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GENE EXPRESSION: FROM GENE TO PROTEIN - Coggle Diagram
GENE EXPRESSION: FROM GENE TO PROTEIN
GENES SPECIFY PROTEINS VIA TRANSCRIPTION AND TRANSLATION
Gene Expression
Process where DNA directs synthesis of proteins
2 Stages
Translation
Transcription
Evidence from Studying Metabolic Defects
Garrod - suggested genes dictate phenotypes through enzymes, proteins that catalyze specific chem reactions in the cell
"inborn errors of metabolism"
One gene-One enzyme hypothesis
Garrod's hypothesis that a gene dictates the production of a specific enzyme
Cells synthesize and degrade most organic molecules thru metabolic pathways
Drosophila
each mutation affecting eye color blocks pigment synthesis at a specific step preventing enzyme production
Nutritional Mutants in Neurospora: Scientific Inquiry
X rays where known to cause mutations
change in a mutants phenotype
wild type neurospora has modest food requirements
Minimal medium - wild type mold cells use their metabolic pathways to produce moleules for growth
Basic Principles of Transcription and Translation
In DNA/RNA, monomers are the 4 types of nucleotides that differ in nitrogenous bases
bridge bt DNA and protein synthesis is the nucleic acid RNA
genes give instructions for making specific proteins
gene DOES NOT build a protein directly
DNA - A, G, C, T
RNA - A, G, C, U
Transcription
synthesis of RNA using info in the DNA
carries genetic message from DNA to protein synthesis machinery of the cell
Messenger RNA (mRNA)
carries genetic message from DNA to protein synthesis machinery of the cell
Translation
synthesis of a polypeptide using the info in mRNA
change in language
cell must translate nucleotide sequence of mRNA into amino acid sequence of a polypeptide
Ribosomes
sites of translation
molecular complexes that facilitate the orderly linkage of amino acids in to polypeptide chains
Primary Transcript
initial RNA transcript form any gene, even specifying RNA that isn't translated into protein
DNA ---> RNA ---> PROTEIN
The Genetic Code
Triplets of nucleotide bases are the smallest units of uniform length that can code for all the
amino acids.
With a
triplet code
, three consecutive bases specify an amino acid, creating (64) possible code words.
The genetic instructions for a polypeptide chain are written in DNA as a series of
nonoverlapping three-nucleotide words, which are then translated into an amino acid chain.
During transcription, one DNA strand, the
template strand
, provides a template for ordering
the sequence of nucleotide bases in an mRNA transcript.
A given DNA strand can be the template strand for some genes along a DNA molecule, while for other genes in other regions, the complementary strand may function as the template.
The complementary mRNA molecule is synthesized according to base-pairing rules, except that uracil is the complementary base to adenine.
Like a new strand of DNA, the mRNA molecule is synthesized in an antiparallel direction to the template strand of DNA.
The mRNA base triplets are called
codons
. They are written in the 53 direction.
The term codon is also used for the DNA base triplets along the nontemplate strand.
These codons are complementary to the template strand and thus identical in sequence to the mRNA except that they have T instead of U.
For this reason, the nontemplate DNA strand is called the “coding strand.”
To extract the message from the genetic code requires specifying the correct starting point.
The starting point establishes the
reading frame
; subsequent codons are read in groups of three nucleotides.
Concept 17.2 Transcription is the DNA-directed Synthesis of RNA: A Closer Look
Termination of Transcription
bacteria and eukaryotes differ in the way they terminate transcription
Bacteria
transcription proceeds thru a terminator sequence in DNA
Eukaryotes
RNA polymerase II transcribes a sequence on the DNA called polyadenylation signal sequence
Transcription Unit
The Stretch of DNA downstream from the promoter that's transcribed into an RNA molecule
Molecular Components of Transcriptions
RNA Polymerase
an enzyme that prise the 2 strands of DNA apart & joins RNA nucleotides complementary to the DNA template strand, elongating the RNA polynucleotide
"Downstream" & "Upstream"
Promoter
the DNA sequence where RNA polymerase attaches and initiates transcription
in bacteria, the sequence that signals the end of transcription is called
terminator
Bacteria have a single type of RNA polymerase
synthesizes mRNA & other types of RNA that function in protein synthesis
EX: Ribosomal RNA
figure 17.8
RNA Polymerase Binding and Initiation of Transcription
Transcription Factors
mediate the binding of RNA polymerase and the initiation of transcription
in eukaryotes, a collection of proteins
Transcription Initiation Complex
the whole complex of of transcription factors and RNA polymerase II bound to the promoter
TATA Box
DNA sequence in forming the initiation complex at a eukaryotic promoter
Elongation of the RNA Strand
The promoter of a gene includes within it the transcription start point-- the nucleotide where RNA polymerase actually begins synthesis of the mRNA -- & extends nucleotide pairs upstream form the start point
Concept 17.3 Eukaryotic Cells Modify RNA After Transcription
enzymes in eukaryotic nucleus modify pre-mRNA before the genetic message is dispatched to the cytoplasm
RNA Processing*
both ends of the primary transcript are altered
produce an mRNA molecule ready for translation
In most cases, certain interior parts of the molecule are cut out and the remaining parts
are spliced together
These modifications help form an mRNA molecule that is ready for translation
Split Genes and RNA Splicing
RNA Splicing
a remarkable stage of RNA processing in the eukaryotic nucleus
where large portions of the RNA molecules are removed and the remaining portions are reconnected
The average length of a transcription unit along a human DNA molecule is about 27,000 nucleotide pairs.
However, it takes only 1,200 nucleotides to code for an average-sized protein of 400 amino acids.
Introns
non coding segments of nucleic acid that lie bt coding regions
Exons
the other regions, translated into amino acid sequences
Spliceosome
the removal of introns is accomplished by a large complex made of proteins and small RNAs
The spliceosome interacts with certain sites along an intron, releasing the introns and joining
together the two exons that flanked the introns.
snRNAs catalyze these processes as well as participating in spliceosome assembly and
splice site recognition.
Structure & Function of Ribosomes
Alternative RNA Splicing
many genes are known to give rise to 2 or more diff polypeptides, depending of which segments are treated as exons during RNA processing
Domains
proteins often have a modular architecture consisting of discrete structural and functional regions
One domain of an enzyme may include the active site, while another might allow the
protein to bind to a cellular membrane.
Concept 17.4 Translation is the RNA-Directed Synthesis of a Polypeptide
Transfer RNA (tRNA)
the message is a series of codons along an mRNA molecule, & the translator is called tRNA
process of translations "reads" a genetic message and builds a polypeptide accordingly
Structure & Function of tRNA
the key is the fact that each tRNA molecule enables translation of a given mRNA codon to a certain amino acid
Anticodon
the loop extending from the other end of the L includes the anticodon
the particular nucleotide triplet that base-pairs to a specific mRNA codon
Aminoacyl-tRNA Synthesis
the correct matching up of tRNA and amino acid is carried out by a family of related enzymes
The 20 different synthetases match the 20 different amino acids.
Each has active sites for only a specific tRNA–amino acid combination.
Wobble
the flexible base pairing at this condon position
Wobble explains why the synonymous codons for a given amino acid most often differ in their third base, not in their other bases.
Structure & Function of Ribosomes
a ribosome consists of a large subunit and a small subunit, each made up of proteins and one or more rRNAs
in eukaryotes, the subunits are made in the nucleus
P Site
Peptideyl-tRNA - holds the tRNA carrying the growing polypeptide chain
A Site
Aminoacyl-tRNA - holds the tRNA carrying the next amino acid to be added to the chain
E Site
Exit site - where the discharged tRNAs leave the ribosome from
The ribosome holds the tRNA and mRNA in close proximity and positions the new amino
acid for addition to the carboxyl end of the growing polypeptide.
It then catalyzes the formation of the peptide bond.
As the polypeptide becomes longer, it passes through an exit tunnel in the ribosome’s large
unit and is released to the cytosol through the exit tunnel.
Evidence supports the hypothesis that rRNA, not protein, is responsible for the ribosome’s
structure and function
Ribosomal RNAs (rRNAs)
○ A ribosome consists of a large and a small subunit, each made up of proteins and
ribosomal RNA (rRNA).
In eukaryotes, the subunits are made in the nucleolus
rRNA genes are transcribed and the RNA is processed and assembled with proteins
imported from the cytoplasm.
The subunits are exported via nuclear pores to the cytoplasm
A single mRNA may be used to make many copies of a polypeptide simultaneously as multiple ribosomes,
polyribosomes or polysomes,
trail along the same mRNA.
Polyribosomes can be found in bacterial and eukaryotic cells.
figure 17.19
figure 19.20
Concept 17.5 Mutations of One or a Few Nucleotides ca Affect Protein Structure and Function
Nucleotide pair sub
the replacement of one nucleotide and its partner w another pair of nucleotides
Mutations
changes to the genetic info of a cell
responsible for the huge diversity of genes found among organisms bc mutations are the ultimate source of new genes
Point Mutations
small scale mutations of one or a few nucleotide pairs including point mutations
changes in a single nucleotide pair of a gene
silent mutation
no observable effect on the phenotype
substitutions that change one amino acid to another one
Missense mutations
change one amino acid for another with little effect on protein function.
In some cases, the mutation switches one amino acid for another with similar properties.
Other mutations occur in a region where the exact amino acid sequence is not essential
for function.
Nonsense mutations
change an amino acid codon into a stop codon, causing premature
termination of translation and nearly always leading to a nonfunctional protein.
Insertions and deletions are additions or losses of nucleotide pairs in a gene
Insertions and deletions
are more likely than substitutions to have a disastrous effect on the
resulting protein.
frameshift mutation
Unless insertion or deletion mutations occur in multiples of 3, they cause a frameshift mutation
All the nucleotides downstream of the deletion or insertion will be improperly grouped
into codons.
The result will be extensive missense, ending sooner or later in nonsense—premature
termination
Mutagents
a number of physical and chemical agents
interact w DNA in ways that cause mutations
