3.02: cell cycle, mitosis, & meiosis. 3.02: cell cycle cell division, also called cell...
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3.02: Cell Cycle, Mitosis, & Meiosis
3.02: Cell Cycle
Cell division, also called cell reproduction, occurs in all organisms. This is how we grow and how we repair our cells.
Why do cells divide instead of just growing bigger and bigger?
The larger a cell becomes, the more demands it places on its DNA.
The larger a cell becomes, the more trouble it has moving nutrients and wastes across the cell membrane.
Cell Cycle
Each cell has only one set of genetic information stored in its DNA. Before cell division occurs, the cell must replicate (copy) all of its DNA.
In prokaryotes, cell division is simple because they have no nucleus or organelles to organize, they just split each cell in half in a process called binary fission. Eukaryotic cell division is much more complex and happens in stages.
The first 3 phases of the cycle are known as interphase. This part makes up most of a cell’s life.
Cell Cycle
1. G1, First growth phase: During G1 a cell grows rapidly and carries out routine functions. This phase occupies the major portion of a cell’s life.
2. S, Synthesis phase: A cell’s DNA is replicated during this phase.
3. G2, Second growth phase: Preparations are made for the nucleus to divide.
4. M, Mitosis: the division phase during which the nucleus of a cell is divided into 2 nuclei. Each nucleus has the same DNA as the parent cell they came from.
5. Cytokinesis: the process during cell division in which the cytoplasm divides. This results in two daughter cells, each with the exact DNA of the parent cell.
Cell Cycle
G1S
G2Cytokinesis
Mitosis
Cell growth
DNA synthesis
Growth and
preparation for mitosis
Division of cytoplasm
Division of nucleus
Cell Cycle
Cells know when to divide because of checkpoints at different stages of the cell cycle. If one of these checkpoints fails to do its job because of mistakes in the DNA called mutations, then the regulation of cell growth and division is disrupted. Cancer, the uncontrolled growth of cells, may result from these mutations.
Chromosomes
Chromosomes are DNA and proteins tightly coiled together. Humans have 46 chromosomes in the nucleus of each cell of our bodies.
During interphase, the DNA is not visible (this is called chromatin). As a eukaryotic cell prepares to divide, these chromosomes become visible and the DNA is replicated.
The two exact copies of the DNA that make up each chromosome are called sister chromatids. The two chromatids are attached a point called the centromere.
Chromosomes
The chromatids separate during cell division and each one is placed into a new daughter cell so that each new cell has the same genetic information as the original cell.
3.02: Mitosis
During mitosis, the nucleus divides to form two nuclei, each containing a complete set of the cell’s chromosomes.
Mitosis involves 1 cell dividing 1 time to produce 2 identical daughter cells.
Mitosis is called asexual reproduction, because the new daughter cells are genetically identical to the parent cell. This means that the number of chromosomes in each daughter cell is the same as the number in the parent cell.
Steps of Mitosis: PMAT
1. Prophase: chromosomes coil up and become visible, the nuclear envelope surrounding the nucleus dissolves, and a spindle forms.
Steps of Mitosis: PMAT
2. Metaphase: the chromosomes move to the middle of the cell and line up along the equator, spindle fibers link the chromatids of each chromosome to the opposite poles.
Steps of Mitosis: PMAT
3. Anaphase: centromeres divide, the two chromatids (now called chromosomes) move toward opposite poles as the spindle fibers attached to them shorten.
Steps of Mitosis: PMAT
4. Telophase: a nuclear envelope forms around the chromosomes at each pole, the chromosomes uncoil and the spindle dissolves.
Steps of Mitosis: PMAT
Cytokinesis follows mitosis. This is where the cytoplasm divides to produce two identical daughter cells.
Please add this to your notes:
In Telophase of plant cells, instead of pinching in half the cell forms a cell plate.
3.02: Cell Types and Chromosome Number
In an organism, there are somatic cells and there are sex cells.
o Somatic cells are all of the body’s cells that are not sex cells.
o Sex cells are sperm and egg.
Somatic cells have the diploid number of chromosomes, which means one set from the mother and one set from the father.
o Diploid = full set (2n)
Cell Types and Chromosome Number
Humans have 46 chromosomes in their somatic cells, 23 from Mom and 23 from Dad, that are paired together. These paired chromosomes are called homologous, meaning they are similar in size, shape, and genetic content.
Cell Types and Chromosome Number
Gametes (sex cells, egg or sperm) contain only one set of chromosomes. Since a gamete contains only one set of chromosomes, it is said to be haploid.
o Haploid = half set (n)
The fusion of two haploid gametes is called fertilization and results in a zygote. A zygote is a fertilized egg cell, the first cell of a new individual.
3.02: Meiosis
Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells, such as gametes or spores.
Meiosis involves 2 divisions of the nucleus – Meiosis I and Meiosis II. Before meiosis begins, the DNA in the original cell is replicated (just like before mitosis).
Meiosis
Meiosis I (kind of like mitosis, but with pairs of chromosomes instead of singles):
1. Prophase I : the chromosomes become visible, the nuclear envelope dissolves, and homologous chromosomes pair up forming a tetrad—2 chromosomes joined together. Crossing-over occurs when portions of a chromatid on one chromosome are broken and exchanged with the same chromatid portions on the homologous chromosome.
Meiosis
2. Metaphase I : pairs of homologous chromosomes are moved by the spindle to line up at the middle of the cell.
3. Anaphase I : The homologous chromosomes separate and each is pulled to the opposite poles of the cell by the spindle fibers. The chromosomes do not separate at the centromere as in mitosis, each chromosome is still composed of 2 chromatids.
4. Telophase I : Individual chromosomes gather at each of the poles of the cell, then the cytoplasm divides (cytokinesis) forming 2 new cells.
Meiosis
Meiosis II (looks just like mitosis, but the chromosomes are genetically different than the parent cell because of crossing over):
5. Prophase II : a new spindle forms around the chromosomes.
6. Metaphase II : chromosomes line up along the middle and are attached at their centromeres to spindle fibers.
7. Anaphase II : centromeres divide, chromatids move to opposite poles of the cell.
8. Telophase II: nuclear envelope forms around each set of chromosomes, the cell undergoes cytokinesis.
Meiosis I
Meiosis II
Meiosis
The end result of meiosis is 4 haploid cells from 1 diploid cell. Each new cell has half the genetic material as the original cell.
3.02: Gametogenesis
In males, these 4 haploid cells are called sperm. This process by which sperm are produced in male animals is called spermatogenesis.
In females, the cytoplasm of these 4 haploid cells divides unevenly so that there is one larger cell with nearly all the cytoplasm and 3 smaller cells. The larger cell becomes and egg cell, or ovum, while the smaller cells are called polar bodies. The process by which gametes are produced in female animals is called oogenesis.
Nondisjunction
Sometimes mistakes are made when gametes are formed in the body. During meiosis, if the homologous chromosomes do not separate properly then the gametes will have the wrong number of chromosomes. This is called nondisjunction.
Then this mutated gamete is used to make a zygote. That developing organism will have too many or too few chromosomes in each of its cells.
Nondisjunction
Often, this mistake is corrected by nature: the mother will have a miscarriage.
Babies born with nondisjunction can display certain serious diseases, such as Down’s Syndrome, Turner’s Syndrome, or Klinefelter’s Syndrome.
Down’s Syndrome
3 copies of chromosome 21; leads to learning disabilities, developmental disabilities and a specific facial appearance.
Turner’s Syndrome
a female has only one X chromosome instead of the normal two; these girls are short, have a low hairline, a webbed neck, and are sterile.
Klinefelter’s Syndrome
a male has two X chromosomes and a Y instead of the normal X and Y; these boys have small testicles, reduced fertility, and physical and behavioral problems.
Sexual vs. Asexual Reproduction
In asexual reproduction, a single parent cell passes on copies of all its DNA to its offspring
An individual produced by asexual reproduction is a clone – an organism that is genetically identical to its parent.
Sexual vs. Asexual Reproduction
Sexual reproduction involves the formation of reproductive cells called gametes that have half the chromosome number as the original cell.
A diploid mother has haploid eggs, a diploid father has haploid sperm. In the way the egg and sperm combine during fertilization to make a to make a zygote. haploid (n) + haploid (n) = diploid (2n)
Since both parents contribute genetic material, the offspring have traits of both parents but are not exactly like either parent. Sexual reproduction causes genetic variation.
Sexual vs. Asexual Reproduction
Asexual reproduction is simple and efficient. It allows organisms to reproduce without forming gametes or finding a mate. However, in an environment that is constantly changing, some organisms need variation in order to survive. Sexual reproduction provides a way to quickly make different combinations of genes among individuals and is how organisms evolve over time.
Sexual vs. Asexual Reproduction
Asexual reproduction = genetically identical offspring (clones) = mitosis 1 cell divides 1 time to produce 2 identical
cells
Sexual reproduction = genetic variation = meiosis 1 cell divides 2 times to produce 4 cells with
half the chromosomes
Mitosis Meiosis
Sexual or Asexual?
Asexual Sexual
Number of divisions
1 2
Number of cells formed
2 4
Type of cell Somatic (body cell)
Gamete (sex cell)
Identical to or different from parent?
Identical Different
Crossing over? No Yes
Genetic variation?
No Yes