Chapter 3 – Part 2– Cell Cycle Interphase G1 (gap 1) – metabolic activity and vigorous growth G0 – cells that cease dividing S (synthetic) – DNA replication G2 (gap 2) – preparation for division Mitotic phase Mitosis and cytokinesis

Interphase – G0 G0 represents not simply the absence of signals for mitosis but an active repression of the genes needed for mitosis. Cancer cells cannot enter G0 and are destined to repeat the cell cycle indefinitely Most cells will leave the cell cycle, temporarily. They remain in G1 until properly stimulated to begin dividing again. Often G0 cells are terminally differentiated: they will never reenter the cell cycle but remain in G1 until they die. (muscle and nerve cells)

Interphase – S phase (DNA replication) _________untwists the double helix and exposes complementary strands Helicase Each nucleotide strand serves as a template for building a new ______________ strand complementary RNA primers _____________are added to begin DNA synthesis DNA polymerase III __________________continues from the primer and covalently adds complementary __________ to the template nucleotides Since DNA polymerase only works in one direction: 5` to 3` A continuous leading strand is synthesized A discontinuous lagging strand is synthesized __________ splices together the short segments of the discontinuous strand DNA ligase PLAY

Mitosis – Nuclear Division Essential for body growth and tissue repair The phases of mitosis are: Prophase Anaphase Metaphase Telophase Cytokinesis – division of the cytoplasm Cleavage furrow formed in late anaphase by contractile ring Cytoplasm is pinched into two parts after mitosis ends PLAY

Early and Late Prophase Asters are seen as chromatin condenses into chromosomes Nucleoli disappear Centriole pairs separate and the mitotic spindle is formed PLAY PLAY

Metaphase Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate PLAY

Anaphase Centromeres of the chromosomes split Motor proteins in kinetochores pull chromosomes toward poles PLAY

Telophase and Cytokinesis New sets of chromosomes extend into chromatin New nuclear membrane is formed from the rough ER Nucleoli reappear Generally cytokinesis completes cell division PLAY PLAY

Control of Cell Division Surface-to-volume ratio of cells Contact inhibition Chemical signals such as growth factors and hormones Cyclins and cyclin-dependent kinases (Cdks) complexes

Cell destruction – death Necrosis "cell homicide" -Inflammation with secondary injury to surrounding normal tissues.. Apoptosis "cell suicide". -No Inflammation or secondary tissue injury

Control of Cell Reproduction Most cells divide an average of about 50 times Mitotic clock -tips of chromosomes (telomeres) about 12,000 nucleotides 25-200 nucleotides lost with each division When repeats are gone cell quits dividing. Cell Differentiation - when cells develop different structures and specialized functions. Stem cell can divide to form a progenitor cell Click screen to add animated notes. totipotent – can give rise to any cell type pluripotent – can give almost any cell type in the body multipotent – can differentiate into a limited range of cells

Loss of Cell Division Control - Cancer Division to frequent dysplasia neoplasmtumor benign -->lump in place malignant - lump extends into surrounding tissue resembling a crab -->metastasis Genes that cause cancer oncogenes activate other genes that increase cell division tumor suppressor gene – normally regulate mitosis; if inactivated they will not regulate mitosis Environmental causes? Cancer treatments are directed at procedures that kill fast dividing cells. What normal cells do they also effect?

Protein Synthesis Each triplet specifies coding for an amino acid DNA serves as master blueprint for protein synthesis Genes are segments of DNA carrying instructions for a polypeptide chain Triplets of nucleotide bases form the genetic library Each triplet specifies coding for an amino acid

Transcription Transfer of information from the sense strand of DNA to RNA Transcription factor Loosens histones from DNA in the area to be transcribed Binds to promoter, a DNA sequence specifying the start site of RNA synthesis Mediates the binding of RNA polymerase to promoter DNA triplets are transcribed into mRNA codons by RNA polymerase

Translation to Polypetide Roles of the Three Types of RNA Form Initiation complex Messenger RNA (mRNA) carries the genetic information from DNA in the nucleus to the ribosomes in the cytoplasm Transfer RNAs (tRNAs) bound to amino acids base pair with the codons of mRNA at the ribosome to begin the process of protein synthesis Ribosomal RNA (rRNA) is a structural component of ribosomes

Information Transfer from DNA to RNA Return

Centrioles Small barrel-shaped organelles located in the centrosome near the nucleus Pinwheel array of nine triplets of microtubules Form the bases of cilia and flagella Organize mitotic spindle during mitosis kinetochore Return

Back