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Cell Biology

Cell Biology. Cell Cycle –Series of stages that takes place during cell division and forms two new daughter cells Consist of two phases Interphase – includes no. of stages Cell Division stage or Mitotic stage – Mitosis and Cytokinesis Interphase Cell cycle spent most of the time (90%)

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Cell Biology

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  1. Cell Biology

  2. Cell Cycle –Series of stages that takes place during cell division and forms two new daughter cells • Consist of two phases • Interphase – includes no. of stages • Cell Division stage or Mitotic stage – Mitosis and Cytokinesis • Interphase • Cell cycle spent most of the time (90%) • Can be divided into subphases • G1 phase • S phase • G2 phase Cell Cycle

  3. Cell Cycle • G1 Phase (First gap): • Recovery from previous division • Cell doubles its organelles, eg. mitochondria, ribosomes • Accumulates raw materials for DNA synthesis • S Phase (Synthesis): • - DNA replication (synthesis) • G2 Phase (Second gap): • Cell synthesizes proteins necessary for cell division

  4. T A A A A T A T T T T A G C C G G C C C G G G C A T T T A A A T A A T T T A T A T T A A A T T A C G C G C C G G G C C G (a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. (c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand. (d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand. (b) The first step in replication is separation of the two DNA strands. • In DNA replication, two new strands of DNA are made • The parent molecule unwinds • Each strand of parent molecule acts as a template for building a new strand in replication • Two new daughter strands are built based on base-pairing rules DNA Replication

  5. DNA Replication • DNA replication is semiconservative • Each of the two new daughter molecules will have one old strand, derived from the parent molecule, and one newly made strand

  6. DNA Replication • DNA`s sugar-phosphate backbone • runs in opposite direction • Each strand has 3` (three –prime) • end and 5` end • Prime no. refers to the carbon • atoms of nucleotide sugar • At one end of DNA strand , sugar`s • 3` carbon atom is attached to an • – OH group • At other end, the sugar`s carbon • has a phosphate group

  7. DNA Replication • Enzyme helicases unwind the double helix • DNA polymerases add nucleotides – help of RNA Primer • RNA polymerase adds short amount of RNA – RNA Primer • That is complementary to parent strand of DNA • Then DNA polymerase add nucleotide to the 5`3end of a growing strand and synthesize complimentary strand • Leading strand • For formation of other strand of DNA - Lagging strand • DNA polymerase work away from the replication fork Helicase   Primase

  8. The lagging strand • Is synthesized as a series of small segments (100 to 200 nucleotides) called Okazaki fragments • Only one primer is needed for synthesis of the leading strand • But for synthesis of the lagging strand, each Okazaki • fragment must be primed separately • DNA polymerase removes old RNA primer – replaces • with DNA adjacent to the • new Okazaki segment • Gaps between them are • joined together by DNA ligase • Two identical DNA molecules • are produced DNA Replication Helicase   Primase

  9. Cell Cycle • M (Mitotic Stage): • Mitosis • Cytokinesis • Mitosis: • Division of the nucleus into two daughter nuclei • Has same amount and type of DNA as parent or original nucleus • Cytokinesis: • - Cytoplasm division • - Results in two genetically identical daughter cells

  10. Mitosis • Mitosis • nuclear division of a cell • produces 2 identical nuclei • Each nucleus has the same amount and type of DNA as the original nucleus • Is preceded by Interphase • 4 stages: • Prophase • Metaphase • Anaphase • Telophase

  11. Interphase • Interphase • Time between cell division • period of cell growth • chromosomes not clearly visible in nucleus • Found as thin threads of chromatin in the nucleus • cell may contain pair of centrioles

  12. Mitosis • Prophase • chromatin in nucleus condenses into chromosomes, becomes visible • Each chromosome consists of two chromatids joined at centromere • nucleolus disappears • centrioles begin moving to opposite ends of cell • fibers extend from centrosomes • some fibers cross cell to form mitotic spindle • In late prophase, nuclear membrane dissolves • chromosomes begin moving

  13. Mitosis • Metaphase • Centrosomes are at opposite end of the cell • spindle fibers align chromosomes along middle of nucleus (metaphase plate) • Spindle fibers are attached to kinetochores in the centromere of each chromosome • Anaphase • Chromosomes separate at kinetochores • separated chromatids move to opposite sides of cell

  14. Telophase • chromatids arrive at opposite poles of cell • Nuclear envelops around the chromosomes, nucleoli reappear • chromosomes disperse, no longer visible • spindle fibers diappear

  15. Cytokinesis • Typically occurs during telophase. • Is the division of the cytoplasm • forming a cleavage furrow or an indention of plasma membrane • mother cell becomes two daughter cell • each daughter cell has one nucleus

  16. Meiosis • Sperm cell development and oocyte development • (gamete formation) involve meiosis • this cell division occurs only in testes and ovaries • consists of two nuclear division • Four daughter cells are produced • Each has half no. of chromosomes as parent cell

  17. Chromosomes in Human Cells • In humans • Each somatic cell has 46 chromosomes • called diploid no. or 2n (2n=46) • made up of two pairs of 23 chromosomes – homologous pair • One set of chromosomes comes from each parent • Gametes • Gametes (sperm and oocyte cells) contain only one set of chromosomes • called haploid cells (n) • For humans, haploid no. is 23 (n=23) • Fertilized egg (zygote) • Fusion of haploid gametes in fertilization forms a diploid zygote • reestablishes 2n no. of chromosomes

  18. Chromosomes in Human Cells • Set of 23 chromosomes consists of • - 22 autosomes • - 1 sex chromosome • In oocyte cell sex chromosome is – X chromosome • In sperm cell sex chromosome is – X or Y chromosome • Meiosis: • cell division leading to production of gametes (sperm & oocyte) • Meiosis reduces the number of chromosome sets from diploid to haploid Human Karyotype

  19. Like mitosis,Meiosis is preceded by chromosome duplication • Takes place in two sets of divisions, meiosis I and meiosis II • End with 4 haploid daughter cells instead of • 2 diploid daughter cells like in mitosis • Meiosis I • Reduces the number of chromosomes from diploid to haploid • Meiosis II • Produces four haploid daughter cells Meiosis

  20. Gene and Gene Expression • Gene • Gene is a segment of DNA molecule • Functional unit of heredity • Heredityis the transfer of characteristics from parent to offspring through their genes • Gene expression • Genetic information in DNA is transcribed into RNA and then translated into proteins • Production of proteins from the • information stored in DNA is called • Gene expression • Gene expression involves two steps: • Transcription • Translation

  21. Two Steps in Protein Synthesis • Transcription • Synthesis of RNA under the direction of DNA in nucleus • Information is copied from • DNA – mRNA • Translation • Is the actual synthesis of a • polypeptide (protein), which occurs under the • direction of mRNA in cytoplasm • Occurs on ribosomes Transcription Translation DNA RNA Protein

  22. Transcription • Transcription • transfer of genetic information from DNA into RNA • RNA: uses U instead of T • And ribose instead of deoxyribose • RNA: 3 types • mRNA = messenger RNA • Has protein-building instructions • rRNA = ribosomal RNA • Major component of ribosomes • tRNA = transfer RNA • Delivers amino acid to ribosome uracil (base) phosphate group sugar (ribose)

  23. Transcription • During transcription, a segment of DNA serves as template for RNA production • RNA Polymerase binds to Promotor (DNA nucleotide sequence) with the help of proteins – Transcription factors and unwind DNA strand • Complementary RNA molecule synthesizesfrom DNA by following the rules of complimentary base pairing, A with U, C with G • When RNA polymerase encounter stop signal, it detaches from DNA and release newly formed mRNA

  24. Transcription • RNA transcribed in the nucleus is modified before moving to the cytoplasm for translation • The RNA that encodes an amino acid sequence is messenger RNA (mRNA) • RNA Splicing • Non protein coding segments called introns are cut out • Remaining exons, protein coding sections are joined to form a continuous coding sequence • A 7-methyl guanosine cap and a adenine nucleotides called tail are added to the ends

  25. Genetic Code • Genetic Information • Is contained in mRNA as a sequence of three nucleotides units, or codons • And translated into amino acid sequences • Each word codes for one amino acid in a polypeptide • Nearly all organisms use exactly the same genetic code

  26. Second mRNA base U C A G U UAU UUU UCU UGU Tyr Cys Phe UAC UUC UCC UGC C U Ser UUA UCA UAA Stop Stop UGA A Leu UAG UUG UCG Stop UGG Trp G CUU CCU U CAU CGU His CUC CCC CAC CGC C C Arg Pro Leu CUA CCA CAA CGA A Gln CUG CCG CAG CGG G Third mRNA base (3 end) First mRNA base (5 end) U AUU ACU AAU AGU Asn Ser C lle AUC ACC AAC AGC A Thr A AUA ACA AAA AGA Lys Arg Met or start G AUG ACG AAG AGG U GUU GCU GAU GGU Asp C GUC GCC GAC GGC G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G The Genetic Code • 64 codons of life = GENETIC CODE • Usually more than 1 codon for same amino acid • Eg. Glutamate = GAA or GAG • All new polypeptide chains start with AUG • =methionine • =first point of mRNA transcript • UAA, UAG, UGA = STOP SIGNALS • Don’t code for amino acid • Instructions to stop adding amino acids to polypeptide chain

  27. Translation • Translation • Is the conversion from the nucleic acid language to the protein language • Messenger RNA (mRNA) • Is the first ingredient for translation • Transfer RNA (tRNA) • Acts as a molecular interpreter • Carries amino acids • Matches amino acids with codons in mRNA using anticodons

  28. Ribosomes • Are organelles that actually make polypeptides. • Are made up of two protein subunits. • Contain ribosomal RNA (rRNA) • Ribosome has three binding sites to facilitate pairing between tRNA and mRNA • The E (for exit) site • The P (for peptide) site • The A (for amino acid) site

  29. Translation • Translation is divided into three phases: • Initiation • Elongation • Termination

  30. Initiation • mRNA enters ribosome • Initiatior tRNA binds to “small unit” of ribosome • tRNA scans mRNA for “start codon” • =AUG • UAC anticodon of tRNA binds to AUG of mRNA • Forms Methionine amino acid • Small unit of ribosome combines with large unit and create functional ribosomes • Initiator tRNA occupies the P site

  31. Elongation • Codon recognition • The anticodon of an incoming tRNA pairs with the mRNA codon at A site • Peptide bond formation • The peptide bond formation takes place between amino acids • And attaches the polypeptide to the tRNA in the A site • Translocation • mRNA moves forward, and peptide-bearing tRNA moves to P site, empty tRNA exits from E site • Then at A site new codon is ready to receive another tRNA

  32. Termination • Elongation continues until the ribosome reaches a stop codon, UGA • Termination occurs at a stop codon • Termination is the final step in protein synthesis • mRNA and polypeptide chain releases from ribosome • Ribosome subunits dissociate

  33. Summary of Gene Expression

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