Cell Cycle Lecture Notes PDF

BIO 11 UNIFYING CONCEPTS OF BIOLOGY CELL CYCLE LECTURE 7 Why is cell reproduction important? Multicellular organisms depend on cell division for Development from a fertilized cell Growth Repair Cell division is...

BIO 11 UNIFYING CONCEPTS OF BIOLOGY CELL CYCLE LECTURE 7 Why is cell reproduction important? Multicellular organisms depend on cell division for Development from a fertilized cell Growth Repair Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division Cell division involves the distribution of genetic material to daughter cells Most cell division results in daughter cells with identical genetic information, DNA The exception is meiosis, a special type of division that can produce sperm and egg cells Cellular Organization of the Genetic Material All the DNA in a cell constitutes the cell’s genome A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) DNA molecules in a cell are packaged into chromosomes Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Somatic cells (non-reproductive cells) have two sets of chromosomes Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells Distribution of Chromosomes During Eukaryotic Cell Division Each duplicated chromosome has two sister chromatids (joined copies of the original chromosome), which separate during cell division The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached Phases of the Cell Cycle The cell cycle consists of Mitotic (M) phase (mitosis and cytokinesis) Interphase (cell growth and copying of chromosomes in preparation for cell division) Interphase Interphase (about 90% of the cell cycle) can be divided into subphases G1 phase (“first gap”) S phase (“synthesis”) G2 phase (“second gap”) The cell grows during all three phases, but chromosomes are duplicated only during the S phase Mitosis Mitosis is conventionally divided into five phases Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis overlaps the latter stages of mitosis The Mitotic Spindle: A Closer Look The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis The spindle includes the centrosomes, the spindle microtubules, and the asters Cytokinesis: A Closer Look In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow In plant cells, a cell plate forms during cytokinesis Binary Fission in Bacteria Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart The plasma membrane pinches inward, dividing the cell into two Molecular Control System The cell cycle appears to be driven by specific chemical signals present in the cytoplasm Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei The Cell Control System For many cells, the G1 checkpoint seems to be the most important If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a non-dividing state called the G0 phase Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) Cdks activity fluctuates during the cell cycle because it is controled by cyclins, so named because their concentrations vary with the cell cycle MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase Internal and External Signals An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide A clear example of external signals is density-dependent inhibition, in which crowded cells stop dividing Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide Cancer cells exhibit neither density- dependent inhibition nor anchorage dependence Loss of Cell Cycle Controls in Cancer Cells Cancer cells may not need growth factors to grow and divide They may make their own growth factor They may convey a growth factor’s signal without the presence of the growth factor They may have an abnormal cell cycle control system A normal cell is converted to a cancerous cell by a process called transformation If abnormal cells remain only at the original site, the lump is called a benign tumor Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form additional tumors Meiosis Offspring acquire genes from parents by inheriting chromosomes Genes are the units of heredity, and are made up of segments of DNA Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) Asexual vs Sexual Reproduction In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes A clone is a group of genetically identical individuals from the same parent In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Fertilization and Meiosis alternate in sexual life cycles A life cycle is the generation-to- generation sequence of stages in the reproductive history of an organism Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologs Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters The sex chromosomes, which determine the sex of the individual, are called X and Y Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome The remaining 22 pairs of chromosomes are called autosomes Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A diploid cell (2n) has two sets of chromosomes For humans, the diploid number is 46 (2n = 46) The Stages of Meiosis After chromosomes duplicate, two divisions follow Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes Meiosis II (equational division) sister chromatids separate The result is four haploid daughter cells with unreplicated chromosomes LINK TO THE VIDEO: https://www.youtube.com/watch?v=kQu6Yfrr6j0 A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell What happens when a sex cell does not undergo proper meiosis? NONDISJUNCTION Occurs when the chromosomes fail to separate properly as the cell divides Gametes (sex cells) will contain the wrong number of chromosomes sperm or egg cell may contain an extra chromosome (= 24) or may be missing a chromosome (= 22) Cases of Nondisjunction (in Humans) Of a sex chromosome Turner’s syndrome (monosomy for chromosome 23) Klinefelter’s syndrome (trisomy for chromosome 23) Of an autosomal chromosome Down’s syndrome (trisomy for chromosome 21) - Turner’s Syndrome Inheritance of only one X (XO) 98% spontaneously aborted Survivors are short, infertile females No functional ovaries Secondary sexual traits reduced May be treated with hormones, surgery - Klinefelter’s Syndrome XXY condition Results mainly from nondisjunction in mother (67%) Phenotype is tall males Sterile or nearly so Feminized traits (sparse facial hair, somewhat enlarged breasts) Treated with testosterone injections - Down’s Syndrome Trisomy of chromosome 21 Mental impairment and a variety of additional defects Can be detected before birth Risk of Down syndrome increases dramatically in mothers over age 35 Cases of Nondisjunction (in non-humans) Monoploidy and Polyploidy Individuals have one (n), three or more of each type of chromosome (3n, 4n) Common in flowering plants sometimes it makes larger flowers and fruits; are usually sterile and sometimes seedless Polyploidy is less common in animals (esp. mammals) but if present result in bigger animals Polyploidy is lethal for humans (99% die before birth; newborns die soon after birth) - Polyploids vs. Normal (plants) - Polyploids vs. Normal (animals) Sources of Genetic Variability INDEPENDENT ASSORTMENT For any species, the total number of combinations of chromosomes that meiosis can produce in gametes is 2n, where n is the haploid number n of humans; n=23 There are about 8 million possible chromosome combinations for humans. RANDOM FERTILIZATION During sexual reproduction, the male gamete and female gamete that fuse to produce an offspring are selected randomly from the pool of male and female gametes How many possibilities are there when a gamete from one individual unites with a gamete from another individual in human fertilization? 64 trillion (8 million x 8 million) combination of chromosomes CROSSING-OVER Crossing over is an exchange in corresponding segments between two non sister chromatids of homologous chromosomes During crossing over, homologous chromosomes are closely paired all along their lengths with precise gene-by-gene alignment END OF DISCUSSION