Cell Cycle and Cellular Reproduction

Eukaryotic Cells: Reproduction

Ø  Even the most complex eukaryotic organisms begin life as a single cell, the fertilized egg.

l  Usually, this cell divides to form two cells, and each of these two cells divide repeatedly, eventually forming enough cells to make up all the complexities of the organism.

Eukaryotic Cells: Somatic Cells

Ø  Cells that form the body tissue are called somatic cells. Examples include nerve, muscle, blood, root, and leaf cells.

Ø  Somatic cells reproduce by a process called mitosis. Mitosis ensures that each cell carries inside the same genetic information.

Eukaryotic Cells: Sex Cells

Ø  Animal cells that are used to produce offspring are called gametes, or sex cells. Males have sperm cells and females have egg cells.

Ø  Sex cells reproduce by a process called meiosis. Meiosis reduces the number of chromosomes in the cell to half the normal number. This ensures that when the sperm and egg meet, the offspring will end up with the same total number of chromosomes as are found in its parents. 

Eukaryotic Cells: Chromosomes

Ø  A given species of a eukaryotic organism will have a characteristic number of chromosomes.

l  Chromosomes are small packages of genetic information that contain the genes.

l  The number of chromosomes has no simple relationship to the size or complexity of the organism.

•          Humans have 46 chromosomes.

•          Chimpanzees have 48 chromosomes.

•          Cabbage has 20 chromosomes.

Eukaryotic Cells: Cell Reproduction

Ø  During the reproduction of both somatic and sex cells, the new cells inherit genetic information, but not in the same way. In MITOSIS, each new cell inherits the exact copy of genes in the founding cell. In MEIOSIS, each new cell inherits half of each of the founding cell’s genes.

Ø  In somatic cells, chromosomes are paired. For example, humans have 46 chromosomes = 23 pairs of chromosomes. So, one set of 23 chromosomes (maternal chromosomes) was given by the mother, and the other set of 23 chromosomes (paternal chromosomes) was given by the father. 23 +23 = 46 total 

Ø  These chromosomes pair up together to form what are known as: homologous chromosomes. (Example: Chromosome 1 from the mother joins with Chromosome 1 from the father)

Diploid Cells

Ø  Somatic cells contain 1 pair of each type of chromosome and are referred to as diploid cells. The diploid number (2n) is 2 times the number of chromosomes in each set of the pair. In humans, the diploid number is 46 (2 X 23).

Haploid Cells

Ø  Sex cells only contain 1 of each type of chromosome and are referred to as haploid cells. The haploid number (n) is the number of chromosomes an organism has in one set. In humans, the haploid number in sperm and egg cells is 23. 

The Cell Cycle

Ø  In order for an organism to survive, cells must generate new cells.  This process requires that the genetic information directing the cell process be copied into every cell that is produced.

Ø  Every cell has a Cell Cycle, which are the different stages a cell goes through while it is alive.   

Ø  In somatic cells, the cell cycle is the period from the beginning of one division to the beginning of the next division.

l  The time to complete one cell cycle is called the generation time.

•          Generation times can range from 20 minutes to several hours or longer.

Cell Division: Interphase

Ø  The Cell Cycle has three main stages: Interphase-Mitosis-Cytokinesis

l  Most of a cell’s life is spent in Interphase. Chromosomes are duplicated during this phase.

•          G1 Phase (first gap phase): cell grows and makes the enzymes necessary to form DNA.

•          S phase (synthesis phase): DNA is doubled; centrosomes are also duplicated.

•          G2 phase (second gap phase): many proteins are manufactured in preparation for mitosis.

l  Eukaryotic Cell division

•          A very small portion of a cell’s life is spent in the Cell Division stage.  This is when a new cell is formed from the pre-existing cell.

•          Cell division involves two main processes:

Ø  Mitosis

Ø  Ensures that each new cell nucleus contains the same number and types of chromosomes present in the original nucleus

Ø  Cytokinesis

Ø  The division of the cytoplasm to form two new cells

Ø  Cytokinesis usually follows mitosis, but there are times when cytokinesis doesn’t occur, and the cell ends up with several nuclei (multinucleate cell).

Cell Division: Mitosis

Ø  A eukaryotic somatic cell leaves Interphase and goes into Mitosis once it has the material necessary for replication and when the environmental conditions are correct. 

Ø  Mitosis is the process by which the two identical sets of chromosomes are distributed into different nuclei.

Ø  Stages of Mitosis

l  Prophase

l  Prometaphase (sometimes indistinguishable from prophase)

l  Metaphase

l  Anaphase

l  Telophase

Ø  Word Clarification! There are some words that cause confusion because of their similarity. Pay close attention to the definitions of the following words:

l  Chromatin: masses of thin strands of DNA

l  Chromatid: a daughter strand of a replicated chromosome

l  Centromere: a constricted region on sister chromatids that attaches them together

l  Kinetochore: protein complexes on either side of the centromere where spindle fibers attach

l  Centrosome: microtubule organization center of the cell; contains centrioles and asters

l  Centriole: barrel-shaped organelle found in the centrosome

l  Aster: small microtubules that extend from centrioles to cell membrane—offers support for spindle fibers

Mitosis: Prophase

Ø  Prior to prophase, chromosomes were duplicated, and upon entering prophase, exist in the cell as a pair of identical sister chromatids. These sister chromatids are joined together by a centromere. The centromere has a structure known as the kinetochore, where the microtubules attach.

Ø  Prophase begins when the chromatin in the chromatids condenses into visible chromosomes.

Ø  The two centrosomes (each has one pair of centrioles) and their associated asters begin to migrate towards opposite sides of the cell.

Ø  Nucleolus disappears, and the nuclear envelope disentigrates.

Mitosis: Prometaphase

Ø  Kinetochores on the centromeres attach sister chromatids to spindle fibers.

l  One sister chromatid is attached to one side of the cell, and the other sister chromatid is attached to the opposite side.

l  These sister chromatids begin to move toward the equator (center) of the cell.

Mitosis: Metaphase

Ø  Chromosomes become aligned at the center of the cell.

l  Numerous spindle fibers extend from each pole to the center of the cell.

l  Some of the spindle fibers remain unattached to the chromosomes.

l  Chromosomes are now completely condensed and can be seen more clearly than at any other time.

Mitosis: Anaphase

Ø  Begins as the sister chromatids separate at the centromere. Each chromatid is now an independent chromosome.

Ø  Separated chromosomes are pulled toward opposite poles by spindle fibers.

Ø  The centrosome is pulled with its kinetochore first, and the two “arms” lag behind.

Ø  Phase ends when a complete set of chromosomes reaches each pole.

Mitosis: Telophase

Ø  Cell returns to Interphase conditions.

Ø  Chromosomes elongate by uncoiling, becoming chromatin threads once more.

Ø  Nuclear envelope forms around each set of chromosomes.

Ø  Nucleoli reappear and spindle fibers disappear.

 

Mitosis in Plant Cells

Ø  Mitosis is similar in plant cells, except that plant cells do not have centrioles or asters assisting in cell division. Plants go through the same stages of mitosis as do animal cells.

Cytokinesis:
Separation of daughter cells

Ø  Cytokinesis usually begins during telophase and ends soon after the completion of mitosis.

Ø  The division of animal cells is accomplished by a cleavage furrow.

l  Encircles the cell around the middle and begins to squeeze it, forcing the cell to stretch apart. The contractile ring (actin filaments) pulls and separates the cytoplasm until there are two daughter cells.

Ø  The division of plant cells is accomplished by the formation of a cell plate that is produced by organelles called phragmoplasts.

l  Plant cells build new cell walls between the daughter cells.

l  Complete separation occurs when the cell plate is formed. The cell plate becomes new plasma membrane between the daughter cells. Molecules within the new membrane will then build cell walls for the daughter cells.

 

Apoptosis: Cell death

Ø  Cells harbor enzymes that can cause their own death (apoptosis). These enzymes are stimulated to function by internal or external signals.

Ø  An example of apoptosis is the loss of the tail on tadpoles.

l  Enzymes become active at certain a certain point during a frog’s development when having a tail is no longer necessary. These enzymes cause tail cells to self-destruct, and the tail gradually disappears.

Ø  Cell dying through apoptosis, or cell suicide, undergoes distinctive changes.

l  Shrinks and pulls away from its neighbors (top right).

l  Blebs (pink spheres) appear on the surface (making the cell appear to boil), and the chromatin condenses at the edges of the nucleus.

l  Soon the nucleus, and then the cell itself, breaks up, and the cell fragments are quickly ingested by other cells in the vicinity.

Cancer Cells

Ø  Damaged cells are naturally eliminated by apoptosis.

Ø  Cancer cells are able to avoid apoptosis and replicate.

The Cell Cycle and Cancer

Ø  Cancer cells are engaged in uncontrolled mitosis.

Ø  Cancer is caused by a mutation of genes that regulate the cell cycle.

l  Mutations in several different classes of genes have been identified as causing cancer.

•          Proto-oncogenes and tumor-suppressor genes

•          Genes that produce enzymes for DNA repair

•          Genes that produce the enzyme telomerase

ASEXUAL REPRODUCTION

Asexual reproduction

·         There are several types of asexual reproduction.

·         All prokaryotes and most unicellular organisms reproduce by binary fission.

·         Some eukaryotes reproduce asexually through various processes.

·         Many eukaryotes that reproduce asexually also reproduce sexually (ex: plants).

Cellular Reproduction in Prokaryotes

·         Prokaryotes and other unicellular organisms reproduce by binary fission.

·         Result is genetically identical offspring

·         One single chromosome is duplicated and distributed to the daughter cells.

**Binary Fission

1.       Chromosome attaches to a binding site.

2.       Cell begins to enlarge.

3.       DNA replicates, producing two chromosomes.

4.       Two chromosomes are pulled apart as the cell elongates.

5.       New plasma membrane and cell wall divide the cell into two.  

Protists, some fungi, and even a few animals can asexually reproduce by producing haploid cells through mitosis. Asexual reproduction can be achieved in varying ways, including budding, fragmentation, or spore formation.

Asexual reproduction is very common in plants.

·         Some plants can send out runners above ground to form new plants (Ex: strawberries).

·         Some plants produce tubers or bulbs below ground for reproduction (Ex: potatoes, onions).

·         Asexual reproduction in crop plants is often used to human’s advantage.

 

Sexual Reproduction

·         Most eukaryotes are sexually reproducing organisms.

·         Sexual reproduction requires each parent to donate half of their chromosomes to the offspring.

·         Meiosis is the type of nuclear division that reduces the diploid number (in each parent) to the haploid number (in each sex cell).

Meiosis

·         Meiosis consists of two cell divisions, the first and second meiotic divisions, called Meiosis I and Meiosis II.

·         Meiosis I separates homologous pairs of chromosomes into sister chromatids.

·         Meiosis II separates the sister chromatids into individual chromosomes.

·         Each phase of meiosis includes PMAT stages.

·         As with mitosis, chromosomes are duplicated during the S phase of interphase before meiosis actually begins.

·         Upon entering meiosis, these duplicated chromosomes exist as chromatids joined by the centromere.

Prophase I

·         As with prophase in mitosis, chromosomes condense and become visible, centrioles form and move toward the poles, and the nuclear membrane begins to dissolve.

·         The homologs pair up (synapsis), forming a tetrad.

o   Each tetrad is comprised of four chromotids - the two homologs, each with their sister chromatid.

·         Homologous chromosomes will swap genetic material in a process known as crossing over (XO). All genes on a chromosome are said to be linked together and tend to be inherited together. However, genetic material from the homologous chromosomes can be randomly swapped, creating four unique chromatids.  This is called genetic recombination.

o   Since each chromatid is unique, the overall genetic diversity of the gametes is greatly increased.

Metaphase I

·         Microtubules grow from the centrioles and attach to the centromeres at the kinetochores.

·         The tetrads line up along the equator of the spindle (metaphase plate).

Anaphase I

·         Homologous chromosomes of each pair separate, moving toward opposite poles.

·         Chromatids of each chromosome are still joined at their centromere regions.

Telophase I

·         As in mitotic telophase, the nuclei reorganize, the chromatids begin to elongate, and cytokinesis generally takes place.

Interkinesis

·         Occurs between meiosis I and meiosis II.

·         Similar to interphase in mitosis—the cell’s state before the next division.

·         However, DNA replication does not occur.

·         Interkinesis is very brief or entirely absent in some organisms.

Prophase II

·         Prophase II is also a brief stage in which centrioles form and move toward the poles and the nuclear membrane dissolves.

·         Only one of each pair of chromosomes is present, which means no synapsis or crossing over.

Metaphase II

·         Microtubules grow from the centrioles and attach to the centromeres.

·         The sister chromatids again line up along the cell equator. (Only one of each chromatid is present this time!)

Anaphase II

·         Centromeres split and sister chromatids (now complete chromosomes) separate and move to opposite poles.

Telophase II

·         The chromosomes may decondense (depends on species).

·         One of each kind of chromosome is located at each end of the cell.

·         Cytokinesis begins, eventually creating four haploid daughter cells .

After meiosis II…Haploid cells mature and become gametes in animals and spores in plants.

Mitosis/Meiosis Comparison

·         In meiosis, there are TWO successive nuclear and cell divisions, producing a total of four daughter cells. In mitosis, there is only one nuclear division, producing two daughter cells.

·         Each of the four cells produced in meiosis contains the haploid number of chromosomes (only one member of each homologous pair). In animal cell mitosis, each daughter cell contains the diploid number.

·         During meiosis, homologous chromosomes containing genetic info from each parent are shuffled, and one chromosome of each pair is randomly distributed to each new cell. Resulting haploid cells have new combinations of genes and chromosomes. In mitosis, however, daughter cells contain identical sets of chromosomes, and these chromosome sets are identical in every way to the parent cell.

·         During meiosis, there may be some exchange of parts between homologous chromosomes (XO), so that even the genes originally located together on one chromosome do not always stay together. Crossing-over further increases the genetic reshuffling of meiosis. In mitosis, there is little opportunity for crossing over.

Gamete production in humans

·         In humans, meiosis occurs only during gamete production.

·         Male gamete production is called spermatogenesis.

o   One primary spermatocyte (2n) becomes 2 secondary spermatocytes (n) which each become 2 spermatids (n), for a total of 4 sperm.

·         Female gamete production is called oogenesis.

o   Generally, one primary oocyte (2n) becomes either one secondary oocyte (n) plus three polar bodies (after fertilization) or one secondary oocyte (n) plus two polar bodies (no fertilization).