collectively called the mitotic spindle, which serves to equally divide the chromatids between the two offspring cells during cell division. Kinetochore fibers attach to the chromatids, while Polar fibers elongate the cell.
Metaphase
The kinetichore fibers move the chromosomes to the center of the cell.
Anaphase
During anaphase, the chromatids of each chromosome seperate at the centromeres and slowly move, centromere first, toward opposite poles of the dividing cells. Centromere breaks.
Telophase
After chromosomes reach opposite ends of the cell, spindle fibers disassemble, and the chromosomes become chromatin. A nuclear envelope forms around each set of chromosomes. Telophase is often grouped with Cytokinesis.
Cytokenesis
A cleavage furrow forms and pinches the cell into two individual cells. In plant cells, vesicles from the golgi apparatus join together to form a cell plate.
8.3 Meiosis
In animals, meiosis produces gametes, which are haploid. Meiosis is characterized by 2 stages of cell division.
Meiosis I
Prophase IJust like in mitosis, spindle fibers appear, and the nuclear membrane disassembles.Each chromosome lines up next to its homologue. This pairing of chromosomes is called synapsis. Each pair is a tetrad and line up in lengthwise (figure 6) . During Synapsis, the chromatids undergo crossing-over. The genetic material between the chromatids crosses over and is recombined.
Metaphase I
During Metaphase I, the tetrads line up randomly along the midline of the cell, with spindle fibers attaching to the centromeres.
Anaphase I
During anaphase, each homologous chromosome moves to an opposite side of the cell. The random seperation of the homologous chromosomes is called independent assortment.
Telophase I and Cytokenisis I
The chromosomes reach opposite sides and the cells split. However, each cell still has 2 copies (2n), in the form of chromatids, of the original cell DNA. The new cells are genetically different from the mother cell and each other.
Meiosis II
Prophase II, Metaphase II, Telophase II and Cytokenisis IISpindle fibers form, and the centromere breaks as the chromatids seperate and move towards opposite poles.
Telophase II and Cytokenisis II
The cells split. At the end, now there are 4 cells. Organisms vary in timing and structures associated with gamete formation. Each male gamete is called sperm, and each of the 4 cells develop into a mature sperm in a process called spermatogenesis. In females, oogenesis, creating eggs, results in 4 cells, just like males, but only one of them becomes an egg. The other 3 become polar bodies that eventually degenerate. Sexual reproduction allows for evolution through the traits passed through crossing over.