Download One Gene-one polypeptide:

One Gene-one polypeptide:
- Each gene codes for the production of a specific polypeptide
-Beadle and Tatum first showed a direct relationship between genes and enzymes,
which they put forward as the one gene-one enzyme hypothesis
-Since a different gene encodes each distinct polypeptide, their hypothesis was
restated as the one gene-one polypeptide hypothesis
Template strand: is one of the two DNA strands that are used in transcription to copy the
DNA into an RNA strand, and only part of the DNA sequence of the
genome is copied in any cell at any given time
Messenger RNA: the RNA strand that is transcribed from a protein-coding gene
Genetic code: written in 3 letter words using a four letter alphabet. The nucleotide bases for
DNA are Adenine, Thymine, Guanine, and Cytosine (A, T, G,C). The RNA nucleotide bases are
Adenine, Uracil, Guanine, and Cytosine (A, U, G, C). There are four RNA bases, but there are
20 amino acids. The genetic code is the nucleotide information that specifies the amino acid
sequence of a polypeptide.
Codon: the three-letter word (triplet) of the code. The three-letter codons in DNA are first
transcribed into complementary three-letter RNA codons. Understanding codons is more
important for understanding translation.
Degeneracy: when an amino acid is represented by more than one codon, some by as many as
6.
Comma less: the code can be read correctly only by starting at the right place—at the first
base of the first three-letter codon at the beginning of a coded message—and
reading three nucleotides at a time from this beginning codon.
Start codon: start codon is AUG, which is the first codon read in an mRNA in translation in
both prokaryotes and eukaryotes
Stop codon: acts as periods, indicating the end of a polypeptide-encoding sentence. When a
ribosome reaches one of the stop codons, the polypeptide synthesis stops and the
new polypeptide chain is released from the ribosome
Introns: non-protein coding sequences that interrupt the protein-coding sequence.
Exons: the amino acid—coding sequences that are retained in finished mRNAs
Promoter: a control sequence, that initiates transcription, RNA polymerase binds to the
promoter, unwinds the DNA in that region, and starts synthesizing a new RNA
molecule at the transcription start point. The part of the gene that is copied into
RNA is called the transcription unit.
Initiation: RNA polymerase binds to the promoter, unwinds the DNA, and initiates the
transcription at the start point
Elongation: RNA polymerase moves along the DNA, unwinding it and adding new RNA
nucleotides to the transcript in the 5’—3’ direction. Behind the enzyme, the DNA
strands reform into a double helix
Termination: the complete RNA molecule is released from the template DNA, RNA
polymerase leaves the DNA, and the double helix reforms
Alternative splicing: when pre-mRNAs are processed by reactions that join exons in
different combinations to produce different mRNAs from a single gene. This greatly increases
the number and variety of proteins encoded in the cell nucleus without increasing the size of the
genome. As a result of the alternative splicing process, the number of proteins produced far
exceeds the number of genes, and it is proteins that direct an organisms functions. This increases
the coding capacity of existing genes.
Exon shuffling: a process by which existing protein regions or domains, already selected for
their functions by the evolutionary process, are mixed into novel
combinations to create new proteins. This generates new proteins.
Eukaryotic and prokaryotic transcription is very similar. Major difference is the RNA processing
and that transcription occurs in the nucleus of a eukaryote and in the cytosol of a prokaryote.
Similarities between DNA replication and RNA transcription
1. Only one of the 2 DNA strands acts as a template to make a complementary RNA copy,
instead of both as in replication
2. Only a small part of a DNA molecule—the sequence encoding a single gene—serves as a
template, rather than all of both strands as in replication
3. RNA polymerases catalyze the assembly of nucleotides rather than DNA polymerases in
replication
4. RNA molecules resulting from replication a single-stranded rather than double as in
replication
5. Where adenine appears in the DNA template chain, a uracil is matched to it in the RNA
transcription instead of thymine as in DNA replication
Transcription
1. An RNA polymerase molecule binds to the DNA at the beginning of the gene to be
transcribed…and the DNA begins to unwind at the front of the RNA polymerase, which begins
moving along the DNA
2. During transcription, RNA nucleotides are base paired one after another with the template
DNA bases
3. The RNA copy is released when the entire gene has benn transcribed. The unwound region of
the DNA rewinds into a double helix
mRNA splicing—occurs in the nucleus and removes introns from pre-mRNAs and joins the
exons together. This is part of pre-mRNA processing which takes place in the nucleus of
eukaryotes. Takes place in a spliceosome. The snRNPs are what actually removes the introns.
The snRNPs that have small nuclear RNA or snRNA, recognize the pair with sequences at the
junctions of the intron with the adjacent exons. The complex then recruits the other snRNPs,
producing a larger complex that loops out the intron and brings the two exon ends close together.
At this point the active spliceosome has been formed. The spliceosome cleaves the pre-mRNA at
the junction between the 3’ end of the exon and the 5’ end of the intron, and the intron loops
back to bond itself near its 3’ end. The spliceosome then cleaves the pre-mRNA at the junction
between the 3’ end of the first intron and the exon, releasing the intron and joining together the
two exons. Enzymes then degrade the intron and the snRNPs are released.
RNA molecule elongates as new nucleotides are added one by one. Elongation continues until
the RNA synthesis terminates and the completed RNA transcript and RNA polymerase are
released from DNA.
In prokaryotes, the promoters are immediately upstream of the site where transcription initiates.
RNA polymerase itself recognizes the key DNA sequences in the promoter, binds to the
promoter and begins transcription.
In eukaryotes, RNA polymerase II transcribes protein-coding genes. RNA polymerases I and III
transcribe genes for non-protein coding RNAs, such as rRNAs in the ribosomes, and tRNAs.