Transcript From Gene to Protein

Ch. 17: From Gene to Protein
The Connection Between Genes
and Proteins
• The study of metabolic defects provided evidence that
genes specify proteins
– Garrod, suggested phenotypes had to do with expression of
genes for enzymes
• Transcription and translation are the two main processes
linking gene to protein
– Copy the information and interpret the information
• In the genetic code, nucleotide triplets specify amino
acids
– Sequence of nucleotides = primary protein structure
• The genetic code must have evolved very early in the
history of life
– DNA code is universal…..all cells use the same codons and
amino acids to make their various proteins
Back to Mendel
• One of Mendels “factors” for peas was stem length. We
say “height” and the alleles are tall and short. Actually it’s
“length” and the stems are long or not-long
• Normal peas have a gene for the hormone called
gibberellin which stimulates stem elongation.
• Dwarf peas lack this gene and do not make gibberellin
and are therefore not tall.
• Dwarf peas will grow to normal height if gibberellins are
added to their water
• PROTEINS ARE THE LINKS BETWEEN
GENOTYPE AND PHENOTYPE.
Scientific Evidence
• 1909 – “inborn errors of metabolism”
– alkaptonuria
• 1930 – Beadle and Ephrussi, eye color in flies is due to
an enzyme for pigment production
• Beadle and Tatum – minimal medium Neurospora crassa
(bread mold), used x-rays to create mutations, complete
media had 20 amino acids, looking for inability to
metabolize amino acids from a limited source
– Mutants had defects in metabolism
– Must be enzymes related
– Enzymes are proteins
– One gene – one enzyme hypothesis
– now modified to one gene – one (protein) polypeptide
Beadle and Tatum’s Neurospora crassa Experiment
Transcription and Translation
• Genes have instructions for making proteins, but
genes do not make proteins directly
• Transcription is the synthesis of RNA under the
direction of DNA. DNA provides the template. Get
an accurate copy; mRNA
• Translation is the actual synthesis of a polypeptide,
at the ribosome, under the direction of the mRNA
• DNA  RNA  protein (polypeptide)
Terminology
• Triplet code: three DNA nucleotides = a word
• mRNA: carries message from DNA to ribsome
• tRNA: transports amino acids within cytoplasm
rRNA: ribosomes are composed of rRNA and proteins
• Ribosome: solid organelle found in cytoplasm of ALL cells;
used to manufacture protein
• Template strand: for each gene only one side of the DNA is
transcribed
• Codon: mRNA triplets are called codons
• Reading frame: 5’3’, starting at beginning, groups of three
– The red dog ate the cat
– xHer edd oga tat hec atx
Cracking the Code
• 1960 – Marshall Nirenberg at NIH
– National Institute of Health
– www.nih.gov
– Human genome projects library
• Translated all the possible codons into amino
acids
• Found codons for “start” and for “stop”
• Several amino acids can be coded for with more
than one codon (redundancy) but no codons are
for multiple amino acids (no ambiguity)
• “wobble effect”
Code Evolved Early in the History
of Life on Earth (and any life anywhere else too)
• Code is (near) universal to all
know/studied organisms…. Bacteria can
translate human genetic information
• All modern organisms have a common
ancestor
• Few exceptions are found in protista and
in mitochondria DNA (…. Endosymbiont hypothesis….)
• Sketch a DNA molecule with chemically correct
details
• Show how it would replicate and
• How it would transcribe and
• List the amino acids in the short polypeptide it
forms
• It has the following template strand sequence of
DNA triplets
– TAC TTT GAG ATT
Genomic like information
• Stthegeneticcodeisnearlyuniversalpsharedbyorg
anismsfromthesimplestbacteriatothemostcomple
xplantsandanimalspstthernacodonccgpforinstan
cepistranslatedastheaminoacidprolineinallorgani
smswhosegeneticcodehasbeenexaminedpstinla
boratoryexperientspgenescanbetranscribedandtr
anslatedaftertheyaretransplantedfromonespecie
stoanotherpfponeimportantapplicationisthatbacte
riacanbeprogrammedbytheinsertionofhumangen
estosynthesizecertainhumanproteinsthathaveim
portantmedicalusesp
The Synthesis and Processing of
RNA
• Transcription is the DNA directed
synthesis of RNA
• Eukaryotic cells modify RNA after
transcription
Transcription
• RNA polymerase fits onto DNA (3’) and moves in
a 5’  3’ direction for the synthesis of the RNA
strand.
• C with G and this time, A with U (uracil)
• DNA acts as a template
• DNA is only opened at a small region (gene or
genes of interest)
• DNA helix reseals as RNA polymerase passes
by…. Completely intact and undiluted.
Bacterial transcription
• Eukaryotic cells have 3 kinds of RNA
polymerase (I, II – used in RNA synthesis
and III)
• Bacteria have one kind – it makes not only
mRNA but also other types of RNA
• Bacteria have one chromosome and many
plasmids. Information is constantly being
sent to ribosomes for translation into
proteins needed by the bacterial cell
Steps of Transcription
1.
Initiation
Promoter – region where polymerase attaches and a dozen bases
upstream; start here and use this side of the helix.
Collection of transcription factors initiate the “complex” – TATA box
2.
3.
Elongation
DNA exposed 20 bases at a time
5’  3’ synthesis of RNA strand
RNA peels away from DNA as completed
rate is 60 nucleotides per second
Termination
DNA contains a terminator sequence
polymerase continues to a AAUAAA sequence and 10-35
nucleotides later the preRNA is cut free
other details are still ‘murky’
Modification of RNA
• Initially RNA is called preRNA
• The 5’ end (transcribed 1st) is capped with
special guanine – provides protection and
a start here signal for translation
• Other end gets a ploy A tail (AAA-AAA) –
in addition to ribosomal attachment and
protection, it seems to facilitate RNA as it
leaves the nucleus
• These regions are nontranslated
Further modification of RNA
• Most of the pre RNA is actually
removed…. It didn’t code for information
about how to make a protein. We are
uncertain of the function of this info, which
does not make the info unimportant.
• Initially the RNA can be 8000 bases,
actual info for protein that goes to
ribosomes is about 1200 or 400 amino
acids (1200 bases/ 3 bases per codon)
“Cut and Paste”
• Called RNA splicing
• Introns (intervening segments) are removed
– they are noncoding, short, repetitive sequences, unique – cause
restriction enzymes to cut segments differently and create the DNA
fingerprint
– Probably have a role in gene expression and activity
– May be place where new proteins evolve
– Increase odds of crossing over during synapsis of tetrads (meiosis II)
• Exons (expressed)
– these are translated into amino acids for the polypeptide
– 150 nucleotides
• 5’ cap + exon + exon + exon + …. + poly A tail
• Process requires snRNP’s - small nucleotide ribonucleoproteins….
Sites to bind
• Ribozymes = RNA that functions as an enzyme.