Chapter 19- Cell Division

Anaphase

*Centromeres divide and sister chromatids move toward opposite poles of the spindle

1.Prophase

*Centrosomes have duplicated and move toward opposite ends of the nucleus *Spindle fibers appear *The nuclear envelope begins to fragment; nucleolus disappears *The chromosomes condense (are now visible and distinct) *Each is composed of two sister chromatids held together at a centromere

Cell Cycle Control

*Checkpoints delay the cell cycle until certain conditions are met 1.G1 checkpoint 2.G2 checkpoint 3.Mitotic checkpoint

Metaphase

*Chromosomes align at the metaphase plate equidistant from the spindle poles

Telophase

*Chromosomes near the pole and become indistinct chromatin again *The spindle disappears *The nuclear envelope *The nucleolus reappears

Mitosis

*Creates identical diploid daughter cells *Occurs in all dividing cells other than during the formation of gametes

Turner Syndrome (XO)

*Female, has a single X chromosome *Short stature, with a broad chest *Ovaries, uterine tubes, and uterus are very small and underdeveloped; they do not undergo puberty or menstruate, and their breasts do not fully develop *Normal intelligence can lead to fairly normal lives if they receive hormone supplements

Refresher on chromosome nomenclature

*Homologous chromosomes (same genes, different alleles) (one originally from mom and the other one from dad) *Non-homologous chromosomes (different genes) *Sister chromatids (same genes and alleles held together at centromere) (result from DNA replication in S phase of cell cycle)

Meiosis I Highlights

*Homologous chromosomes pair=synapsis *Homologous chromosomes exchange genetic material=crossing over or recombination *Alignment of homologous chromosomes is random=independent assortment *Homologous chromosomes separate while sister chromatids remain attached *DNA does not replicate after meiosis I; cells are haploid after cytokinesis

Chromosomes

*Humans have 46 chromosomes, in 23 pairs -22 pairs=autosomes *Numbered 1-22 (from largest to smallest) -One pair= sex chromosomes (typically differ between males and females) -XX (female) and XY (male) *Organisms with a pair of chromosomes are diploid

Sex chromosomes and X-inactivation

*Humans need 1 and only 1 active X chromosome *In XX females, one X chromosome is inactivated early in development -Inactivated X= Barr body *In XY males the single X is not inactivated] *In XY males the single X is not inactivated] *In 45m XO (Turner female), the single X is not inactivated (1 Barr body)

Significance of Meiosis

*Keeps chromosome number constant from generation to generation *Results in genetic variation

Klinefelter Syndrome (47, XXY)

*Male, with two X chromosomes and one Y chromosome *May have subtle speech and language delays *Slightly feminized physique *Required assisted reproduction to father children *Receive testosterone supplements beginning at puberty

Chromosome Number

*Most people have 46 chromosomes (2n=46) -22 pairs of autosomes -1 pair of sex chromosomes are born with too few or too many chromosomes *Caused by nondisjunction during oogenesis or spermatogenesis -Trisomy: one type of chromosome is present in three copies (2n+1) [in humans=45 chromosomes]

Monosomy and trisomy examples in humans

*No viable autosomal monosomies *Trisomy 21 (Down Syndrome) *Several sex chromosome examples

Meiosis I Stages

*Prophase I (synapsis and crossing over) *Metaphase I (homologous chromosomes randomly align) *Anaphase I (homologous chromosomes separate) *Telophase I (homologous chromosomes move to the poles)

Meiosis highlights

*Reduction division- parent cell is diploid (2n) and produces haploid (1n) daughter cells *Has two consecutive cell divisions without an interphase in between -Meiosis I and Meiosis II *Occurs only in testes or ovaries to produce gametes (eggs or sperm) *Produces genetic variation *Daughter cells are different from the parent cell and from each other

Meiosis is different in males and females

*Spermatogenesis produces sperm in males -in testes -continual starting at puberty -300,000 sperm/minute (over 400 million per day) *4 daughter cells becomes sperm *Oogenesis produces eggs in females -in ovary -discontinuous -starts in fetal development, arrest at Prophase I -resumes meiosis at puberty (ovulation) -completes meiosis only if fertilized -uneven cell divisions -1 egg (ovum) created

2.Prometaphase

*The spindle fibers attach to the centromeres as the chromosomes continue to shorten and thicken *Chromosomes are randomly placed in the nucleus

Cell Cycle Stages

1.Interphase (longest phase) *Substages: G1, S, G2 *Cell growth, organelles carry out functions, cell is active *DNA replicates (S phase) 2.Cell division or M-phase *Substages: mitosis and cytokinesis *Chromosomes split *Cytoplasm divides

Phases of Mitosis

1.Prophase 2.Prometaphase 3.Metaphase 4.Anaphase 5.Telophase

Meiosis II

Anaphase II: daughter chromosomes move towards the poles Telophase II: spindle disappears, nuclei form, and cytokinesis takes place Daughter Cells: Meiosis results in four haploid daughter cells 4 stages: Prophase II, Metaphase II, Anaphase II, Telophase *During anaphase II, the centromeres divide *Sister chromatids separate and move toward opposite poles of the spindle -The daughter cells become gametes

Down Syndrome=Most Common Autosomal Trisomy

Characteristics: short stature, eyelid fold, flat face, stubby fingers, a wide gap between the first and second toes, a large, fissured tongue, a round head, a palm crease (simian line), intellectual disability

Chromatin & Chromosomes Chart

Chromatin: nature of packaging is "relaxed", it is observed in interphase, and may be active with respect to making RNA Chromosomes: nature of packaging is highly condensed, it is observed in cell division, and is inactive in respect to making RNA

Cytokinesis

Division of the cytoplasm and organelles *An indentation called a cleavage furrow passes around the circumference of the cell

A Karyotype of Human Chromosomes= display of chromosomes

Homologous chromosomes-pairs *Same shape, size, same genes in same order *BUT, may have different versions of these genes *Likely, NOT identical *Exception: X and Y chromosomes *Chromosomes and chromatin *Many proteins associate with DNA and assist in the organizational structure -Collectively, the DNA and proteins are called chromatin

External control of cell cycle

Hormones and growth factor signals can regulate cell division 1.Signal binds to a receptor in the target cell 2.Signal is relayed to proteins inside the cell (signal transduction pathway) 3.Activates gene(s) in nucleus to stimulate (or inhibit) cell cycle 4.Cell responds