Cell Cycle PDF

CELL CYCLE  gap 2, or G2, stage: prepares to divide, Cell Cycle FAQ’s  mitosis, or M, stage: divides How many cells are lost each day?  Every day, more than 50 billion cells die in our bodies.  Abou...

CELL CYCLE  gap 2, or G2, stage: prepares to divide, Cell Cycle FAQ’s  mitosis, or M, stage: divides How many cells are lost each day?  Every day, more than 50 billion cells die in our bodies.  About 300 million cells die every minute in our bodies Do dead cells disappear?  Once the neutrophils and macrophages (the phagocytes) get to where the dead cells are, they start eating them. How many cells are produced in a day?  According to biologists Ron Sender and Ron Milo of the Weizmann Institute of Science in Israel, your body replaces around 330 billion cells per day. At that rate, your body is Why do cells divide? making over 3.8 million new cells  For the survival and growth of every second. organisms. How many cells multiply in a day?  Maintaining chromosome numbers.  In human bodies, nearly two trillion  Damaged cells are renewed. cells divide every day What cells regenerate the fastest? 3 types of Cell Division  stomach walls and intestine last 1. Prokaryotic (Bacteria) around five days before regeneration. Binary fission divides forming two  Skin cells are replaced every two to new identical cells four weeks. is the division of a single entity into What cells reproduce the slowest? two or more parts and the  nerve and muscle cells regeneration of those parts to How many cells can reproduce? separate entities resembling the  human cells can reproduce up to 50 or original. 60 times at most. What cells in the human body live the 2. Mitosis longest?  is the process by which a cell o Brain cells: Lifetime replicates its chromosomes and then o Eye lens cells: Lifetime segregates them, producing two o Egg cells: 50 years identical nuclei in preparation for cell o Heart muscle cells: 40 years division. o Intestinal cells (excluding lining): 15.9 i. Cell or organism years growth o Skeletal muscle cells: 15.1 years ii. Replacement or repair o Fat cells: 8 years of damaged cells o Hematopoietic stem cells: 5 years o Liver cells: 10-16 months o Pancreas cells: 1 year Cell Cycle- refers to the series of events that take place in a cell leading to its maturity and subsequent division The cell cycle is a four-stage process:  gap 1, or G1, stage: cell increases in size  synthesis, or S, stage: copies its DNA 3. Meiosis  a type of cell division in sexually G0 Phase reproducing organisms that reduces is a resting phase where the cell has the number of chromosomes in left the cycle and has stopped gametes (the sex cells, or egg and dividing. sperm). Cells that are completely differentiated may also enter G0. Situation: Some cells stop dividing when issues of sustainability or viability of their daughter cells arise, such as with DNA damage or degradation, a process called cellular senescence. Cellular senescence occurs when normal diploid cells lose the ability to divide, normally Interphase after about 50 cell divisions. period of growth and DNA replication S (Synthesis) Phase between cell divisions  DNA replication occurs during this Interphase is when the cell grows, and phase. the organelles double prior to the Replication of chromosomes actual splitting of the nucleus. Now two strands 93% of a cell’s life is spent in called sister interphase. chromatids joined by Interphase has three parts a centromere Growth 1 (G1) G2 (GAP 2) Phase Synthesis (S)  organelles are reproduced or Growth 2 (G2) manufactured.  Parts necessary for mitosis and cell division are made during G2, including microtubules used in the mitotic spindle. organelles double new cytoplasm forms All other structures needed for mitosis form G1 (GAP 1) Phase  During this phase, a cell undergoes rapid growth and performs its routine functions.  cell increases in size  If a cell is not dividing, the cell enters the G0 phase from this phase.   Nutrients Summary  Growth factors State Name Description  DNA damage Quiescent Resting phase A resting phase  Size. Is the cell large enough to divide? Senescent (G0) where the Cell  Nutrients. Does the cell have enough has left the energy reserves or available nutrients cycle and has to divide? stopped dividing.  Molecular signals. Is the cell receiving positive cues (such as growth factors) Interphase 1st growth Cells increase in from neighbors? phase (G1 ) size in G1. Cells perform their  DNA integrity. Is any of the DNA normal activities damaged? G2 Checkpoint (Restriction Point) Synthesis DNA replication  It prevents cells from entering mitosis phase (S) occurs during this phase when DNA is damaged, providing an opportunity for repair and stopping 2nd growth The cell will the proliferation of damaged cells. phase (G2 ) continue to grow and many  G2 checkpoint helps to maintain organelles will genomic stability. divide during  G2 checkpoint, the cell checks for: their phase  DNA damage Cell cycle checkpoints`  DNA replication completeness A checkpoint is a stage in the  If errors or damage are detected, the eukaryotic cell cycle at which the cell cell will pause at the G2 checkpoint to examines internal and external cues allow for repairs. and "decides" whether or not to move  If the damage is irreparable, the cell forward with division. may undergo apoptosis, or three most important ones are: programmed cell death. G1 Checkpoint  This self-destruction mechanism G2 Checkpoint ensures that damaged DNA is not Spindle Checkpoint passed on to daughter cells and is important in preventing cancer. Spindle Checkpoint (Restriction Point)  the cell examines whether all the sister chromatids are correctly attached to the spindle microtubules.  This checkpoint ensures that all the chromosomes are properly aligned before the cell is allowed to divide.  the cycle will not proceed until all the chromosomes are firmly attached to at least two spindle fibers from opposite poles of the cell.  How does this checkpoint work? G1 Checkpoint (Restriction Point)  If a chromosome is misplaced,  Ensures that the cell is large enough the cell will pause mitosis, to divide and that nutrients are allowing time for the spindle available to support the resulting to capture the stray daughter cells. chromosome.  At the G1 checkpoint, cells decide whether or not to proceed with division based on factors such as:  Cell size  What are Chromosomes? DNA containing cell’s genetic code Each chromosome has a matching pair -- Homologous Pair During interphase, each chromosome copies itself. Who regulates the check points in Cell Cycle? Regulated by: EUKARYOTIC CELL DIVISION Cyclin (A, B, D, E) DNA found on chromosomes located Cyclin dependent kinase (CDK) in nucleus of cell CDK1, CDK2, CDK4, CDK6 Cell cycle continuous process Tumor suppressors Cells grow pRb- protein retinoblastoma DNA replicated p53 Organelles duplicated Divide to form daughter cells 2 Main steps: 1: Mitosis (4 steps—Prophase, Metaphase, Anaphase, Telophase) Nucleus divides 2: Cytokinesis—Cytoplasm divide, forming 2 cells Each new daughter cell is genetically identical to parent cell. MITOSIS Stages: (PMAT) Prophase Metaphase Anaphase Telophase MITOSIS Process that divides cell nucleus to produce two new nuclei each with a Anaphase complete set of chromosomes 1. sister chromatids separate Continuous process 2. centromeres divide Four phases (PMAT) 3. sister chromatids move to opposite Prophase poles Metaphase Anaphase Telophase Telophase 1. chromosomes uncoil now chromatin 2. nuclear membranes reform 3. spindle disappears Prophase  Chromatin condenses into chromosomes.  Nuclear envelope disappears until nucleus is gone  centrioles migrate to opposite sides of the cells  Microtubules appears between two Cytokinesis pairs of centrioles ( spindle) -Occurs at end of Mitosis. --division of the cytoplasm to form 2 new daughter cells --organelles are divided -Daughter cells are genetically identical Cytokinesis – Plant vs Animal Cell Plant cells undergo cytokinesis by forming a cell plate between the two daughter nuclei. Animal cells undergo Metaphase cytokinesis through the 1. chromosomes line up on the equator formation of a cleavage of the cell. furrow. A ring of 2. spindles attach to centromeres. microtubules contract, pinching the cell in half. MEIOSIS Genetics Terminology: Ploidy Meiosis is a type of cell division in sexually Refers to the number of sets of chromosomes reproducing organisms that reduces the in cells. number of chromosomes in gametes (the sex Haploid – one copy of each cells, or egg and sperm) chromosome – designated as “n”, the number of History of Meiosis chromosomes in one “set” Meiosis was discovered and – gametes described for the first time in sea - are formed by the process of urchin eggs in 1876 by the German meiosis. Biologist Oscar Hertwig Diploid – two sets of It was described again in 1883, at the chromosomes (two of each level of chromosomes by the Belgian chromosome) Zoologist Edouard Van Beneden in – designated as “2n” Ascaris worms’ egg – somatic cells The term meiosis was coined by J.B - Diploid cells undergo mitosis. Farmer and J.B Moore in 1905 Diploid organisms receive one of each type of Sperm surrounding chromosome from female parent (maternal an egg chromosomes) and one of each type of chromosome from male parent (paternal chromosomes) Homologous Chromosomes Pair of chromosomes similar in shape , size, and types of genes.  Each locus (location of the gene) in same position on chrom. Humans have 23 pairs of homologues  Housefly – 6 pairs  Chicken – 39 pairs  Apple – 17 pairs  Dog – 39 pairs  Cat – 19 pairs This is a karyotype (an image of an organism’s chromosomes) Prophase I - Crossing Over Homologous Chromosomes Crossing over may occur in the tetrad: between nonsister chromatids, ends break and reattach  4. Diplotene- The crossing-over process is completed by this stage. The homologous chromosomes remain attached at the point of chiasma. Stages of Prophase 1 1. Leptotene 5. Diakinesis- The homologous chromosomes 2. Zygotene start to separate and synaptonemal complex 3. Pachytene disappears. The nuclear membrane also 4. Diplotene disappears. 5. Diakinesis 1. Leptotene- This phase is the start of prophase-I. It is marked by the condensation of the chromosomes. Metaphase I Shortest phase; paired homologues align. INDEPENDENT ASSORTMENT occurs 2. Zygotene- In this phase the homologous pairs of homologues line chromosomes start pairing up, called the up independently of synapsis. The synaptonemal complex starts other pairs’ orientation building up. This complex is required to hold toward the poles -- the homologous chromosomes at a place close random. Adds variation to each other. Bivalent chromosomes are visible at this stage. Anaphase I Homologous chromosomes separate towards the poles (Tetrads separate) Sister chromatids remain attached 3. Pachytene- In this stage, this non-sister chromatids of homologous chromosomes Telophase I exchange their parts, the process is called the Each pole now crossing over. The attachment point of the has haploid set of crossing over of the non-sister chromatids is chromosomes called chiasma. (however – still doubled). Cytokinesis occurs: two haploid daughter cells formed. Meiosis II 3 other cells are small “polar No interphase II (no more DNA bodies”, break down (extra replication) chrom. lost). Remember: Meiosis II is similar to mitosis Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs (2nd time). Four haploid daughter cells produced (chromosomes now back to single condition). gametes ~ sperm or egg; ovule or pollen grain Mitosis vs Meiosis Nondisjunction  When the tetrad (in Anaphase I) or the sister chromatids (in Anaphase II) do not separate, creating an abnormal # of chrom. to occur in the gametes.  Lethal most of the time Gamete Formation in Animals Diff. bet. male and female gametes. Male: spermatogenesis all 4 develop into sperm cells. Female: oogenesis cytokinesis in meiosis is uneven. most of cytoplasm goes into 1 of the 4 meiotic products (forms large egg cell) Trisomy 21: Down Syndrome Trisomy 21 is the most common chromosomal anomaly in humans, affecting about 5,000 babies born each year. Causes Down syndrome occurs because of the extra copy of chromosome 21, which can cause the body and brain to develop differently than a child without the syndrome. Trisomy 18 – Edward syndrome In Victoria, Edward syndrome affects about one in 1,100 pregnancies. Edward syndrome is also known as Trisomy 18, because the person has three copies of chromosome 18 instead of two. Some of the characteristics of Edward syndrome may include: physical irregularity of the kidneys, ureters, heart, lungs and diaphragm cleft lip or cleft palate small skull (microcephaly) malformations of the hands and feet – including missing thumbs, club feet and webbing between the fingers and toes (syndactyly) neural tube defect, where the spinal cord, meninges and blood vessels protrude through a gap in the vertebrae (myelomeningocele) malformations of the sex organs. Trisomy 13 – Patau syndrome In Victoria, Patau syndrome affects around one in 3,000 pregnancies. Patau syndrome is also known as Trisomy 13, because the person has three copies of chromosome 13 instead of two. Some of the characteristics of Patau syndrome may include: small skull (microcephaly) an abnormal opening in the skull malformations of part of the brain structural defects of the eyes cleft lip or cleft palate additional toes or fingers (polydactyly) congenital heart disorders, such as ventricular septal defect neural tube defect, where the spinal cord, meninges and blood vessels protrude through a gap in the vertebrae (myelomeningocele) malformations of the sex organs