The Cell Cycle, Mitosis, and Reproduction Practice Flashcards

The Cell Cycle and Interphase

• The Cell Cycle Overview: In animals, autosomal cells are diploid ($2n$), meaning they contain two copies of each chromosome, forming homologous pairs (one from each parent). Eukaryotic cells replicate through a specific series of phases known as the cell cycle, during which the cell grows, synthesizes DNA, and ultimately divides.

• Active Division Phases: For cells that are actively dividing, the cell cycle consists of four distinct stages: $G1$, $S$, $G2$, and $M$.

Interphase

• Interphase Definition: The collective term for the $G1$, $S$, and $G2$ stages. During interphase, DNA is uncoiled in the form of chromatin to allow for transcription and replication.

• $G0$ Phase: A state where a cell exits the cycle temporarily or permanently. The cell remains alive and performs its normal functions but is not preparing to divide.

  • Example: Neurons typically reside in $G0$.

• $G1$ Phase (Presynthetic Gap):

  • The cell begins growing and carrying out normal functions.

  • Cells produce more organelles (like mitochondria and ribosomes).

  • Checkpoint (Restriction Point): A critical checkpoint exists here where the cell ensures everything is in order (specifically the quality of DNA) before proceeding to the $S$ phase.

• $S$ Phase (Synthesis of DNA):

  • DNA replication occurs during this phase.

  • Chromosomes make identical copies of themselves called sister chromatids.

  • Sister chromatids are connected at a specialized region called the centromere.

• $G2$ Phase (Postsynthetic Gap):

  • The cell continues to grow and replicates its organelles and other cellular components to prepare for division.

  • Checkpoint: A second checkpoint ensures that DNA replication was successful and the cell has attained adequate size to support two daughter cells.

Control of the Cell Cycle

• Checkpoints:

  • $G1/S$ Checkpoint (Restriction Point): The cell determines if the condition of the DNA is good enough for synthesis. The main protein in control of this checkpoint is $p53$.

  • $G2/M$ Checkpoint: The cell ensures it has achieved adequate size and that organelles have been properly replicated. $p53$ also plays a role here.

• Molecules of Regulation:

  • Cyclins and Cyclin-Dependent Kinases (CDK): These are the molecules responsible for the cell cycle.

  • Mechanism: Cyclins are regulatory proteins whose concentrations fluctuate throughout the cycle. CDKs are enzymes present in the cell that remain inactive until they bind to specific cyclins. Once bound, the Cyclin-CDK complex is activated.

  • Phosphorylation: The active complex phosphorylates specific transcription factors, which then promote the transcription of genes required for the next stage of the cell cycle.

• Cancer: When the control of the cell cycle becomes deranged or damaged, allowing cells to undergo mitosis without proper checks, cancer may result.

Mitosis

• Definition: The process used by somatic cells to divide their genetic material evenly between two daughter cells. Mitosis takes one diploid cell ($2n$) and produces two identical diploid daughter cells ($2n \rightarrow 2n$).

• Preparation (Interphase): Before mitosis, the cell has already completed $G1$, $S$, and $G2$. It possesses two sister chromatids for every chromosome held at the centromere, and centrosomes (which organize mitotic spindles) have been duplicated.

• 1. Prophase:

  • Chromatin condenses into visible chromosomes.

  • Centrosomes (containing centrioles) migrate to opposite poles and begin organizing the mitotic spindle.

  • Centriole Composition: Centrioles are made of fused microtubules composed of alpha-tubulin ($\alpha$-tubulin) and beta-tubulin ($\beta$-tubulin).

  • Spindle fibers form and attach to the kinetochores located on the chromosomes.

• 2. Metaphase:

  • Chromosomes align along the metaphase plate (the center/equator of the cell).

  • Each chromosome is attached to a spindle fiber from both sides of the cell via its kinetochore.

• 3. Anaphase:

  • Sister chromatids are pulled apart at the centromere.

  • The separated chromatids are now considered individual chromosomes.

  • Shortening of the spindle fibers drags the chromatids to opposite ends of the cell.

  • Chromosome Count: The chromosome count temporarily doubles during this phase (e.g., 2 chromosomes become 4 because the doubles are separated).

• 4. Telophase:

  • A new nuclear envelope forms around each set of chromosomes at either side of the cell.

  • Chromosomes decondense back into chromatin.

  • The spindle fibers break down.

• 5. Cytokinesis:

  • The physical splitting of the cytoplasm and organelles.

  • Result: Two separate daughter cells, each with its own nucleus and organelles, both being diploid ($2n$) like the parent.

Meiosis

• Definition and Purpose: Meiosis occurs only in gametes (cells that become sperm or eggs). The purpose is to produce four non-identical haploid cells ($n$), each containing half the number of chromosomes of the original cell.

• Chromosome Counts:

  • Haploid ($n$) = 23 chromosomes in humans.

  • Diploid ($2n$) = 46 chromosomes in humans.

  • Reduction to haploid is necessary so that when egg and sperm unite, the resulting zygote has the correct diploid number.

• Process Overview: Meiosis involves one round of replication followed by two rounds of division (Meiosis I and Meiosis II). The starting cell is a diploid germ cell.

Meiosis I

• Prophase I:

  • Synapsis: Homologous chromosomes pair up to form a tetrad.

  • Crossing Over: Homologous chromosomes exchange segments of DNA at points called chiasmata.

  • Recombinant Chromosomes: This exchange results in recombinant chromosomes, which increases genetic variation in offspring.

  • Gene Linkage: Two genes located close together on the same chromosome tend to be inherited together. Crossing over can separate these linked genes, promoting diversity.

• Metaphase I:

  • Homologous pairs (not individual chromosomes) line up at the metaphase plate.

  • Orientation is random, further adding to genetic variation.

• Anaphase I:

  • Homologous Chromosomes are pulled apart to opposite poles (Disjunction).

  • Note: Unlike mitosis, sister chromatids remain attached at the centromere here.

• Telophase I & Cytokinesis:

  • Nuclear membranes form and the cell splits into two cells.

  • Result: Each cell is now haploid ($n$), containing one chromosome from each original pair.

Meiosis II

• Prophase II: Spindle fibers reform in both haploid cells.

• Metaphase II: Individual chromosomes (composed of sister chromatids) line up at the center of each cell.

• Anaphase II: Sister chromatids are finally pulled apart and dragged to opposite poles (similar to mitotic anaphase).

• Telophase II & Cytokinesis:

  • Nuclear membranes reform and cells divide again.

  • Final Result: Four genetically unique haploid daughter cells.

  • Sex Differences: In males, all four become functional sperm. In females, only one becomes a functioning ovum, while the other three become non-functional polar bodies.

Nondisjunction and Comparison

• Nondisjunction: Failure of chromosomes to separate properly during Meiosis.

  • Anaphase I: Homologous chromosomes don't separate; one cell receives both, the other receives none.

  • Anaphase II: Sister chromatids don't separate; one cell gets an extra chromatid, the other gets none.

  • Consequences: Results in gametes with abnormal chromosome numbers. If fertilized, the zygote will have the wrong chromosome count.

  • Example: Down Syndrome is caused by an extra copy of Chromosome 21.

• Comparison: Mitosis vs. Meiosis:

  • Mitosis: $2n \rightarrow 2n$; somatic cells; no pairing of homologous chromosomes; no crossing over.

  • Meiosis: $2n \rightarrow n$; germ cells; homologous chromosomes pair and cross over; two rounds of division.

The Reproductive System

• Chromosomal Sex: Humans have 23 pairs of chromosomes. The 23rd pair determines sex.

  • Females: $XX$

  • Males: $XY$

• The X Chromosome: Large and carries many genes. Mutations cause X-linked disorders.

• The Y Chromosome: Small and carries the $SRY$ gene, which triggers the development of testes and male characteristics.

• Hemizygosity: Males are hemizygous for X-linked genes because they only have one X. They will express any recessive mutation on the X chromosome since there is no second X to "cover it up."

Male Reproductive Anatomy

• Testes: Housed in the scrotum; responsible for producing sperm and secreting testosterone.

• Seminiferous Tubules: The "sperm factory" where long-structure sperm production occurs.

  • Sertoli Cells: Provide structural and nutritional support to developing sperm.

• Leydig Cells: Located outside the tubules; secrete testosterone and other androgens.

• Scrotum: External sac that regulates temperature; sperm development requires a temperature lower than normal body temperature.

• Epididymis: A coiled tube where sperm mature, are stored, and gain motility (ability to swim).

• Vas Deferens: A muscular tube transporting sperm from the epididymis to the ejaculatory duct.

• Ejaculatory Duct: Formed by the junction of the vas deferens and the seminal vesicle duct; carries sperm into the urethra.

• Urethra: Carries both semen and urine through the penis and out of the body.

• Accessory Glands:

  • Seminal Vesicle: Produces fluid rich in fructose for sperm energy.

  • Bulbourethral (Cowper's) Gland: Secretes clear mucus to lubricate the urethra and flush out leftover urine.

  • Prostate Gland: Produces fluid to protect sperm from the acidic vaginal environment.

• Sperm Pathway Mnemonic: SEVE(N) UP:

  1. Seminiferous tubules

  2. Epididymis

  3. Vas deferens

  4. Ejaculatory Duct

  5. (Nothing)

  6. Urethra

  7. Penis

Spermatogenesis and Sperm Structure

• Developmental Sequence:

  1. Spermatogonium: Diploid stem cell ($2n$) undergoing mitosis.

  2. Primary (1°) Spermatocyte: Daughter cell that enters Meiosis I.

  3. Secondary (2°) Spermatocyte: Result of Meiosis I; haploid cells that enter Meiosis II.

  4. Spermatid: Result of Meiosis II (4 haploid cells).

  5. Spermatozoan: Fully formed, mature sperm.

• Sperm Structure:

  • Head: Contains the nucleus (DNA).

  • Acrosome: A cap filled with enzymes to penetrate the egg.

  • Midpiece: Packed with mitochondria to produce ATP for movement.

  • Tail: A flagellum used for swimming.

Female Reproductive Anatomy

• Ovaries: Gonads that produce ova (eggs) and secrete estrogen and progesterone.

  • Follicles: Thousands of fluid-filled sacs that protect and nourish immature oocytes.

• Ovulation: Process where a mature egg is released into the peritoneal sac and then drawn into the Fallopian tube.

• Fallopian Tube (Oviduct): Lined with an infundibulum and fimbriae to sweep the egg inward; this is the primary site of fertilization.

• Uterus: Muscular organ for fetal development.

  • Endometrium: Inner lining that thickens each cycle; sheds during menstruation if no fertilization occurs.

  • Cervix: The lower part of the uterus that opens into the vagina; it softens/opens during ovulation and dilates during childbirth.

• Vagina: Muscular tube for menstrual flow, intercourse, and the birth canal.

• Vulva: External anatomy including labia majora and minora.

• Greater Vestibular (Bartholin's) Gland: Secretes lubricating fluid.

Oogenesis

• Difference from Sperm: Females produce a finite number of eggs during fetal development.

• Developmental Sequence:

  1. Oogonium: Diploid stem cells formed during fetal development.

  2. Primary (1°) Oocyte: Cells paused in Prophase I until puberty.

  3. Secondary (2°) Oocyte: Formed after Meiosis I (at ovulation), paused in Metaphase II. Completion of Meiosis II only occurs upon fertilization.

  4. Ovum: The mature egg produced after fertilization, along with a polar body.

• Polar Bodies: Small cells with a nucleus but very little cytoplasm; they are non-functional and serve to discard extra sets of chromosomes.

• Protective Layers of Oocytes:

  • Zona Pellucida: Directly around the plasma membrane; involved in sperm binding.

  • Corona Radiata: Outer layer for nourishment and communication with the follicle.

Hormonal Regulation and the Menstrual Cycle

• Hormonal Axis: Hypothalamus releases GnRH (Gonadotropin-Releasing Hormone) → Pituitary gland releases FSH (Follicle-stimulating hormone) and LH (Luteinizing hormone).

• Hormones in Males:

  • FSH: Stimulates Sertoli cells and drives spermatogenesis.

  • LH: Acts on Leydig cells to produce testosterone.

• Hormones in Females:

  • FSH: Stimulates development of follicles.

  • LH: Triggers ovulation.

• Menstrual Cycle (Approx. 28 days):

  1. Follicular Phase: FSH facilitates ovum maturation.

  2. Ovulation: Marked by a peak in LH around Day 14; egg is released.

  3. Luteal Phase: Progesterone and estrogen peak; endometrium reaches maximum thickness.

  4. Menstruation: If no fertilization, the endometrial lining is shed, marking the start of a new cycle.

• Menopause: With age, ovaries become less sensitive to FSH and LH, leading to ovarian atrophy and the cessation of the menstrual cycle.