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Cell Reproduction Chapter 8 Topics: Chromosomes Cell Division Meiosis DNA: deoxyribonucleic acid Found in nucleus of eukaryotes, DNA loop in prokaryotes Codes for proteins, cell functions DNA + proteins = chromosome Chromosomes composed of genes; gene is a specific segment of DNA coding for one particular protein The protein(s) coded for determines an organism’s characteristics DNA structure is a coiled double helix; has associated proteins – Histones – proteins that help maintain the coiled structure – Non-histones – proteins that control DNA activities In a non-dividing cell, DNA is diffuse & difficult to see; called chromatin In a dividing cell, DNA becomes condensed into chromosomes, much easier to see; called a chromosome DNA DNA Double helix showing base pairs DNA with associated proteins Chromosome Chromosome – tightly coiled DNA + proteins, seen during cell division Consist of 2 identical halves called chromatids Chromotids are connected by a Genes located on chromosome centromere – What’s a gene again? Chromosome Number The number of chromosomes vary from species to species; no correlation between chromosome number and species sophistication Some examples: (page 145 text) – Human 46 Cat 32 Dog 78 – Lettuce 18 Gorilla 48 Fern 1,262 Animal chromosomes divided into autosomes or sex chromosomes Sex Chromosomes Chromosomes that determine sex of individual These chromosomes also carry genes determining other characteristics Humans: female XX; male XY Humans: have 2 sex chromosomes Autosomes All other chromosomes Carry genes determining characteristics of individual Humans have 44 autosomes Karyotype “picture of chromosomes” Female Male Chromosomes Focus on human – Has 46 chromosomes: 2 sex chromosomes & 44 autosomes or 23 pairs of chromosomes: 1 pair sex chromosomes & 22 pairs of autosomes – Each individual receives 23 chromosomes from female parent & 23 chromosomes from male parent – The 23 chromosomes from mom match the 23 chromosomes from dad; called homologues or homologous pairs – The genes on each chromosome code for the same traits Each homologous pair contain gene sequences coding for the same traits – Look at karyotypes of normal individuals on prior slide Karyotype – term for pictomicrograph of chromosomes Diploid v. Haploid Diploid – description of cells having 2 sets of chromosomes – Human: all body cells are diploid – Abbreviated as “2n” – Human diploid number is 46 Haploid – description of cells having 1 set of chromosomes – Humans: only sex cells are haploid (egg, sperm) – Abbreviated as “1n” – Human haploid number is 23 Prokaryotes Prokaryotic chromosomes consist of a circular DNA molecule with associated proteins Most prokaryotes have only 1 chromosome Chromosome DNA loop attached to cell membrane Prokaryotic DNA is haploid Prokaryotic Cell Division Called Binary Fission Steps in process – DNA loop copies itself; so cell has 2 copies – Cell continues to grow, making copies of cellular contents – When cell reaches approx 2X original size, a cell wall forms between 2 copies of DNA – As wall forms, it divides cell into 2 equal halves – Called daughter cells, each identical to other Eukaryotic Cell Division Two types of cell division in eukaryotes Mitosis – Results in production of daughter cells that are identical to parent cell – Results in 2 diploid daughter cells Meiosis – Results in production of daughter cells that are not identical to parent cell – Results in 4 haploid daughter cells Mitosis Occurs in body cells (somites) for growth & repair Occurs in unicellular organisms for reproduction Yields 2 diploid daughter cells that are genetically identical to parent cell Mitosis is only a small part of the life cycle of a cell Cell cycle is the repeating events that make up the life cycle of a cell Cell Cycle Interphase (time between cell divisions) – G1 phase (cell growth) – S phase (DNA copied) – G2 phase (cell growth; prep for cell division) M phase (division of cell nucleus) – – – – Prophase Metaphase Anaphase Telophase Cytokinesis (division of cell cytoplasm) Cell Cycle Interphase G1 phase – “G” stands for “gap”, referring to the gap between cell divisions – Cell grows, organelles copied, necessary proteins manufactured/stored S phase – DNA copied during this phase; if cell is to divide then each cell needs complete set of DNA G2 phase – any further cell growth occurs here, cell prepares for division Interphase Cells in interphase Onion cells & whitefish cells Prophase First stage of cell division Occurrences: – DNA coils into typical chromosome shapes, can be seen using light microscope – Remember chromosome composed of 2 chromatids connected by centromere (but the DNA amount is doubled!) – Nuclear membrane & nucleolus break down – 2 centrosomes (containing 2 each centrioles) form [Exception: plant cells have no centrioles] – Spindle fibers radiate between centrosomes attaching to chromosomes and other cell organelles – Centrosomes migrate to poles (opposite sides of cell) with the mitotic spindle arrayed between them Prophase Cells in prophase Interphase & Prophase Metaphase Second stage of cell division Occurrences: – Chromosomes migrate to equator of cell – Kinetochore – disk shaped protein found in the center of each centromere (remember the centromere connects the chromatids); kinetochore fibers connect the centromere to one of the centrosomes – Polar fibers connect centromeres of adjacent chromatids along equator – It is the kinetochore & polar fibers that help align the chromosomes along the cell’s equator Metaphase Cells in metaphase Anaphase Third step in cell division Occurrences: – The centromere of each chromatid separates – One chromatid pulled to one pole; other chromatid of that pair pulled to opposite pole – Chromatid considered a complete chromosome now – Remember the DNA amount was doubled in the S phase of Interphase, so the “chromatid” that migrated to the pole has 2n DNA amount – The mitotic spindle ensures the proper migration of DNA to poles Anaphase Cells in anaphase Telophase Fourth step in cell division Occurrences: – Spindle fibers disassemble – Chromosomes become diffuse (now called chromatin) – Nuclear membrane reforms – Nucleolus reforms Telophase Cells in telophase Telophase Cytokinesis Step where the cell physically divides Animal cell – Cleavage furrow forms where cell membrane begins to pinch together – Cell components equally divided to both daughter cells – Each daughter cell has own nucleus that formed during telophase Plant cell – Cell plate forms to separate cell contents – Each cell has own nucleus formed in telophase Cytokinesis Cells undergoing cytokinesis Meiosis Occurs in germ cells of the reproductive organs, testes or ovaries Produces 4 haploid gametes (sperm, ova) Unlike mitosis where there is one division phase, meiosis has 2 divisions – Meiosis I – Meiosis II Cells undergoing meiosis prepare by going through interphase (same events occur here, mainly duplication of chromosomes) Meiosis I Prophase I – Cell enters with duplicated chromosomes – Events of prophase I are same as prophase of mitosis except that synapsis occurs – Synapsis – lining up of homologues; so the chromatids of homologous chromosomes are lined up next to each other; these paired homologues called tetrads – Important because this aligns corresponding genes on the chromatids, the chromatids touch and genetic material is exchanged – Crossing-over is the term for this exchange of genetic material between homologous chromosomes – This results in genetic recombination Meiosis I, con’t Metaphase I – Tetrads line up randomly along equator of cell, just as the chromatids did in mitosis metaphase – Spindle fibers form, extend from centrioles to centromeres of each tetrad Anaphase I – Tetrads (homologous pairs) pulled apart; one to one pole, one to the other – Because they are randomly aligned along equator, the homologue pulled to a pole is random; called Independent Assortment Telophase I – Homologues reach their respective pole – Cell begins to pinch together; cleavage furrow forms Cytokinesis – Cell splits into two, both cells are haploid (1n) – Because each cell received only one of each homologous pair Meiosis I Remember this as meiosis continues… When cytokinesis occurs in meiosis I, 2 haploid cells formed, each having only one of the homologous chromosomes (but remember that the chromosome carries with it a copy) The nuclear membrane does NOT reform and cells does NOT begin another interphase Cells immediately go into prophase II Meiosis II Prophase II – Spindle fibers form and move chromosomes to equator of each cell Metaphase II – Chromosomes aligned along equator of each cell Anaphase II – Chromatids now separate and are pulled to one of the poles Telophase II – Nuclear membrane reforms Cytokinesis – Cleavage furrow forms and each cell splits into two – Yield 4 haploid cells Meiosis II Gametogenesis In females, meiosis occurs in ovaries – Process called oogenesis – Yields 4 haploid cells: 1 egg and 3 polar bodies In males, meiosis occurs in testes – Process called spermatogenesis – Yields 4 haploid cells: 4 spermatids that mature into 4 sperm cells Spermatogenesis Begins with the onset of puberty and release of male sex hormone, testosterone Continues throughout the lifetime of the individual It takes only a few weeks for each gamete to reach maturity and become a sperm – So, every sperm in a male is only a few weeks old Occurs in the testes Each mature sperm is haploid, 1n Oogenesis Begins during gestation in the fetal ovaries, but stops at Prophase I; cells are primary oocytes So female infant is born with ovaries containing primary oocytes that are in “arrested” development awaiting “activation” Meiosis does not restart until puberty about 10 – 14 years later With each 28 day cycle (mensus), only one primary oocyte matures to become 1 egg (1n) and 3 polar bodies The egg receives all the cellular contents making it quite large; the polar bodies are “discarded” cells which die and are resorbed by the body Note: females are born with a finite number of primary oocytes which formed during gestation; this makes the egg as old as the woman; older women more likely to produce ova with mutations Fertilization Fertilization – union of egg and sperm Occurs in the fallopian tube of female Fertilized egg – zygote; is diploid or 2n Zygote implants into uterine lining; which became engorged with blood during the 28 day cycle – If zygote implants, the ovaries & other endocrine glands produce hormones to maintain the pregnancy – In human, gestation is ~9 months No zygote, uterine lining sloughed off as menstrual flow