Storage and use of genetic information - PowerPoint PPT Presentation

About This Presentation

Storage and use of genetic information


Translation Proteins and rRNAs ... prokaryotic differences mRNA lifetime is different Cap, tail, introns in eukaryotes Shine-Delgarno vs. Kozak sequence & cap Size of ... – PowerPoint PPT presentation

Number of Views:34
Avg rating:3.0/5.0
Slides: 31
Provided by: DGILMORE


Transcript and Presenter's Notes

Title: Storage and use of genetic information

Storage and use of genetic information
  • The genetic code
  • Three bases (in a row) specify an amino acid
  • Transcription
  • The synthesis of a mRNA, complementary to one of
    the DNA strands, containing the genetic code
  • Translation
  • Proteins and rRNAs in the ribosome along with
    tRNAs translate the genetic code into proteins.
  • Post-translational modification
  • Proteins are altered after synthesis

The Genetic Code
  • Four bases taken how many at a time? Need to code
    for 20 different amino acids.
  • Each base 1 amino acid only 4
  • Every 2 bases 1 a.a. 16 combinations, 4
  • Every 3 bases 64 combinations, enough.
  • Every 3 bases of RNA nucleotides codon
  • Each codon is complementary to 3 bases in one
    strand of DNA
  • Each codon (except for T ?U switch) is the same
    as 3 bases in the other DNA strand.

More about the Genetic Code
  • The code is
  • Unambiguous each codon specifies 1 amino acid
  • Degenerate a particular amino acid can be coded
    for by several different codons.
  • Ordered similar codons specify the same amino
  • Commaless, spaceless, and non-overlapping each
    3 bases is read one after the other.
  • Punctuated certain codons specify start and
  • Universal by viruses, both prokaryotic domains,
    and eukaryotes (except for some protozoa,

The Genetic Code-2
  • Cricks Wobble Hypothesis
  • The code is ordered
  • The first 2 positions are more important
  • When lining up with the anticodon of the tRNA,
    the third position doesnt bind as tightly, thus
    a looser match is possible.
  • Because of this flexibility, a cell doesnt need
    61 different tRNAs (one for each codon).
  • Bacteria have 30-40 different tRNAs
  • Plants, animals have up to 50.

  • Archibald Garrod (1908) determined that genes
    must code for enzymes by studying metabolic
    diseases (black urine). But what relationship?
  • Beadle and Tatum (1941) one gene one enzyme,
    there is a one to one correspondence between DNA
    and protein
  • Mutation in DNA changes nucleotide sequence,
    changes amino acid sequence in protein.
  • Not completely true in eukaryotes because of

The code is colinear with protein sequence
  • Bacteriophage MS2
  • An RNA virus with 3 genes, including a viral coat
  • Amino acid sequence of coat protein determined,
  • Sequence of coat protein gene determined, 1972
  • Correspondence between codons and amino acids
    exactly as predicted.

Ribosome structure
Large and small subunits. Eukaryotic 60S 40S
80S Prokaryotic 50S 30S 70 S
Large subunit 2 -3 rRNAs and many proteins Small
subunit 1 rRNA and many proteins
Size in Svedbergs
  • S is a unit based on ultracentrifugation
  • What affects how fast particles move toward the
    bottom of the tube?
  • Mass, density, and shape (which affects friction)
  • Example supercoiled DNA moves faster than
    relaxed DNA.
  • Because of these factors, S units are not
  • 50S 30S prokaryotic subunits make a 70S

Components of ribosomes
  • Multiple copies exist of ribosomal genes
  • Def. of gene extended to DNA that codes for
  • Multiple copies means moderately repetitive DNA
  • Cells can crank out lots or rRNA (and r-proteins)
  • Transcription results in RNA requiring processing
  • Pre-RNA cut into rRNAs (and tRNAs)
  • rRNA NOT just structural are ribozymes, carry
    out the actual protein synthesis.

About tRNAs-1
  • Coded for by tRNA genes
  • Post-transcriptional modification
  • tRNAs have some bases changed
  • tRNAs interact with rRNA during protein synthesis
  • tRNAs must have amino acid attached
  • Enzymes aminoacyl tRNA synthetases
  • 20 different enzymes, one for each aa.
  • Enzymes recognize shape of tRNA

About tRNAs-2
  • tRNA structure must be highly conserved
  • Must be folded so that both the synthetases
    recognize it, and it fits properly on the
  • Errors in translation result in bad proteins
    mutations in tRNA genes are selected against.

3D structure The familiar loops of the 2D
structure are labeled.
3 end Attaches to amino acid.
Decoder end Complementary to codon. tRNA/trna20s1.jpg
  • mRNA provides message to be translated.
  • Ribosomes functional workbench for synthesis.
  • tRNA bring aa to ribosome, decode mRNA.
  • Aminoacyl tRNA synthetases enzymes that attach
    amino acids to tRNAs.
  • Protein factors help move process along
    initiation, elongation, and termination.
  • Process is similar, but different between
    prokaryotes and eukaryotes.

Initiation and Termination of protein synthesis
  • AUG is always the first codon (initiator codon)
  • Establishes an open reading frame (ORF)
  • Ribosome begins synthesis with a methionine
  • In bacteria, it is N-formylmethionine (fMet)
  • After synthesis , either formyl group is removed
    or entire fMet is removed (Met in eukaryotes)
  • Three codons serve as termination codons
  • UGA, UAG, UAA any one can be a stop signal
  • Do NOT code for an amino acid
  • Cause translation to end protein is completed

  • Initiation
  • Small subunit, mRNA, met-tRNA, IFs, GTP
  • mRNA sequence for binding to ribosome needed
  • prokaryotes Shine-Delgarno
  • Eukaryotes Cap and Kozak sequence
  • (GCC)RCCATGG where R is a purine
  • First tRNA is fMet-tRNA in prokaryotes
  • IFs are protein Initiation Factors
  • GTP needed for energy
  • When all have come together, Large subunit added

  • Ribosome has 3 sites
  • AA site where tRNA-aa first sits in
  • P site where tRNA with growing peptide sits
  • E or Exit, site transiently occupied by used tRNA
  • Elongation, with help of EFs and GTP
  • tRNA with new aa sits in A site
  • Stays in A site if anticodon on tRNA is
    complementary to codon on mRNA.
  • tRNA in P site transfers growing chain to new aa
  • Catalyzed by rRNA
  • Ribosome moves relative to mRNA and tRNAs
  • tRNAs now in new sites, new codon lined up

Ribosome schematic
  • Termination
  • When stop codon is in A site, no tRNA binds
  • GTP-dependent release factor (protein) removes
    polypeptide from tRNA in P site. All done.
  • Ribosomal subunits typically dissociate.
  • Do a Google Search for translation animation
  • Many hits. Note presence, absence of E site
  • Note shape of ribosomes
  • Note whether role of rRNA in catalysis is shown

Nonsense mutations and suppressors
  • A mutation may change a normal codon to a stop
    codon protein synthesis ends prematurely.
    (nonsense mutation)

A second mutation can cure the original a
suppressor. If the gene for a tRNA is mutated
in the anticodon so that the stop codon is now
read by the tRNA.
Polysomes and Polycistronic mRNA
  • In eukaryotes, when mRNA enters the cytoplasm,
    many ribosomes attach to begin translation. A
    mRNA w/ many ribosomes attached polysome.
  • In eukaryotes, the mRNA for a single gene is
    processed and translated in prokaryotes, mRNA
    can be polycistronic, meaning several genes are
    on the same mRNA and are translated together
  • With no nucleus, translation can begin in
    prokaryotes before transcription is over.

Multiple ribosomes attach to the mRNA and begin
translating. Strings of ribosomes can be seen
attached to the mRNA.
ms/ cppe/traducti/tpoly.html
Eukaryotic, prokaryotic differences
  • mRNA lifetime is different
  • Cap, tail, introns in eukaryotes
  • Shine-Delgarno vs. Kozak sequence cap
  • Size of ribosomes 70S vs. 80S
  • fMet-tRNA vs. Met-tRNA
  • Eukaryotic attachment of ribosomes to ER
  • Polypeptides extruded through tunnel in large
    subunit, directly into ER

Proteins vs. polypeptide
  • Common usage
  • Polypeptide is a string of amino acids
  • Doesnt imply function
  • Doesnt imply 3D shape
  • Protein implies functionality and 3D shape
  • Thus a protein can have a quaternary structure
    and be made of several different polypeptides.

Review of protein structure
String of amino acids, covalently attached by
peptide bonds directional (N terminus, C
Primary structure
The particular amino acids and the order that
they are in.
20 different amino acids connected by peptide
bonds 100-1000 amino acids in a peptide chain.
Secondary structure
Amino acid chain twists in space, held in place
by hydrogen bonds. Forms alpha helix or beta
pleated sheet.
Tertiary structure
3-D folding of the protein chain in space the
shape is determined from the primary structure
and from the secondary structure (which itself
depends on the primary structure. The primary
structure depends on the info encoded in the DNA
Protein structure slides from
Quaternary structure
Individual polypeptide chains aggregate to form a
single functional unit. Individual polypeptides
(protein subunits) may be switched out to give
the multipart protein a different function or
Many proteins that act on DNA or RNA have a
quaternary structure.
Different exons actually code for parts of
proteins that fold into discrete areas. May be
involved in evolution of proteins and their
Write a Comment
User Comments (0)