Mitosis and Meiosis

• The expected learning outcomes are to describe each phase of mitosis and meiosis, and how a cell can transform into a cancer cell, including its unusual features.

Key Terms in Cell Division

• Mitosis results in two daughter cells with the same number and kind of chromosomes as the parent nucleus.
• Meiosis results in four daughter cells, each with half the number of chromosomes as the parent cell, as in gamete production.
• Cyclins are proteins associated with the cycle of cell division, thought to initiate certain processes of mitosis.
• Interphase is the longest stage in the eukaryote cell cycle where the cell acquires nutrients, creates and uses proteins/molecules, and replicates DNA.
• Cytokinesis is the cytoplasmic division of a cell at the end of mitosis or meiosis, separating into two daughter cells.
• Haploid refers to having a single set of unpaired chromosomes, such as in egg and sperm cells.
• Nondisjunction is the failure of paired chromosomes to separate during cell division, resulting in both chromosomes going to one daughter cell and none to the other.

Cell Division and Reproduction Basics

• Cell division makes new cells to replace old, dead, or damaged ones; it allows organisms to grow by producing more cells, not by cells getting larger.
• Nearly two trillion cells divide every day in human bodies.
• The dividing cell is the "parent" cell, which divides into two "daughter" cells, repeating in the cell cycle.
• Cell division is regulated through chemical signals from proteins called cyclins, acting as switches to start and stop division.
• Failure to stop dividing can lead to cancer.
• Skin cells divide constantly, 50 million die a day, while nerve and brain cells divide less often.
• Mitosis makes new body cells while meiosis creates egg and sperm cells.
• During mitosis, a cell duplicates its contents and splits into two identical daughter cells.

Mitosis vs. Meiosis

• Meiosis ensures humans have the same number of chromosomes in each generation; it’s a two-step process reducing chromosomes by half (46 to 23) to form sperm and egg cells.
• In conception, sperm and egg contribute 23 chromosomes each, resulting in the usual 46 in the embryo.
• Meiosis additionally allows genetic variation through DNA shuffling.
• Mitosis involves the division of body cells one time, while meiosis involves the division of sex cells twice.
• Two daughter cells are produced after mitosis, while four daughter cells are produced after meiosis.
• Mitosis makes more cells that are genetically the same as the parent cell for the development of embryos, and growth/development.
• Before mitosis, a cell undergoes interphase, which is often included in discussions of mitosis technically is not a part, but rather encompasses G1, S, and G2 of the cell cycle

Stages of Cell Division

• During interphase, the cell engages in metabolic activity and prepares for mitosis by replicating DNA.
• Each chromosome reorganizes into paired structures called sister chromatids, with each carrying a full copy of the DNA sequence
• Mitosis has four stages: prophase, metaphase, anaphase, and telophase + cytokinesis
• Prophase: Chromatin condenses into visible chromosomes , the nucleolus disappears, centrioles move to opposite ends, and the mitotic spindle forms.
• Metaphase: Spindle fibers align chromosomes along the metaphase plate ensuring each new nucleus receives one copy of each chromosome.
• Anaphase: Paired chromosomes separate at the kinetochores and move to opposite sides of the cell via kinetochore movement along the spindle microtubules and polar microtubules.
• Telophase: Chromatids arrive at opposite poles, new membranes form around daughter nuclei, chromosomes disperse, spindle fibers disperse, and cytokinesis begins.

Cytokinesis

• First visible change in animal cells is the appearance of cleavage furrow.
• For plant cells, a cell plate is synthesized between the two daughter cells.
• Meiosis involves a single cell dividing twice to produce four cells, each containing half the original genetic information
• Four daughter cells are haploid- meaning half the number of chromosomes from parent cell.
• Meiosis produces sex cells/gametes- eggs in females and sperm in males.
• Meiosis can be divided into nine stages, split into two divisions: meiosis I and meiosis II

Meiosis 1 Stages

• Interphase: DNA in the cell copied, resulting in two identical full sets of chromosomes.
• Prophase I: Copied chromosomes condense into X-shaped structures, each composed of two sister chromatids; chromosomes pair up, exchanging bits of DNA through recombination/crossing over; the nuclear membrane dissolves away, and the meiotic spindle extends.
• Metaphase I: Chromosome pairs line up along the center of the cell as centrioles at opposite poles extend the meiotic spindles outward, the spindles then attach to one chromosome of each pair.
• Anaphase I: The chromosomes pair are pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell while the other chromosome goes to the opposite pole; the sister chromatids stay together.
• Telophase I and Cytokinesis: The chromosomes reach opposite poles, and a full set gathers together at each pole; then a membrane forms around each set of chromosomes, creating two new nuclei. The single cell then pinches in the middle to form two separate daughter cells, each containing a full set of chromosomes within a nucleus in a process called Cytokinesis.

Meiosis 2 Stages

• There are now two daughter cells, each with 23 chromosomes (23 pairs of chromatids).
• Prophase II: Chromosomes condense again into visible X-shaped structures in each daughter cell, with the membrane around the nucleus dissolving away, releasing the chromosomes, the centrioles duplicate, and the meiotic spindle forms again.
• Metaphase II: The chromosomes (pair of sister chromatids) line up end-to-end along the equator of each of the two daughter cells.
• Anaphase II: Sister chromatids are pulled to opposite poles because of the action of the meiotic spindle, the separated chromatids are now individual chromosomes.
• Telophase II and Cytokinesis: At each pole of the cell, a full set of chromosomes gather together; a membrane forms around each set of chromosomes creating two new cell nuclei.
• Once cytokinesis is complete there are now four granddaughter cells, each with half a set of chromosomes (haploid): in males all four cells are sperm cells, but in females one of the cells is an egg cell while the other three are polar bodies which don't turn into eggs.

Nondisjunction

• This is the failure of paired chromosomes to separate during cell division.
• This results with both chromosomes going to one daughter cell and none to the other.
• This causes errors in chromosome number, such as trisomy 21 (Down syndrome) and monosomy X (Turner syndrome).
• A common cause of early spontaneous abortions.

Cancer Cell Biology

• Key terms related to cancer cell include
  • Cancer is a disease caused by an uncontrolled division of abnormal cells
  • Apoptosis/the death of cells occurs as a normal mechanism of normal growth
  • Metastasis is the process by which cancer spreads from the primary tumor to distant locations
  • Mutation occurs when a DNA gene is damaged to alter a genetic message
  • Mutagen is an agent that causes genetic mutation
  • Carcinogen causes cancer
  • Benign refers to growths that are not cancerous
  • Malignant refers to tumors made of cancer cells that can invade nearby tissues
• Cancer cells are cells gone wrong that no longer respond to cellular growth control signals, and they evade programmed cell death, breaking through normal tissue boundaries to metastasize.
• Normal cell growth requires a balance between genes that promote and suppress cell division, relying on genes that signal damaged cells to undergo apoptosis.
• After mutations accumulate in genes cells become cancerous.

Development of Cancer

• In cancer, cells possess 60+ mutations that challenge medical researchers to identify which are responsible for cancer.
• Research reveals that certain genes are mutated in cancer cells more often than others.
• Growth-promoting genes, like Ras, can become super-active, and certain chemotherapy drugs counteract these mutations by blocking growth-signaling proteins.
• Other cancer-related mutations inactivate tumor suppressor genes, which normally function like brakes on proliferation, where both copies within a cell must be mutated for uncontrolled division to occur.
• Cancer comes from accumulated damage to genes either by chance or by exposure to cancer-causing substance called carcinogens where the causes can be environmental, viral, or in some cases due to genetic factors.
• Majority of cancer cases cannot be attributed to a single cause.

Cancerous Risk

• Risks can be divided into:
  • Biological or internal factors like age, gender, inherited genetic defects, and skin type.
  • Environmental exposure: radon, UV radiation
  • Occupational risk factors: carcinogens such as chemicals, radioactive materials and asbestos
  • Lifestyle-related factors that cause cancer: tobacco, alcohol, UV radiation, and food-related factors, like nitrites and poly aromatic hydrocarbons from barbecuing food.
• Cancer-causing factors related to work/living environments include asbestos fibers, tar and pitch, metal compounds, and some plastic chemicals.
• Bacteria and viruses that can cause cancer: Helicobacter pylori, HBV, HCV, HPV, EBV
• Radiation can cause cancer: ionizing radiation, non-ionized radiation
• Some drugs may increase the risk of cancer: certain antineoplastic agents, hormones, and medicines that cause immune deficiency.
• Mutations accumulate over time as a result of independent events.
• The progression of cancer: a micro evolutionary process where advantageous mutations allow offspring to outperform noncancerous cells.

Growing Tumors

• During early stages, tumors will be likely to be begnin, then may eventually turn malignant.
• Cancerous cells enter the bloodstream or the lymphatic system and travel to a new location in the body, dividing and laying the foundation for secondary tumors.
• For metastasis to occur, the cells must penetrate the normal barriers of the body and enter and exit blood/lymph vessels.
• Uncontrolled cell division is related to DNA replication.
• As each generation of cells produces more non-functional proteins from uncorrected DNA damage, the small, uncorrected errors are passed from parent to daughter cells and accumulate.
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Test your knowledge of mitosis and meiosis. This quiz covers the key differences between mitosis and meiosis, DNA replication, nondisjunction, the role of cyclins, chromosome number, and cytokinesis.