Cell Division Cycle: Mitosis and Meiosis

• Prokaryotes reproduce via binary fission, which involves DNA replication and cell splitting, resulting in two identical cells and leading to the evolution of mitosis.
• Eukaryotes use mitosis for growth and repair, producing two identical cells.
• Meiosis in eukaryotes produces four non-identical gametes (sex cells) with half the DNA for sexual reproduction.

Cell Division Cycle

• The three main categories of the cell division cycle are interphase, M phase (mitosis), and cytokinesis, with interphase being the longest.

Interphase Subphases

• The three subphases of interphase are G1, S, and G2.
• G1 involves cell growth and organelle creation.
• The S (DNA synthesis) phase is dedicated to DNA replication.
• G2 involves further cell growth and final preparations for cell division.
• Cells exiting the M phase can enter a G0 phase, ceasing preparation for further division.

S Phase Enzymes

• Four enzymes are active during the S phase: helicase, RNA primase, DNA polymerase, and DNA ligase.
• Helicase breaks hydrogen bonds between parent strands, unwinding DNA into two separate strands.
• RNA primase creates an RNA primer, providing a starting point for DNA polymerase on the two new template strands.
• DNA polymerase creates a new DNA strand in the 5' to 3' direction complementary to the parent strand.
• The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously in Okazaki fragments.
• DNA polymerase replaces the RNA primer with DNA and helps anneal fragments.
• DNA ligase ensures the correct bonding of new and old DNA fragments.
• Parent and daughter strands form a double helix, resulting in two identical daughter molecules.
• Multiple replication bubbles on DNA speed up replication.
• DNA replication is semiconservative, producing two DNA copies, each with half of the original DNA.
• Meselson and Stahl's experiments proved that DNA replication is semiconservative.
• The three original theories of DNA replication were conservative, semiconservative, and disruptive.
• New DNA molecules consist of one parent strand and one daughter strand.
• After DNA replication, two sister chromatids are attached at the centromere.

Chromosome Structure

• The centromere is the center of the chromosome.
• A single-armed chromosome is an unreplicated, uncondensed DNA strand.
• A double-armed chromosome consists of two identical sister chromatids after replication.
• Sister chromatids are identical chromosome copies.

Chromosome Numbers

• A karyotype is a picture of paired maternal and paternal chromosomes from a cell.
• Human somatic cells (body cells) have 46 chromosomes.
• Human gametes (sperm/egg cells) have 23 chromosomes.
• Autosomes are any chromosomes that are not X or Y.
• Sex chromosomes are X or Y chromosomes.
• Homologous chromosomes are matching pairs from each parent with different gene versions (alleles).
• In males, XY chromosomes are an exception as they are not identical.
• Diploid refers to the total number of chromosomes in somatic cells.
• Haploid refers to the number of chromosome pairs.

Mitosis Stages

• The stages of mitosis are prophase, metaphase, anaphase, and telophase, with cytokinesis overlapping the latter.

Mitosis: Prophase

• The nuclear envelope disintegrates and DNA condenses into chromosomes during prophase.
• Spindle fibers form between centrioles.
• Cells package DNA into chromosomes for mitosis (M phase).
• Kinetochores are proteins around the centromere where spindle fibers attach to sort DNA copies.

Mitosis: Metaphase

• Chromosomes attach to spindle fibers at kinetochores and align in the middle of the cell at the metaphase plate.

Mitosis: Anaphase

• Chromosomes separate, with one sister chromatid on each side, pulled by spindle fibers moving in opposite directions and pushing fibers apart, causing elongation.

Mitosis: Telophase

• Each pole has a copy of the chromosome, DNA uncoils into chromatin, the nuclear envelope reforms, and a cleavage furrow forms in animal cells.

Mitosis: Cytokinesis

• Cytokinesis involves the division of cytoplasm, resulting in two identical diploid cells.
• In plants, a cell plate forms instead of a cleavage furrow.

Cell Cycle Controls

• Cell cycle controls are checkpoints that ensure cells divide correctly.
• Contact inhibition stops cell division when cells touch, preventing overgrowth.
• External regulatory proteins signal cells to start dividing (e.g., healing) or stop dividing when enough cells exist.
• Internal regulatory proteins at G1, G2, and M pause the cycle to fix problems; if unfixable, apoptosis (programmed cell death) occurs.

Cancer

• Cancer is uncontrolled cell division leading to a tumor, a mass of cells.
• Benign tumors are not harmful, whereas malignant tumors are harmful and can spread.
• Tumors are caused by mutated cell cycle genes due to environmental damage or hereditary factors.
• Cancer development starts with a single abnormal cell and requires multiple mutations.
• Carcinogens are mutagenic environmental agents, like viruses, radiation, or chemical exposure.
• Mutations causing cancer must affect genes controlling the cell cycle, such as the p53 tumor suppressor gene.
• An oncologist diagnoses cancer.
• Metastasis is the spread of cancer to new locations.
• Cancer cells bypass normal controls, don't self-destruct, use enzymes to penetrate tissues, and spread via the bloodstream or lymphatic system, forming secondary tumors.

Cancer Treatment

• Cancer can be treated via surgery, chemotherapy (chemicals destroying cancer cells, interfering with DNA synthesis, and killing rapidly dividing cells), and radiation therapy.

Gametogenesis

• Gametogenesis is the making of gametes
• Spermatogenesis is sperm production in males from puberty until death.
• Oogenesis is egg production in females from embryonic development until menopause, with babies born with eggs in meiosis I and only one functional egg after meiosis II.

Eukaryotic Cell Division

• Mitosis makes somatic cells and involves one division round, creating two identical diploid daughter cells.
• Meiosis makes gametes for sexual reproduction, involves two division rounds, and creates four genetically different haploid gamete cells.

Stages of Meiosis

• Two steps: meiosis I and meiosis II
• Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase/Cytokinesis I
• Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase/Cytokinesis II

Meiosis I: Prophase I

• Homologous pairs find each other and form tetrads, crossing over DNA sections at chiasmata.
• Prophase I introduces variation through crossing over.
• Random alignment happens at Metaphase I/Metaphase II
• Separation happens at Anaphase I (tetrads separate)/Anaphase II (sister chromatids separate)

Fertilization and Embryo Development

• Cleavage is rapid cell division/mitosis of the first cell into a multicellular embryo
• Stem cells are non-specialized cells of the initial embryo.
• A zygote is the term after sperm and egg fuse during fertilization.
• Differentiation is cells maturing from stem cells into specialized cells.

Male Reproductive System

• Testis: site of sperm and testosterone production; contained in the scrotum.
• Epididymis: sperm storage.
• Vas deferens: tube transferring sperm from the epididymis to the urethra.
• Seminal vesicles: add liquid to sperm.
• Prostate: secretes an alkaline solution to semen to counter vaginal acidity.
• Bulbourethral gland: also adds seminal fluid.
• Penis: contains the urethra, which transports semen and urine.

Female Reproductive System

• Ovary: holds and develops eggs (ova).
• Fallopian Tube: site of fertilization where a fertilized egg will travel.
• Uterus: develops the endometrium lining for embryo implantation and menstruation if no baby.
• Cervix: separates the uterus from the vagina and keeps the baby in the womb until delivery.
• Vagina: birth canal.
• The two main female hormones are progesterone and estrogen.

Monthly Female Hormone Cycle

• Hypothalamus monitors estrogen and progesterone levels via negative feedback.
• Low estrogen and progesterone signal the release of GnRH, which stimulates the pituitary gland to release FSH and LH.
• FSH and LH mature the follicle/egg inside the ovary.
• The follicle releases estrogen and a mature egg is released from the ovary = ovulation via positive feedback.
• The follicle becomes the corpus luteum after ovulation and is developed via LH which will secrete estrogen and progesterone.
• Estrogen and progesterone stimulate the thickening of the endometrium (uterus lining).
• The hypothalamus and pituitary terminate the reproductive cycle via negative feedback from low estrogen and progesterone.
• The endometrium breaks down in the absence of FSH, LH, estrogen, and progesterone.
• Endometrium stops growing and detaches from the uterine wall, leading to menstruation.
• If the egg is fertilized in the fallopian tube, the embryo implants into the uterus and makes HCG, estrogen, and progesterone, which maintain the endometrium as the baby grows.
• Progesterone prevents further ovulation during pregnancy.
• Pregnancy tests test for HCG, released during the first trimester.
• Birth control pills contain estrogen and progesterone and prevent ovulation via negative feedback.
• Negative feedback: accumulation of the end product stops the process; a lack of the end product increases the process.
• Positive feedback: accumulation of the end product increases the process; a lack of the end product stops the process.
• Hormone spikes most at ovulation.

Nondisjunction and Aneuploidy

• Nondisjunction is the failure of tetrads/homologous chromosomes separation in meiosis I or sister chromatids in meiosis II
• Aneuploidy is a condition where one chromosome is missing or in excess
• Autosomal aneuploidy: Down syndrome (only survivable one)
• Viable human aneuploidies chances increase with the parent's age
• Sex chromosome aneuploidy: Extra or missing X or Y due to nondisjunction
• Turner syndrome: Monosomy X, causes underdeveloped ovaries, causing the female to be sterile
• Klinefelter Syndrome: Underdeveloped testes/low testosterone causing men to be sterile and possibly mild learning difficulties
• Jacob Syndrome: No affect, could result in the person being tall