Cell Division

Cell division

Nuclear division (karyokinesis) followed by cytokinesis

Microtubule organizing centers (MTOCs)

Structures that organize microtubules during cell division

Centrosomes

The primary microtubule organizing centers in animal cells, help form the spindle apparatus and allow for cell segregation. Found outside the nucleus.

Centrioles

Cylindrical structures within centrosomes that assist in the formation of the spindle fibers during cell division. Found in pairs.

Prophase

Chromatin condenses into chromosomes, nucleus disassembles, nucleolus disappears, nuclear envelope breaks down, mitotic spindle forms and microtubules begin connecting to kinetochores

Tubulin

A protein that forms microtubules

Prometaphase

Nuclear membrane breaks down, kinetochore microtubules invade nuclear space and attach to kinetochores, polar microtubules push against each other, moving centrosomes apart.

Metaphase

Chromosomes line up in the center, each chromatid is complete with a centromere and attached kinetochore, centrosomes are at opposite ends of the cell, karyotyping is performed. Cell checks that each chromosome is attached to microtubules with their kinetochore to ensure even split.

Anaphase

Microtubules shorten, each chromosome is pulled apart into two chromatids, chromosomes are pulled to opposite poles, each pole has a complete set of chromosomes, shortest step of mitosis. Chromosomes break at centromeres, and sister chromatids move to opposite ends of the cell. Chromosome number doubles and each pole has a complete set of chromosomes (same number as original).

Disjunction

When chromsomes are pulled to opposite poles during anaphase

Telophase

Nuclear division, nuclear envelope reforms, chromosomes unwind into chromatin, nucleoli reappear

Cytokinesis

Myosin II and actin filament ring contract to cleave cell in two. Forms cleavage furrow in animals and cell plate in plants.

Interphase

Occurs after mitosis and cytokinesis are complete. Consists of G1, S, and G2

Cell cycle

Mitotic phases (mitosis, cytokinesis), interphase (G1, S, G2)

G1 phase

Cell increases size, G1 checkpoint ensures cell is ready for DNA synthesis (produce protein, ribosomes, mitochondria).

S phase

DNA synthesis: second molecule of DNA replicated from the first, provides sister chromatids

G2 phase

Rapid cell growth, preparation of genetic material for cellular division, cell replicates its organelles

Small S/V ratio

Cellular exchange is hard, leads to cell death or cell division to increase SA

Small G/V ratio

Cell exceeds the ability of its genome to produce sufficient amounts of regulation for cellular activities. Some large cells (paramecium, skeletal muscle) are multinucleated to deal with this

G1 checkpoint (restriction point)

Most important checkpoint, cell growth is assessed and favorable conditions are checked. If it fails, cell enters G0. Liver and kidney cells can be induced out of G0, and nerve and muscle cells stay in G0 permanently.

G2 checkpoint

Cell evaluates accuracy of DNA replication and signals whether to begin mitosis. Cell checks for sufficient mitosis promoting factor (MPF) levels to proceed.

M checkpoint (during metaphase)

Mitosis stops if the chromosomes are not attached to spindle fibers. If all are attached, cell is allowed to proceed with anaphase.

Cyclin-dependent kinases (Cdks)

Activate proteins that regulate cell cycle by phosphorylation. Activated by the protein cyclin.

Growth factors

Plasma membrane has receptors for growth factors that stimulate cells for division (like damaged cells)

Density-dependent inhibition

Cells stop dividing when surrounding cell density reaches a maximum.

Anchorage dependence

Most cells only divide when attached to an external surface such as neighboring cells or a side of culture dish

Cancer cells

Defy the 5 cell-specific regulations and are called transformed cells. Manifestation of defective cell differentiation

Cancer drugs

Inhibit mitosis by disrupting the ability of microtubules to separate chromosomes during anaphase, thus stopping replication

Myeloma

Cancerous plasma cell

Hybridoma

An antibody producing plasma cell fused with a myeloma

Meiosis I

Homologous chromosomes pair at the plate, and migrate to opposite poles. NO separation of sister chromatids.

Prophase I

Nucleus disassembles, nucleolus disappears, chromatin condenses, spindle formation. Synapsis occurs as homologous chromosomes pair up to form tetrads. Crossing over occurs at chiasmata, allowing for genetic recombination that results in changes to nucleotide sequence.

Chromosomes condense, nuclear envelope breaks down. Crossing over occurs.

Synapsis

When homologous chromsomes pair up during prophase I. Pair is called a tetrad (group of 4 chromsomes) or bivalent

Chiasmata

The region where crossing over of non-sister chromatids occurs (in prophase I)

Synaptonemal complex

A protein structure that temporarily forms between homologous chromsomes. Gives rise to the tetrad with chiasmata and crossing over

Metaphase I

Homologous pairs are lined up across the plate. Microtubules are attached to kinetochores of one member of each homologous pair.

Anaphase I

Homologous pairs within tetrads uncouple and are pulled to opposite sides (disjunction)

Telophase I

Nuclear envelope develops. Each pole forms a new nucleus that now has half the number of chromosomes - chromosome reduction phase to haploid.

5 sub-steps of Prophase I

Leptotene —> zygotene —> pachytene —> diplotene —> diakinesis

Leptotene

Chromosomes start condensing

Zygotene

Synapsis begins; synaptonemal complex forming

Pachytene

Synapsis complete, crossing over

Diplotene

Synaptonemal complex disappears, chiasma still present

Diakinesis

Nuclear envelope fragments, chromosomes complete condensing, tetrads ready for metaphase

Telophase I and cytokinesis

Chromosomes gather at the poles of cells. Cytoplasm divides

Prophase II

A new spindle forms around the chromosomes. Nuclear envelope disappears, spindle develops, NO chiasmata, NO crossing over

Metaphase II

Chromosomes line up at equator, now there are half the number of chromosomes

Anaphase II

Centromeres divide, chromatids move to the opposite poles of the cells. Each chromosome is pulled into two separate chromatids and migrate to opposite poles of the cell.

Telophase II and cytokinesis

A nuclear envelope forms around each set of chromosomes. Cytoplasm divides. At completion of meiosis II, there are 4 haploid cells.

Meiosis II

Chromosomes spread across the metaphase plate and sister chromatids separate and migrate to opposite poles

Causes of genetic variation

Crossing over during prophase I, independent assortment of homologous chromsomes during metaphase I, random joining of gametes (germ cells)

Independent assortment

Random orientation of homologous chromosomes allows for the production of gametes with many different assortments

Random joining of gametes

Joining of gametes is random, but some sperm cells have a genetic composition that gives them a competitive advantage (not all are equally competitive)

Chromatin

General packaging of DNA around histone proteins, DNA is packaged like this for most of the cell cycle

Chromosome

Additional level of DNA organization that is even denser than chromatin, visible with light microscope (particularly during metaphase). Can be in duplicated or unduplicated state

Chromatid

Chromosome in duplicated state (like at the beginning of mitosis)

Sister chromatids

Genetically identical and attached at the centromere

In mitosis, each chromatid is considered separate, individual chromosomes when…

Sister chromatids separate once anaphase has begun. Once sister chromatids have separated, each chromatid is also considered a chromosome

In mitosis, the normal chromosome number is restored…

After the end of mitosis when the dividing cells have fully separated and the membranes have reformed

Anaphase I only separates…

Homologous chromosomes (neither the chromosome number nor the chromatid number changes during anaphase I since only separation of sister chromatids changes the number)

Meiosis II

Similar to mitosis, but there are now half as many chromosomes as before

During mitosis and meiosis the __ never changes

chromatid count (only number of chromosomes changes by doubling during anaphase wehn sister chromatids separate)

During meiosis I, neither the chromosome number nor the chromatid number change until…

After telophase I is complete