Notes

Chapter 15

The Eukaryotic Cell Cycle, Mitosis, and Meiosis (p. 303-326)

Cell division: The process in which one cell divides into two cells.

*Prior to cell division, chromosomes become tightly compacted so they can be seen and counted with light microscope.

Cytogenetics: The field of genetics that involves the microscopic examination of chromosomes.

Karyotype: A photographic representation of the chromosomes in an actively dividing cell.

*Procedure for making a karyotype Figure 15.1, p. 304

Sex chromosomes: A distinctive pair of chromosomes that are different in males and females.

Autosomes: All of the chromosomes found in the cell nucleus of eukaryotes except for the sex chromosomes.

*Humans have 22 pairs and 1 pair of sex chromosomes (23 pairs total, 46 chromosomes)

Diploid: Refers to cells containing two sets of chromosomes; designated as 2n.

Gamete: A haploid cell that is involved with sexual reproduction, such as a sperm or egg cell.

Haploid: Refers to cells containing one set of chromosomes; designated as 1n.

Homologue: A member of a pair of chromosomes in a diploid organism.

*the DNA of a homologues pair of autosomal chromosomes is 99% similar

Homology: A fundamental similarity that occurs due to descent from a common ancestor.

Cell cycle: The series of phases a eukaryotic cell progresses through from its origin until it divides by mitosis.

Phases of the cell cycle:

G1

First gap

S

Synthesis of DNA, the genetic material

G2

Second gap

M phase

Mitosis and cytokinesis

Interphase: The G1, S and G2 phases of the cell cycle. It is the portion of the cell cycle during which the chromosomes are decondensed and found in the nucleus.

*G0 – The phase in which a cell may enter and never divide again (nerve cells)

G1 phase: The first gap phase of the cell cycle.

*cell growth typically occurs in this phase

S phase: The phase of the cell cycle during which occurs the synthesis of DNA.

*Sister chromatids are present (twice the cell’s DNA)

Sister chromatids: The two duplicated chromatids that are still joined to each other after DNA replication.

G2 phase: The second gap phase of the cell cycle.

*Proteins are made for sorting chromosomes and cell division

M phase:  Includes the mitosis process and the cytokinesis process

*Cell cycles range from several minutes in embryo cells to several months in adult cells.

Mitosis: In eukaryotes, the process in which nuclear division results in two nuclei, each of which receive the same complement of chromosomes.

*look at times of each phase* (p. 307)

Cytokinesis: The division of the cytoplasm to produce two distinct daughter cells.

Cyclins: A protein responsible for advancing a cell through the phases of the cell cycle by binding to a cyclin-dependent kinase.

*These proteins ensure conditions are appropriate for cell division, which minimize the occurrence of mutations.

Cyclin-dependant kinase (cdk): A protein responsible for advancing a cell through the phases of the cell cycle. Its function is dependent on the binding of a cyclin.

*Different cyclins increase during different stages*

*The specific cyclin binds to the cdk to form activated cyclin/cdk complex

*once activated, the cdk functions as a protein kinase to phosphoralates other proteins needed in the next step

Checkpoint: One of three critical regulatory points found in the cell cycle of eukaryotic cells. At these checkpoints a variety of proteins act as sensors to determine if a cell is in the proper condition to divide.

*the cyclin degrades when the phase is over

*checkpoints limit mutations by stopping cell division until the problem is fixed

Restriction point: A point in the cell cycle in which a cell has become committed to divide.

*G1 – checkpoint proteins can sense if the DNA has incurred damage

*G2 – checkpoint proteins check the DNA for damage and ensure all DNA has been replicated

*Metaphase checkpoint senses integrity of the spindle apparatus

Maturation promoting factor (MPF): A factor, now known to be a complex of cyclin and cyclin-dependant kinase, important in the division of all types of eukaryotic cells.

*It was first discovered in leopard frog eggs in the 1960s

15.2 – Mitotic Cell Division

Mitotic cell division: A process whereby a eukaryotic cell divides to produce two new cells that are genetically identical to the original cell.

*mitosis and cytokinesis

Asexual reproduction: A reproduction strategy that occurs when offspring are produced from a single parent, without the fusion of gametes from two parents. The offspring are therefore clones of the parents.

Reasons for mitotic cell division:

(1) reproduction of unicellular organisms

(2) repeated cell divisions of multicelluar organisms

*precise transmission of cells of the body are extremely important

Centromere: The region where the two sister chromatids are tightly associated; the centromere is an attachment site for kinetochore proteins.

Kinetochore: A group of proteins that bind to a centromere and are necessary for sorting each chromosome.

Mitotic spindle: The structure responsible for organizing and sorting the chromosomes during mitosis; also called the mitotic spindle apparatus.

Centrosomes: A single structure often near the cell nucleus of eukaryotic cells that forms a nucleating site for the growth of microtubules; also called the microtubules-organizing center.

*area that organizes the construction of microtubules by rapidly polymerizing tubulin proteins

Three types of spindle microtubes: 1) astral microtubes – extend away from the chromosomes and position he spindle apparatus within the cell. 2) Polar microtubules (interpolar tubules) – extend from one polar region to the other and are used in the separation of the poles. 3) Kinetochore microtubules – attach to kinetochores and centromere of each chromosome.

Pole: A structure of the spindle apparatus defined by each centrosome.

*similar to North and South Poles of the earth

Centrioles: A pair of structures within the centrosome of animal cells. Most plant cells and many protists lack centrioles.

*transmission of chromosomes requires a sorting process known as mitosis

Interphase: The G1, S and G2 phases of the cell cycle. It is the portion of the cell cycle during which the chromosomes are decondensed and found in the nucleus.

*mitosis was first observed by Walter Flemming studying salamander larvae in the 1870s

*Figure 15.9 p. 312-313

Prophase: The phase of mitosis during which the chromosomes condense and the nuclear membrane begins to vesiculate.

Prometaphase: The phase of mitosis during which the mitotic spindle is completely formed and the nuclear envelope completely fragments into small pieces.

1)Centrosomes move apart 2) microtubules rapidly grow out from the poles and attach to the kinetochore it contacts

3) the two kinetochore microtubules from each pole tug at the sister chromatids

Metaphase: The phase of mitosis during which the chromosomes are aligned along the metaphase plate.

*The kinetochore microtubules pulled the sister chromatids into the “middle” of the cell

Metaphase plate: A plane halfway between the poles of the spindle apparatus on which the sister chromatids align during the metaphase stage of mitosis.

Anaphase: The phase of mitosis during which the sister chromatids separate from each other and move to opposite poles; the poles themselves also move further apart.

*The separated chromatids are now chromosomes and are only linked to one pole by one or more kinetochore microtubules

*Polar microtubules lengthen to push the poles further apart

*Kinetochore microtubules short to pull chromosomes to the poles

Telophase: The phase of mitosis during which the chromosomes decondense and the nuclear membrane re-forms.

*two separate nuclei form with identical chromosomes

Cytokinesis: The division of the cytoplasm to produce two distinct daughter cells.

*approx. 20 proteins are involved in cytokinesis in nearly all animal cells

*Read Genomes and Proteomes Connection (group question?)

Cleavage furrow: In animal cells, an area that constricts like a drawstring to separate the cells during cytokinesis.

Cell plate: In plant cells, a structure that forms a cell wall between the two daughter cells during cytokinesis.

15.3 – Meiosis and Sexual Reproduction

Sexual reproduction: A process that requires a fertilization event in which two gametes unite to produce a cell called a zygote.

Fertilization: The union of two gametes, such as an egg cell with a sperm cell, to form a zygote.

Zygote: A diploid cell formed by the fusion of two haploid gametes.

*The zygote grows by mitosis to produce all body cells

Meiosis: The process by which haploid cells are produced from a cell that was originally diploid.

*two key events happen at the beginning of meiosis that do not occur in mitosis

Bivalent: Homologous pair of sister chromatids associated with each other, lying side by side.

Tetrad: See bivalent.

Synapsis: The process of forming a bivalent/tetrad

*a protein structure synaptonemal complex connects homologous chromosomes

*the role of synaptonemal is not understood because some fungi lack this protein

Crossing over:  The exchange of genetic material between homologous chromosomes during meiosis; allows for increased variation in the genetic information that each parent may pass to the offspring.

*the farther from the chiasma, the more often crossing over occurs

Chiasma: The connection at a crossover site of two chromosomes.

Meiosis I: The first division of meiosis in which the homologues are separated into different cells.

*Prophase I  - homologous chromosomes condense

                             Bivalents form

                             Crossing over occurs

                             Nuclear envelope starts to fragment into small vesicles

*Prometaphase I – nuclear envelope completely broken down

                             Spindle apparatus is entirely formed

                             Sister chromatids attach to kinetochore microtubules

                             The pair of sister chromatids are attached to one poles, while mitosis the pair is attached to both poles

*Metaphase I – bivalents are aligned in a double row, not a single row (as in mitosis)

                             *the homologues are randomly aligned (211 or 223 in humans – over 8 million possibilities

*Anaphase I – the bivalent pair break, but not the sister chromatids. The pair of chromatids are pulled by the kinetochore microtubules to opposite poles.

*Telophase I – sister chromatids have reached their respective poles, and they decondense. The nuclear envelope reforms to produce two separate nuclei.

Meiosis II: The second division of meiosis in which sister chromatids are separated into different cells.

*Separation of sister chromatids is similar to mitosis

*Prophase II  - Nuclear envelope breaks sown.

*Metaphase I – chromosomes align at the equator

*anaphase II – sister chromatids are separated

*Telophase I-nuclear envelope reforms

Life cycle: The sequence of events that characterize the steps of development of the individuals of a given species.

*most multi-cellular organisms alternate between haploid and diploid

Diploid-dominant species: Species in which the diploid organism is the prevalent organism in the life cycle. Animals are an example.   *(multi-cellular organisms are haploid)*

Haploid-dominant species: Species in which the haploid organism is the prevalent organism in the life cycle. Examples include fungi and some protists.

Alteration of generations: The phenomenon that occurs in plants and some protists in which the life cycle alternates between multicellular diploid organisms, called sporophytes, and multicellular haploid organisms, called gametophytes.

*Both generations are multi-cellular

Sporophytes: The diploid generation of plants or multicellular protists that have a sporic life cycle; this generation produces haploid spores by the process of meiosis.

Gametophytes: In plants and many multicellular protists, the haploid stage that produces gametes by mitosis.

15.4 – Variation in chromosome Structure and Number

*chromosome structure and number an have major effects on organisms characteristics (look at p. 321 with chromosome numbers)

Metacentric: A chromosome in which the centromere is near the middle.

Submetacentric: A chromosome in which the centromere is off center.

Acrocentric: A chromosome in which the centromere is near one end.

Telocentric: A chromosome in which the centromere is at the end.

*short arm is p or petite, while long arm is q.

G-banding: a result of bands or stripes o chromosomes due to Giesma stain

Ways mutation can alter chromosome structure:

1)  Deletion: A type of mutation in which a segment of genetic material is missing.

2)  Duplication: A type of mutation in which a section of a chromosome occurs two or more times.

3)  Inversion: A type of mutation that involves a change in the direction of the genetic material along a single chromosome.

4)  Translocation: 1. A type of mutation in which on segment of a chromosome becomes attached to a different chromosome. 2. A process in plants in which phloem transports substances from a source to a sink.

a.        Simple translocation: A type of mutation in which a single piece of chromosome is attached to another chromosome.

b.       Reciprocal translocation: A type of mutation in which two different types of chromosomes exchange pieces, thereby producing two abnormal chromosomes carrying translocations.

Variations in the number of chromosome sets and number of individual chromosomes

Euploid: An organism that has a chromosome number that is a multiple of a chromosome set (1n, 2n, 3n, etc.).

Polyploid: An organism that has three or more sets of chromosomes.

Triploid: An organism or cell that has three sets of chromosomes.

Tetraploid: An organism or cell that has four sets of chromosomes.

Aneuploidy: An alteration in the number of particular chromosomes so that the total number of chromosomes  is not an exact multiple of a set.

Trisomic: An aneuploid organism that has one too many chromosomes.

Monosomic: An aneuploid organism that has one too few chromosomes.

Nondisjunction: An event during which the chromosomes do not sort properly during cell division.

Changes in chromosome number have important consequences:

*polyploidy is generally lethal in mammals

             Male bees are haploid while femail bees are diploid.

             Amphibians and reptiles occasionally have diploid species and polyploidy species (p. 324 fig 15.19)

30-35% of ferns and flowering plants are polyploidy

*polyploidy among plants generally produces longer more robust plants

Down syndrome: A human disorder caused by the inheritance of three copies of chromosome 21.