Cell Division and Proliferation

• Cell division occurs in all living organisms
• Cell division produces two daughter cells from one parent cell.
• Cell proliferation leads to exponential increases in cell number.
• Cell proliferation is a rapid mechanism for tissue growth, repair, embryological development, plant tissue/organ production, tissue replacement, and healing.
• Skin cell replacement is an example of cell proliferation.

Cell Cycle Order of Events

• Interphase (longest phase) then Mitosis, then Cytokinesis
• Interphase is when the cell carries out metabolic activities like protein synthesis, transcription, translation, & respiration
• G1: cell grows after mitosis/cytokinesis
• G1 involves protein synthesis, organelle building
• S phase: DNA replication occurs, reducing chromosomes with sister chromatids
• G2: cell prepares for mitosis and cell division by growing and replicating organelles
• M-phase has 4 stages + interphase: prophase, metaphase, anaphase, and telophase.
• chromosomes consist of two elongated DNA molecules called sister chromatids held together until anaphase occurs

Mitosis

• Sister chromatids are two identical copies of a duplicated chromosome.
• During prophase of mitosis and prophase of meiosis, chromatin condenses to form visible chromosomes with sister chromatids.
• Compaction: DNA coils around histone proteins to form nuclear zones.
• Nucleosomes coil to form chromosomes with sister chromatids.
• Chromosome condensation allows segregation during mitosis/meiosis.
• Spindle fibers are composed of protein microtubules attached to chromosome centers via kinetochores during prophase of mitosis and meiosis.
• During anaphase, microtubule motors on kinetochores move chromosomes to the poles.
• Early prophase: chromatin condenses, making chromosomes visible.
• Late prophase: the nuclear envelope breaks down, the centriole moves towards the poles, producing spindle fibers which attach to chromosomes at the centrometers

Metaphase and Anaphase

• In metaphase, chromosomes align at the equatorial plane and attach to spindle fibers
• During anaphase, sister chromatids separate at the centromere to form single-stranded chromosomes.
• Microtubule motors move resulting daughter chromosomes toward opposite poles

Telophase and Cytokinesis

• In telophase, daughter chromosomes reach the pole, uncoil to form chromatin, forming two new genetically identical nuclei
• Cytokinesis splits the cytoplasm of a parent cell to form two cells
• Animal cell cytokinesis involved an actin and myosin contractile ring that forms around the cell equator, deepening until the two daughter cells split.
• Plant cell cytokinesis uses the golgi apparatus to produces vesicles of carbohydrates that line up along the equator to form a cell plate
• Vesicle membranes fuse to form the cell plate
• Cell plate membranes fuse with the existing plasma membrane to divide contents and form two separate cells.
• Cellulose is secreted into the cell plate to form the cell wall
• Cytoplasmic division is usually equal to ensure each daughter cell receives at least one mitochondria
• Unequal cytoplasmic division can occur
• Budding in yeast is an example of unequal division as small daughter cell buds off from a larger cell during asexual reproduction
• Oogenesis produces eggs and involves unequal division of cells during meiosis to produce one large egg cell

Genetics

• Mitosis maintains the chromosome number, but meiosis halves the chromosome number and generates genetic diversity.
• Mitosis and meiosis are forms of nuclear division.
• Nuclear division must occur before cytokinesis.
• Cyclins are proteins that control cell cycle movement.
• Different cyclins increase and decrease to pass checkpoints
• Enzymes are necessary for both mitosis and meiosis.
• Cyclin binds to a specific enzyme.
• G1 checkpoint ensures the cell gets big enough and has enough cytoplasm to divide
• G2 checkpoint ensures that the DNA is doubled
• M-phase checkpoint is in metaphase, which ensures random assortment occurs
• Cyclin E peaks at DNA replication.
• Cyclin E and D are involved in G1
• Cyclin A ensures all organelles are replicated.
• Cyclin B manages nucleus division.
• Cyclin D regulates the whole cell cycle.

Gene Mutation

• Mutations are changes to the DNA or RNA sequence of a cell or virus
• Tumor suppressor genes regulate cell division by inhibiting cell proliferation and tumor development.
• Uncontrolled cell division may result in cancer.
• Proto-oncogenes regulate normal cell growth.
• Oncogenes are genes that change a cell into a tumor cell, leading to cancer.
• The primary tumor is the first tumor produced in the body.
• Metastasis is when cancer cells from the primary tumor spread throughout the body, forming secondary tumors.
• Mitotic Index formula: (Number of cells undergoing Mitosis/Total number of cells) X 100
• The biotic index for cancerous tissue will be higher than the meiotic index of the same tissue without cancer

Meiosis

• Eukaryotes produce genetically varied cells that can develop into gametes via meiosis
• Meiosis produces four daughter cells each with half the number of chromosomes as the parent cell.
• Meiosis produces gametes and spores and is required for sexual reproduction.
• Human sperm and eggs (produced by meiosis) fuse during fertilization to become a zygote
• Two divisions of meiosis produce four haploid nuclei from one diploid nucleus.
• Diploid cells (2n) have a nucleus with two sets of chromosomes and each chromosome has a matching homologue.
• Haploid cells (n) have a nucleus with one set of chromosomes.
• Meiosis is a reduction division because the parent cell is diploid, and the daughter cells are haploid.
• Meiosis reduces the number of chromosomes in the nucleus of the daughter cells.
• Homologous chromosomes contain the same genes in the same locations but may have different alleles

Meiosis Phases

• Before meiosis, both homologous chromosomes are replicated.
• Homologous chromosomes pair to form a bivalent during meiosis one.
• Non-sister chromatids crossover during bivalence formation and exchange sections of non-sister chromatids.
• Meiosis involves two nuclear divisions, each followed by cytokinesis.
• Meiosis one segregates homologous chromosomes to produce two haploid cells, halving the chromosome number.
• Meiosis two divides sister chromatids and produces four haploid cells

Meiosis I

• Prophase 1: Homologous chromosomes pair to form a bivalent, crossing over occurs between non-sister chromatids, the chromatin condenses, the centrioles move towards the poles, spindle fibers form, and then the nuclear membrane breaks
• Metaphase 1: Spindle fibers move bivalents (homologous chromosomes) to the cell equator, also sister chromatids attach to fibers at the centromere. Maternal and paternal homologous chromosomes are line up in the center separately
• Anaphase 1: Homologous chromosomes in the bivalent by the spindle fibers. Microtubule motors then pull and move chromosomes to the opposite poles.
• Telophase 1: Chromosomes (as sister chromatids) arrive at the poles and uncoil. A nuclear membrane forms around sister chromatids at each pole, producing two haploid nuclei. Cytokinesis produces two haploid cells

Meiosis II

• Prophase 2: Chromosomes coil and appear in both haploid cells. Centrioles move towards the poles, producing spindle fiber microtubules that attach to sister chromatids at the centromere
• Metaphase 2: Chromosomes with sister chromatids line up along the center of the cell which are attached to the spindle fibers by their centromere
• Single chromosomes separate at the equator.
• Anaphase 2: Sister chromatids are pulled apart, generating single-stranded chromosomes. Microtubular motors move single-stranded chromosomes towards the poles.
• Telophase 2: Chromosomes reach the poles of each cell and uncoil. A nuclear membrane forms around each set of chromosomes. Cytokinesis in both cells forms four cells with haploid nuclei, which are not identical
• Duplicated chromosomes (including sister chromosomes and duplicated chromosomes) are can count as one chromosome during mitosis and meiosis

Other Genetic Concepts

• Meiosis maintains the diploid number of chromosomes AND also produces haploid eggs/sperm.
• Nondisjunction is when one or more pairs of homologous chromosomes or sister chromatids fail to fully separate during nuclear division
• Nondisjunction can occur during anaphase one or anaphase two of meiosis
• Nondisjunction produces gametes with an extra or missing chromosome.
• If a gamete with an extra chromosome fertilizes a normal gamete, the zygote and the offspring will have three copies of that chromosome
• Down syndrome is an example of an offspring that has three copies of chromosome 21
• If a gamete is missing a chromosome is fertilized by a normal gamete, the zygote and the offspring will have one copy of the chromosome
• Meiosis generates genetic diversity by random orientation of bivalents during metaphase 1 and also by crossing over of non-sister chromatids during prophase 1
• Gametes produced by meiosis are genetically different due to crossing over
• The bivalents (homologous pairs of chromosomes) are sorted independently during meiosis one, increasing genetic variation.