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The Blueprint of Life, From DNA to Protein


The Blueprint of Life, From DNA to Protein. Chapter 7. 7.1 Overview. Genome - complete set of genetic information of a species ... – PowerPoint PPT presentation

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Title: The Blueprint of Life, From DNA to Protein

The Blueprint of Life, From DNA to Protein
  • Chapter 7

7.1 Overview
  • Genome - complete set of genetic information of a
  • Gene - the functional unit of the genome
  • Transmission of heritable information
  • Encodes polypeptides or functional RNA

Characteristics of DNA
  • The central dogma of molecular biology

Characteristics of DNA
  • Deoxyribose-phosphate backbone
  • Nitrogenous bases
  • Double-stranded (complementary)
  • Hydrogen bonds between bases of strands
  • Base pairing
  • AT (two H bonds)
  • GC (three H bonds)
  • Polarity (5 to 3)
  • Strands are anti-parallel

Characteristics of DNA
Characteristics of RNA
  • Ribose-phosphate backbone
  • Usually single stranded
  • Can form intrachain base pairs
  • Can fold into structures with catalytic activity
  • U replaces T
  • RNA is transcribed from DNA
  • A complementary copy
  • Termed transcript

Characteristics of RNA
  • RNA Transcripts
  • Messenger RNA (mRNA) encodes proteins
  • mRNA is read in groups of 3 nucleotides, termed a
    codon, which determines amino acid composition of
    a polypeptide
  • Ribosomal RNA (rRNA) fold into 3D structures with
    catalytic activities
  • Transfer RNA (tRNA) binds to and specifies amino
    acids for translation

Regulating Gene Expression
  • Cells have the ability to regulate the magnitude
    of mRNA synthesis
  • This regulation is mediated by a large group of
    proteins that interact with the DNA to fine-tune
    expression of genes
  • These proteins act as a dimmer switch more than
    an on/off switch
  • Their activities are largely controlled by
    environmental influences

7.2 DNA Replication
  • Bacteria have circular chromosomes of DNA
  • Prior to cell division, the chromosome must be
    replicated (aka - duplicated)
  • This process is bidirectional due to the
    anti-parallel nature of double-stranded DNA
  • The replication is semiconservative because each
    strand acts as a template for the synthesis
  • After replication, each chromosome is composed of
    one template (parental) strand and one new strand

DNA Replication
  • Steps
  • DNA strands separate (denature) and unwind
    because of the activities of helicases
  • RNA primer hybridizes to the template DNA at
    origin of replication sites
  • Primers are short single-stranded
    oligonucleotides no more than a few dozen bases
    in length that are synthesized by the enzyme
  • Hybridization occurs by complementary base pairing

  • Steps (cont.)
  • DNA polymerases bind to the primer-template and
    inserts a new base that is complementary to the
    template base
  • The new bases are deoxynucleotide triphosphates
    (dNTP) and their incorporation results in the
    loss of the ß and ? phosphates
  • The a phosphate becomes the phosphate backbone

DNA Replication
  • Steps (cont.)
  • The template strand is read 3 to 5
  • The new strand is synthesized 5 to 3
  • To accommodate winding tension, DNA gyrases relax
    the coiled DNA by breaking it ahead of the DNA
  • This process of replication results in a
    replication fork where the parental strands are
    separating and the new strands are being

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DNA Replication
  • Okazaki fragments
  • Although one strand can be synthesized
    continuously, the other strand cannot
  • These strands are referred to as the leading
    strand and lagging strand, respectively
  • The lagging strand is synthesized in fragments of
    several hundred nucleotides in length
  • These fragments are termed Okazaki fragments

DNA Replication
DNA Synthesis
DNA Replication
Nucleotide Incorporation
7.3 Gene Expression
  • Transcription
  • The plus strand encodes the genetic information
  • The minus strand is read by the RNA polymerase to
    make RNA
  • This is because nucleic acids are complementary

  • Upstream (i.e, 5) of all genes is the promoter
  • The promoter contains sequences of DNA that are
    recognized by regulatory proteins, termed
    transcription factors
  • These are the dimmer switch sites for controlling
    the magnitude of gene expression

  • In prokaryotes, an mRNA molecule can encode more
    than one polypeptide
  • monocistronic - encodes one polypeptide
  • polycistronic - encodes two or more polypeptides

  • Initiation
  • The sigma factor is a subunit of RNA polymerase
    that recognizes promoters
  • After RNA pol binds to the promoter, sigma
    dissociates, leaving the RNA pol core enzyme
  • Elongation
  • RNA pol reads the minus strand of the gene and
    synthesizes the mRNA 5 to 3
  • At the 3 end of the gene is a termination
    sequence that displaces the RNA polymerase from
    the gene, thus ending transcription

  • Translation is the process of reading the mRNA
    and synthesizing a polypeptide
  • Each mRNA has a short 5 untranslated region
    (UTR), a coding region, and a long 3 UTR
  • The coding region is read three nucleotides at a
    time (codons) to determine the amino acids and
    their order
  • The UTRs contain regulatory elements that control
    several aspects of mRNA activities, including its

  • The Genetic Code
  • 20 amino acids must be encoded by a 4-letter
    alphabet (A, U, G, C)
  • The minimum word size for 20 amino acids is 3
    letters (i.e., 4216 4364)
  • AUG is the universal start codon
  • There are 3 stop codons
  • The code is degenerate because more than one
    codon can encode some amino acids

  • Ribosomes are the sites of polypeptide synthesis
  • Ribosomes are composed of RNAs and proteins
  • They assemble from two large complexes with the
    mRNA to form a single complex
  • The Shine-Delgarno sequence at 5 UTR assures
    proper alignment of the mRNA on the ribosome
  • This allows the ribosome to find the AUG start

  • Transfer RNA molecules contain two important
  • An anti-codon that is complementary to the codon
  • An amino acid that is specific for the codon

  • The anticodon of the tRNAiMet aligns with start
  • The second tRNA aligns with the second codon
  • A ribozyme (catalytic RNA molecule) termed
    peptidyl transferase forms a peptide (covalent)
    bond between the COOH of the first amino acid and
    the NH2 of the second amino acid
  • The first amino acid (Met) is released from its
    tRNA and the tRNA is ejected from the ribosome
  • The ribosome moves to the next codon and repeats
    the elongation
  • When a stop codon is encountered, translation is
    terminated and the polypeptide is released from
    the ribosome

7.4 Differences Between Eukaryotic and
Prokaryotic Gene Expression
7.5 Prokaryotic Gene Regulation
  • Some genes are constitutively expressed
  • Most are modulated
  • Repressors bind to the DNA between the promoter
    and transcriptional start site, thus block RNA
  • Activators interact with the promoter region to
    facilitate RNA pol binding

The lac Operon
  • The lac operon is a sensing pathway for lactose
  • The presence of lactose and absence of glucose in
    culture media stimulates the expression of genes
    that metabolize lactose
  • As glucose levels drop, cyclic AMP increases
  • cAMP binds to catabolite activating protein
    (CAP), allowing it to bind to the promoter
  • Allolactose, an isomer of lactose, binds to the
    lac repressor, preventing from binding to the DNA
  • RNA pol binds to the DNA and begins mRNA synthesis

The lac Operon
7.6 Sensing and Responding to Environmental
  • Microbes must adjust to environmental changes
  • This typically occurs because of the presence of
    specific macromolecular receptors on the surface
  • Ligation to these receptors induces a series of
    signal transduction events inside the cell that
    results in a change in gene expression profile
  • The expressed genes play a role in managing the
    environmental changes

7.7 Genomics
  • A principal focus of genomics is to sequence
    entire genomes of organisms
  • Several microbial genomes have been sequenced and
    has permitted an analysis of important genes
  • The sequencing of medically-important genomes
    will permit better understanding of pathogenesis,
    antibiotic susceptibility and vaccine development

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