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C17 - Coggle Diagram
C17
Genes specify proteins via transcription and translation
A gene
defined as a region of DNA that can be expressed to produce a final functional product that is either a polypeptide or an RNA molecule
The idea of the gene has evolved
a discrete unit of inheritance
a region of specific nucleotide sequence in a chromosome
a DNA sequence that codes for a specific polypeptide chain
Gene expression
the process by which DNA directs protein synthesis, includes two stages: transcription and translation
Proteins are the links between genotype and phenotype
dictating the synthesis of proteins
leads to specific traits
The information content of genes is in the specific sequences of nucleotides
British physician Archibald Garrod
first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions
Basic Principles of Transcription and Translation
RNA is the bridge between genes and the proteins for which they code
Transcription is the synthesis of RNA using information in DNA
Transcription produces messenger RNA (mRNA)
Translation is the synthesis of a polypeptide, using information in the mRNA
In prokaryotes, translation of mRNA can begin before transcription has finished
In a eukaryotic cell, the nuclear envelope separates transcription from translation
Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA
Ribosomes are the sites of translation
Transcription is the DNA-directed synthesis of RNA: a closer look
Molecular Components of Transcription
RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
transcription unit
The stretch of DNA that is transcribed
RNA polymerase
pries the DNA strands apart and joins together the RNA nucleotides
terminator
In bacteria, the sequence signaling the end of transcription
promoter
The DNA sequence where RNA polymerase attaches
Eukaryotic cells modify RNA after transcription
Synthesis of an RNA Transcript
Initiation
Promoters signal the transcription start point and usually extend several dozen nucleotide pairs upstream of the start point
Transcription factors mediate the binding of RNA polymerase and the initiation of transcription
The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex
A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes
Elongation
As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time
Transcription progresses at a rate of 40 nucleotides per second in eukaryotes
A gene can be transcribed simultaneously by several RNA polymerases
Nucleotides are added to the 3′ end of the growing RNA molecule
Termination
The mechanisms of termination are different in bacteria and eukaryotes
In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification
In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence
Translation is the RNA-directed synthesis of a polypeptide: a closer look
Molecular Components of Translation
The Structure and Function of Ribosomes
The three stages of translation
All three stages require protein “factors” that aid in the translation process
Initiation
The start codon (AUG) signals the start of translation
First, a small ribosomal subunit binds with mRNA and a special initiator tRNA
Then the small subunit moves along the mRNA until it reaches the start codon
Proteins called initiation factors bring in the large subunit that completes the translation initiation complex
Elongation
During elongation, amino acids are added one by one to the C-terminus of the growing chain
Each addition involves proteins called elongation factors
Elongation occurs in three steps: codon recognition, peptide bond formation, and translocation
Energy expenditure occurs in the first and third steps
Termination
Elongation continues until a stop codon in the mRNA reaches the A site of the ribosome
The A site accepts a protein called a release factor
The release factor causes the addition of a water molecule instead of an amino acid
This reaction releases the polypeptide, and the translation assembly comes apart
Mutations of one or a few nucleotides can affect protein structure and function
Types of Small-Scale Mutations
Single nucleotide-pair substitutions
def:replaces one nucleotide and its partner with another pair of nucleotides
Silent mutations
Missense mutations
Nonsense mutations
Nucleotide-pair insertions or deletions
Insertions and deletions are additions or losses of nucleotide pairs in a gene
a disastrous effect on the resulting protein more often than substitutions do
frameshift mutation
Insertion or deletion of nucleotides may alter the reading frame
Mutations
changes in the genetic information of a cell
genetic disorder or hereditary disease:a mutation has an adverse effect on the phenotype of the organism
Point mutations
changes in just one nucleotide pair of a gene
The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein
Mutagens
physical or chemical agents that can cause mutations
Most carcinogens (cancer-causing chemicals) are mutagens, and most mutagens are carcinogenic