Introduction to the Cell Cycle and Mitosis:

Phenotype: observable characteristic of physical feature

Chromosomes: threadlike structures containing proteins and nucleic acids

Sister chromatids: duplicated and sister chromosome

Centromere: chromatids attached to a center point

Homologous chromosomes: sister chromatids separate, carry alleles

Mitosis: cell division in somatic cells, which produces daughter cells with exact replicas of the parents DNA

Somatic cells: the body’s non-reproductive cells, two sets of chromosomes

Meiosis: cell division in germ cells, produces daughter cells with DNA that is a combination of the DNA of the two parent cells. Resulting gametes are the body’s reproductive cells, which have half the genetic material that somatic has

The Basic Process of Cell Division

• reproduction: unicellular organism reproduce through cell division

• growth: organisms get larger through cell division

regeneration: some organisms can regenerate entire body parts through mitosis

Two Basic Reproductive Strategies:

• asexual reproduction: offspring identical to parent

  • binary fission: single celled prokaryotes reproduce through this simple process

  • asexual is favorable when: there are not many available mates

  • con: low genetic diversity

• sexual reproduction: two individuals contribute half of their genetic material to the offspring

  • gametes formed through meiosis

  • meiosis: haploid cells

  • combine to produce diploid zygote in fertilization

meiosis: process of cell division resulting in daughter cells with half of the original cell’s genetic material

Comparative Life Cycles:

Diplontic Life Cycle (animals):

1. merger of two haploid (1n) gametes in fertilization, produces a diploid (2n) organism

2. to reproduce, organism reproduces gametes through mitosis

   in diplontic, dominant multicellular organism is diploid

Haplontic Life Cycle (protists, fungi, algae):

1. merger of haploid gametes in fertilization, produces a diploid (2n) zygote

2. zygote undergoes meiosis to form haploid cells called spores

3. haploid cells grow and divide mitotically to form haploid multicellular organism

   • Fungi: can alternate between sexual (to survive with harsh environments) and asexual (to spread quickly)

   in haplontic, dominant multicellular organism is haploid

Cell Division in Prokaryotes: Binary Fission

Binary Fission:

1. Signal to Initiate - some external or internal signal initiates asexual reproduction (environmental)

2. DNA replication - all cellular organisms have DNA-based genomes, which are replicated and passed on to daughter cells

2 regions in circular prokaryotic chromosome:

ori: origin of replication

ter: terminus of replication

3. Segregation of DNA - DNA replication begins at the center of the cell, daughter DNA strand separates, ori regions move toward separate sides of the cell

4. Cytokinesis - cytoplasm is split and two daughter cells result

Cell Division in Eukaryotes:

• Chromosome number: number of chromosomes it has in total

• Ploidy: number of homologous chromosomes that exists for each chromosome type

  • haploid (1n): only one chromosome for each chromosome type; human gametes

  • diploid (2n): two homologous chromosomes for each chromosome type; human somatic cells

• Sister chromatids: two chromatids joined together at a common centromere

• Centromere: region of chromosomes to which microtubules attach during cell division; has protein kinetochores = chromosome movement

• Homologous chromosomes: in diploid organisms, chromosomes exist in homologous pairs

Eukaryotic Cell Structures:

• Nucleus: surrounded by membrane and contains DNA

• Dna packaged by special proteins to form a complex called chromatin

• Interphase: DNA is stretched out

• Nucleolus: specialized structure in nucleus, synthesis of ribosomal RNA and assembly of rRNA into ribosomes

• Centrosomes: regions near the nucleus that are associated with spindle fibers, organizing center for microtubules in the cell, site of synthesis and replication of microtubules

• Centrioles: within centrosomes, pair of hollow microtubule rings in animals

• Cytoskeleton: internal scaffolding, constructed of filaments, structure and mechanical support, dynamic

Eukaryotic Cell Division:

1. Signal to divide - growth factor initiates cell division (internal signal)

2. Replication of DNA - DNA replication occurs during the S-phase

3. Segregation of DNA - each daughter cells must have one copy of each chromosome, segregate into two new nuclei through mitosis

4. Cytokinesis - division of the cell into two daughter cells, plants: cell wall, animal: membrane

Cell Cycle:

1. Interphase: mitosis is not occurring, cell synthesizes RNA, produces proteins, grows in size, nucleus and nucleolus visible, DNA replication occurs

   1. Gap 1 (G1): cell grows, produces protein and RNA, prepares for protein synthesis, cells get stuck (G0)

   2. Synthesis (S): DNA is duplicated through DNA replication upon the receipt of a proper signal

   3. Gap 2 (G2): cell grows, produces proteins, microtubules, in prep for mitosis

2. Mitotic (M): parent cell divides into two daughter cells

   • mitosis: orderly division of the nucleus

   • cytokinesis: division of the cytoplasm

Mitosis:

• prophase: nucleolus fades, chromatin → chromosomes, microtubules disassemble, mitotic spindle forms

• prometaphase: nuclear envelope is completely dissolved and some spindle fibers attach at the kinetochores

• metaphase: chromosomes align at center

• anaphase: chromatids pull apart, move to cell poles, spindle fibers shorten, cell elongates

• telophase: spindle breaks down, nuclear envelopes and nucleoli re-form, chromosomes uncoil and become less compact

Cytokinesis in Animals vs Plants:

• animals: formation of contractile ring of actin and myosin filaments along the metaphase plate

• plants: occurs as vesicles from golgi move along microtubules, forming a cell plate membrane

Cell Cycle Control

• G1 checkpoint: DNA synthesis will be delayed if DNA is damaged, will not continue again until the DNA has been repaired. this checkpoint reduces deleterious effects of DNA damage

• S-phase checkpoint: DNA synthesis will be delayed if the DNA is damaged or if replication is incomplete

• G2 checkpoint: the cell will not progress from the G2 phase to mitosis unless the DNA is undamaged and intact

• M-phase checkpoint: cell will not go into anaphase unless chromosomes are properly aligned along the metaphase plate and kinetochores are attached to the spindle fibers. this checkpoint ensures the cell is ready for division before it proceeds any further

• Regulation of the cell cycle is controlled by proteins called cyclins and cyclin-dependent kinases (CDKs)

  • kinase; phosphorylating enzyme

  • CDK binds to cyclin → allosteric change exposes active site on CDK → CDK phosphorylates proteins that regulate CDK cycle

  • every CDK has a specific cyclin that activates it, so cells can precisely control CDK activity by controlling the production and breakdown of cyclins

  • cyclins are produced in response to various signals, including presence of growth factors

Meiosis:

sexual reproduction: combination of haploid gametes from two parents → diploid

Meiosis 1: reduction phase/reductional division

Prophase I:

• synaptonemal protein complex attaches homologs together, forming a tetrad/bivalent, with 2 homologous chromosomes and four homologous chromatids

• synapsis: fusion of chromosome pairs

• crossing over: DNA is exchanged between homologs at chiasmata = recombinant chromatids, increases genetic variation

• spindle fibers form and attach to kinetochores

• synaptonemal complex begins to break down and nuclear envelope breaks down

Metaphase I:

• chromosomes line up along metaphase plate in homologous pairs

• spindles from each pole attach to one chromosome of each pair

• independent assortment

Anaphase I:

• homologous chromosomes of each tetrad separate and move along the spindle fibers toward each pole as the kinetochore tubules shorten

Telophase I and Cytokinesis:

• homologous chromosomes reach opposite poles

• microtubules disappear

• new nuclear membrane surrounds each haploid set

• chromosomes uncoil

• cytokinesis (pinching of cell membrane) completes creation of 2 daughter cells

Meiosis 2: equitational/restoration phase

sister chromatids separate, cells further divide into four haploid daughter cells with a single chromosome each

Prophase II:

• chromosomes condense, spindle forms, chromosomes move toward metaphase plate

Metaphase II:

• chromosomes meet at the metaphase plate, spindle fibers of each pole attach to the kinetochores of the sister chromatids

Anaphase II:

• centromeres of the sister chromatids separate

• sister chromatids move toward opposite poles

• sister chromatids are now called chromosomes

Telophase II and Cytokinesis:

• splitting of the cell into two daughter cells, chromosomes uncoil

• total of four haploid gametes are formed

Mitosis vs Meiosis:

Mitosis: process of nuclear division in eukaryotic cells that conserves the chromosome number by equally allocating replicated chromosomes to each of the daughter nuclei

Meiosis: two-stage type of cell division in sexually reproducing organisms that results in four cells with half the chromosome number of the original number of the original cell

Timing of Gamete Production:

Males: spermatogenesis, generate gametes throughout their lives, prophase I lasts 1 week

Females: born with all oogonia, prophase I begins before birth, meiotic process stops until puberty

The Origins of Genetic Variation:

Recombination: genetic variation

1. Independent Assortment: during meiosis I, the chromosomes line up in a random fashion along the metaphase plate

2. Crossing Over: prophase I of meiosis 1, crossing over of chromosomes occurs

Nondisjunction, Aneuploidy, Polyploidy, Translocation, Apoptosis, Cancer:

Nondisjunction: occurs when homologous pairs fail to separate in anaphase I, or sister chromatids fail to separate in anaphase II

Aneuploidy: chromosomal abnormality in which a person has an abnormal number of chromosomes in his or her genome

• Monosomy: one chromosome lacks a homologous partner

  • almost always lethal

  • turner syndrome: nonlethal, only one X chromosome

• Trisomy: three copies of one chromosome type

  • trisomy 16: most common, miscarriage

  • trisomy 21 (down syndrome): 3 copies of 21st chromosome

Polyploidy: abnormal amount of chromosome sets

• can result from an extra round of DNA replication before meiosis or lack of spindle formation in meiosis in II

• most common: triploidy (3n) or tetraploidy (4n)

Translocation: occurs when there is a crossover between nonhomologous chromosomes

Apoptosis: programmed cell death

• detach from neighbors, break chromatin apart into small pieces, form blebs: membranous lobes that break into fragments

• remains are ingested by surrounding cells in phagocytosis

• external signals: lack of mitotic signal

• internal signals: recognition of damaged DNA

Cancer: disease caused by uncontrolled cell division of abnormal cells in the body

• oncogene proteins (positive regulators): positively regulate the cell cycle in cancer cells, cause cells to progress through cell cycle more quickly → higher than normal cell division

• tumor suppressors (negative regulators): negatively regulate the cell cycle in normal cells, slow the process of cell division down → inactive in cancer cells