Chapter 17 Flashcards
The information of DNA is in the form of
specific sequences of nucleotides
The DNA inherited by an organism leads to
specific traits by dictating the synthesis of proteins
Proteins are the links between
genotype and phenotype
Gene expression, the process by which DNA directs protein synthesis, includes two stages:
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 happens in the nucleus of eukaryotes.
Transcription produces
messenger RNA (mRNA)
Translation is
the synthesis of a polypeptide, using information in the mRNA
Ribosomes are
the sites of translation
Every kind of cell has
ribosomes and DNA
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
A primary transcript is the
initial RNA transcript from any gene prior to processing
The central dogma is the concept that
cells are governed by a cellular chain of command:
DNA—> RNA —> Protein
There are 20 amino acids, but
there are only four nucleotide bases in DNA
The flow of information from gene to protein is based on a
triplet code: a series of nonoverlapping, three-nucleotide words
The words of a gene are transcribed into
complementary nonoverlapping tree-nucleotide words of mRNA.
These words are then translated into a chain of amino acids, forming a polypeptide
During transcription,
one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript
The template strand is always the same strand for
a given gene
During translation,
the mRNA base triplets, called codons, are read in the 5’ to 3’ direction
Codons along an mRNA molecule are read by
translation machinery in the 5’ to 3’ direction
Each codon specifies the amino acid (one of 20) to be placed at the
corresponding position along a polypeptide
All 64 codons were deciphered by the
mid-1960s
Of the 64 triplets,
61 code for amino acids; 3 triplets are “stop signals to end translation (stop codons)
The genetic code is redundant (more than one codon may specify a particular amino acid) but
not ambiguous; no codon specifies more than one amino acid
Codons must be read in the correct reading frame (correct groupings) in order for
the specified polypeptide to be produced
AUG
start codon
always codes for placement of amino acid called methionine
The genetic code is
nearly universal, shared by the simplest bacteria to the most complex animals
Genes can be transcribed and translated after
being transplanted from one species to another
Transcription is the
DNA-directed synthesis of RNA
Transcription is the
first stage of gene expression
RNA synthesis is catalyzed by
RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides
The RNA is complementary to the
DNA template strand
RNA synthesis follows the same
base-pairing rules as DNA, except that uracil substitutes for thymine
The DNA sequence where RNA polymerase attaches is called the
promoter,
in bacteria, the sequence signaling the end of transcription is called the terminator
The stretch of DNA that is transcribed is called a
transcription unit
The three stages of transcription
- Initiation
- Elongation
- Termination
Promoters signal the transcriptional 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
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
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
Eukaryotic cells modify
RNA after transcription
Enzymes in the eukaryotic nucleus modify
pre-mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm
During RNA processing,
both ends of the primary transcript are usually altered.
Also, usually some interior parts of the molecule are cut out, and the other parts spliced together.
Each end of a pre-mRNA molecule is modified in
a particular way
- The 5’ end and receives a modified nucleotide 5’ cap
- The 3’ end gets a poly-A tail
These modifications to the end of a pre-MRNA molecule share several functions
- They seem to facilitate the export of mRNA to the cytoplasm
- They protect mRNA from hydrolytic enzymes
- They help ribosomes attach to the 5’ end
Most eukaryotic genes and their RNA transcripts have
long noncoding stretches of nucleotides that lie between coding regions
These noncoding regions are called
intervening sequences, or introns
The other regions are called
exons because they are eventually expressed, usually translated into amino acid sequences
RNA splicing removes
introns and joins exons, creating an mRNA molecule with a continuous coding sequence
In some cases,
RNA splicing is carried out by spliceosomes
Spliceosomes consist of
a variety of proteins and several small nuclear rinonucleoproteins (snRNPs) that recognize the splice sites
Ribozymes are
catalytic RNA molecules that function as enzymes and can splice RNA
The discovery of ribozymes rendered
obsolete the belief that all biological catalysts are proteins
There properties of RNA enable it to function as an enzyme:
- it can form a three-dimensional structure because of its ability to base-pair with itself
- some bases in RNA contain functional groups that may participate in catalysis
- RNA may hydrogen-bond with other nucleic acid molecules
Some introns contain sequences that may regulate
gene expression
Some genes can encode more than
one kind of polypeptide, depending on which segments are treated as exons during splicing.
This is called alternative RNA splicing
Consequently, the number of different proteins an organism can produce is
much greater than its number of genes
Proteins often have a modular architecture consisting of discrete regions called
domains
In many cases,
different exons code for the different domains in a protein
Exon shuffling may result in
the evolution of new proteins
Translation is the
RNA-directed synthesis of a polypeptide
Genetic information flows from
mRNA to protein through the process of translation
A cell translates an mRNA message into protein with the help of
transfer RNA (tRNA)
tRNAs transfer
amino acids to the growing polypeptide in a ribosome
Translation is a complex process in terms of its
biochemistry and mechanics
Molecules of tRNA are not identical
- Each carries a specific amino acid on one end
- each has an antiocodon on the other end; the anticodon base-pairs with a complementary codon on mRNA
A tRNA molecule consists of
a single RNA strand that is only about 80 nucleotides long
Flattened into one plane to reveal its base pairing,
a tRNA molecule looks like a cloverleaf
Because of hydrogen bonds, tRNA actually
twists and folds into a three-dimensional molecule
tRNA is roughly
L-shaped
Accurate translation requires two steps
- First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase
- Second: a correct match between the tRNA anticodon and an mRNA codon
Flexible pairing at the third base of a codon is called
wobble and allows some tRNAs to bind to more than one codon
Ribosomes facilitate
specific coupling of tRNA anticodons with mRNA codons in protein synthesis
The two ribosomal subunits (large and small) are
made of proteins and ribosomal RNA (rRNA)
Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences:
some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosome
A ribosome has three binding sites for tRNA:
- the P site
- the A site
- the E site
The P site
holds the tRNA that carries the growing polypeptide chain
The A site
holds the tRNA that carries the next amino acid to be added to the chain
The E site
is the exit site, where discharged tRNAs leave the ribosome
The three stages of translation
- Initiation
- Elongation
- Termination
All three stages require protein “factors” that aid in the translation process
The initiation stage of translation brings together
mRNA, a tRNA with the first amino acid, and the two ribosomal subunits
First, a small ribosomal subunit binds with
mRNA and a special initiator tRNA
initiation stage in translation??
Then the small subunit moves
along the mRNA until it reaches the start codon (AUG)
initiation stage in translation??
Proteins called initiation factors bring in the
large subunit that completes the translation initiation complex
(initiation stage in translation)
During the elongation stage,
amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain
Each addition involves proteins called
elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation
(elongation stage in translation)
Translation proceeds along the
mRNA in a 5’ to 3’ direction
Termination occurs when
a stop codon in the mRNA reaches the A site of the ribosome
termination stage in translation
The A site accepts a protein called a
release factor
termination stage in translation
The release factor causes the
addition of a water molecule instead of an amino acid.
This reaction releases the polypeptide, and the translation assembly then comes apart.
(termination stage in translation)
A number of ribosomes can translate a single mRNA simultaneously forming a
polyribosome (or polysome)
Polyribosomes enable a cell to
make many copies of a polypeptide very quickly
Often translation is not sufficient to make a
functional protein
Polypeptide chains are modified after translation or
targeted to specific sites in the cell
During and after synthesis,
a polypeptide chain spontaneously coils and folds into its three-dimensional shape
Proteins may also require
post-translational modifications before doing their job
Some polypeptides are activated by
enzymes that cleave them
Other polypeptides come together to form
the subunits of a protein
Two populations of ribosomes are evident in cells:
- free ribosomes (in the cytosol)
- bound ribosomes (attached to the ER)
Free ribosomes mostly synthesize
proteins that function in the cytosol
Bound ribosomes make proteins of the
endomembrane system and proteins that are secreted from the cell
Ribosomes are identical and
can switch from free to bound
Polypeptide synthesis always begins in
the cytosol
Synthesis finished in the
cytosol unless the polypeptide signals the ribosome to attach to the ER
Polypeptides destined for the ER for for secretion are marked by a
signal peptide
A signal-recognition particle (SRP) binds to
the signal peptide
The signal-recognition particle (SRP) brings teh
signal peptide and its ribosome to the ER
Messenger RNA (mRNA) function
carries information specifying amino acid sequences of proteins from DNA to ribosomes
Transfer RNA (tRNA) function
serves as adapter molecule in protein synthesis; translates mRNA codons into amino acids
Ribosomal RNA (rRNA) function
plays catalytic (ribozyme) roles and structural roles in ribosomes
Primar transcript function
serves as a precursor to mRNA, rRNA, or tRNA, before being processed by splicing or cleavage
Small nuclear RNA (snRNA) function
plays structural and catalytic roles in spliceosomes
SRP RNA function
is a component of the signal-recognition particle (SRP)
