Chromatin

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AP 151 DNA Replication and Protein Synthesis
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Discovery of the Double Helix

• By 1900components of DNA were known
• sugar, phosphate and bases
• By 1953 x ray diffraction determined geometry of
  DNA molecule
• Nobel Prize awarded in 1962 to 3 men Watson,
  Crick and Wilkins but not to Rosalind Franklin
  who died of cancer at 37 getting the x ray data
  that provided the answers.

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DNA Structure Twisted Ladder
DNA molecule described as double helix.
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Nucleotide Structure

• DNA polymer of nucleotides
• Each nucleotide consist of
• phosphate group
• sugar
• ribose (RNA)
• deoxyribose (DNA)
• nitrogenous base
• in this picture adenine

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Nitrogenous Bases

• Purines - double ring
• guanine
• adenine
• Pyrimidines - single ring
• uracil - RNA only
• thymine - DNA only
• cytosine both
• DNA bases CTAG
• RNA bases CUAG

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Complementary Base Pairing

• Nitrogenous bases united by hydrogen bonds
• DNA base pairings
• A-T and C-G
• Law of complementary base pairing
• one strand determines base sequence of other

Segment of DNA
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DNA Function

• Code for protein synthesis
• Gene - sequence of DNA nucleotides that codes for
  one protein
• Genome - all the genes of one person
• humans have estimated 30-35,000 genes
• other 98 of DNA noncoding junk or regulatory

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DNA Replication 1
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DNA Replication 2

• Law of complimentary base pairing allows building
  of one DNA strand based on the bases in 2nd
  strand
• Steps of replication process
• DNA helicase opens short segment of helix
• replication fork is point of separation of 2
  strands
• DNA polymerase assembles new strand of DNA next
  to one of the old strands
• 2 DNA polymerase enzymes at work simultaneously

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DNA Replication 3

• Semiconservative replication
• each new DNA molecule contains one new helix and
  one conserved from parent DNA
• Additional histones made in cytoplasm
• Each DNA helix winds around histones to form
  nucleosomes
• 46 chromosomes replicated in 6-8 hours by 1000s
  of polymerase molecules

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Errors and Mutations

• Error rates of DNA polymerase
• in bacteria, 3 errors per 100,000 bases copied
• Proofreading and error correction
• a small polymerase proofreads each new DNA strand
  and makes corrections
• results in only 1 error per 1,000,000,000 bases
  copied
• Mutations - changes in DNA structure due to
  replication errors or environmental factors
• some cause no effect, some kill cell, turn it
  cancerous or cause genetic defects in future
  generations

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Cell Division

• Essential for body growth and tissue repair
• Mitosis nuclear division
• Cytokinesis division of the cytoplasm
• Occurs in most cells of the body
• Not in RBCs, neurons, cardiac muscle
• Produces 2 new cells with same number of
  chromosomes as parent cell

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Mitosis

• The phases of mitosis are
• Prophase
• Metaphase
• Anaphase
• Telophase

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Cell Cycle
Cell Cycle

• Interphase
• Growth (G1), synthesis (S), growth (G2)
• Mitotic phase
• Mitosis and cytokinesis

Figure 3.30
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Early and Late Prophase

• Asters are seen as chromatin condenses into
  chromosomes
• Nucleoli disappear
• Centriole pairs separate and the mitotic spindle
  is formed

PLAY
Prophase
PLAY
Prometaphase
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Chromatin

• Threadlike strands of DNA and histones
• Arranged in fundamental units called nucleosomes
• Form condensed, barlike bodies of chromosomes
  when the nucleus starts to divide

Figure 3.29
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Early Prophase
Pair of centrioles
Early mitotic spindle
Centromere
Aster
Chromosome, consisting of two sister chromatids
Early prophase
Figure 3.32.2
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Late Prophase
Fragments of nuclear envelope
Polar microtubules
Kinetochore
Kinetochore microtubule
Spindle pole
Late prophase
Figure 3.32.2
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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
Metaphase
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Metaphase
Metaphase plate
Spindle
Metaphase
Figure 3.32.4
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Anaphase

• Centromeres of the chromosomes split
• Motor proteins pull chromosomes toward poles

PLAY
Anaphase
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Anaphase
Daughter chromosomes
Anaphase
Figure 3.32.5
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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
Telophase
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Cytokinesis

• Cleavage furrow formed in early telophase by
  contractile ring
• Cytoplasm is pinched into two parts after mitosis
  ends

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Telophase and Cytokinesis
Nucleolus forming
Contractile ring at cleavage furrow
Nuclear envelope forming
Telophase and cytokinesis
Figure 3.32.5
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Protein Synthesis

• DNA serves as master blueprint for protein
  synthesis
• Genes are segments of DNA carrying instructions
  for a polypeptide chain
• Triplets of nucleotide bases on DNA form the
  genetic library
• Each triplet specifies coding for an amino acid
  a.a.s are the building blocks of proteins

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Genetic Code

• System that enables the 4 nucleotides (A,T,G,C)
  to code for the 20 amino acids
• Base triplet
• found on DNA molecule
• nucleotides that stand/code for 1 amino acid
• Codon
• mirror-image sequence of nucleotides found in
  mRNA (ex AUG)
• 64 possible codons (43)
• often 2-3 codons represent the same amino acid
• start codon AUG
• 3 stop codons UAG, UGA, UAA
• Remember, Uracil (U) replaces Thymine (T) in all
  RNAs

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Roles of the Three Types of RNA

• Messenger RNA (mRNA)
• carries the genetic information from DNA in the
  nucleus to the ribosomes in the cytoplasm
• Transfer RNAs (tRNAs)
• bound to specific amino acids
• base pair with the codons of mRNA at the ribosome
  to begin the process of protein synthesis
• Triplet of bases on tRNA known as an anticodon
• Ribosomal RNA (rRNA)
• is a structural component of ribosomes

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From DNA to Protein
Figure 3.34
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Genetic Code
mRNA Codons

• mRNA formed by transcription
• DNA mRNA
• mRNA codons code for amino acids according
• to a genetic code on DNA
• Different codons for the same a.a.
• Universal same for all organisms

Figure 3.36
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Transfer RNA (tRNA)

• Activation by ATP binds specific amino acid (to
  green region) and provides necessary energy to
  join amino acid to growing protein molecule
• Anticodon (see Loop 2 above) binds to
  complementary codon of mRNA aka, translation

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Information Transfer from DNA to RNA
Figure 3.39
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DNA and Peptide Formation
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Alternative Splicing of mRNA

• One gene can code for more than one protein
• Exons can be spliced together into a variety of
  different mRNAs.

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Protein Degradation

• Nonfunctional organelle proteins are degraded by
  lysosomes
• Ubiquitin attaches to soluble proteins and they
  are degraded in proteasomes

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Developmental Aspects of Cells

• All cells of the body contain the same DNA but
  develop into all the specialized cells of the
  body
• Cells in various parts of the embryo are exposed
  to different chemical signals that channel them
  into specific developmental pathways
• Genes of specific cells are turned on or off
  (i.e., by methylation of their DNA)
• Cell specialization is determined by the kind of
  proteins that are made in that cell

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Developmental Aspects of Cells

• Development of specific and distinctive features
  in cells is called cell differentiation
• Cell aging
• Wear and tear theory attributes aging to little
  chemical insults and formation of free radicals
  that have cumulative effects throughout life
• Genetic theory attributes aging to cessation of
  mitosis that is programmed into our genes