4.3 Gene Expression Flashcards
What is gene expression?
the process of using a gene sequence to synthesize a protein
What does gene expression rely on?
the participation of several different forms of RNA molecules, most important of which are mRNA, tRNA, and rRNA
What 2 processes does gene expression overall require?
transcription and translation
Where does transcription and translation take place?
in eukaryotes, transcription takes place in the nucleus and translation takes place in the cytoplasm
What is transcription?
the first stage of gene expression, process in which a portion of DNA serves as a template for mRNA formation
What is translation?
during gene expression, the process in which the sequence of mRNA bases determines the sequence of amino acids in a polypeptide
Where does genetic information lie?
in the sequence of the bases in DNA, which through mRNA determines the sequence of amino acids in a protein
What does tRNA do during protein synthesis?
tRNA assists mRNA by bringing amino acids to the ribosomes
How do proteins differ from one another?
by the sequence of their amino acids
What do proteins determine?
the structure and function of cells and the physical characteristics or phenotype of the organism
What is a gene?
a segment that contains encoded information within each individual’s DNA
What happens during transcription?
gene serves as a template for the production of an RNA molecule
What was historically known about genes?
a gene was considered to be a nucleic acid sequence that codes for the sequence of amino acids in a protein
What do we now know about genes?
some genes include instructions for the formation of DNA molecules, such as mRNA, tRNA, and rRNA
What do we know about protein-coding regions?
they can be interrupted by regions that do not code for a protein
What is Mark Gerstein’s suggested new definition for a gene?
“a gene is a genomic sequence (either DNA or RNA) directly encoding functional products, either RNA or protein”
Describe transcription of DNA to form mRNA.
- complementary RNA is made from a DNA template
- at the point of attachment of RNA polymerase, the DNA helix unwinds and unzips, and complementary RNA nucleotides are joined together
- after RNA polymerase has passed by, the DNA strands rejoin and the mRNA transcript is released
What is the purpose of mRNA?
to carry genetic information from the DNA to the ribosomes for protein synthesis
How and where is mRNA formed?
formed by the process of transcription which occurs in the nucleus in eukaryotes
When does transcription begin?
when RNA polymerase binds tightly to promoter
What is promoter?
a region of DNA that contains a special sequence of nucleotides
What is the process of transcription?
- RNA polymerase opens up the DNA helix just in front of it so that complementary base pairing can occur in the same way as in DNA replication
- RNA polymerase inserts the RNA nucleotides, and an mRNA molecule results
- when mRNA forms, it has a sequence of bases complementary to that of the DNA (wherever A, T, G, or C are present in the DNA template, U, A, C, or G are incorporated into the mRNA molecule)
- mRNA is now a faithful copy of the sequence of bases in DNA
When is mRNA processed?
after the mRNA is transcribed in eukaryotic cells, it must be processed before entering the cytoplasm
What happens to mRNA molecule after processing?
the newly synthesized primary RNA molecule becomes a mature mRNA molecule
What are introns?
most genes in humans are interrupted by segments
of DNA that are not part of the gene called introns
What are the 2 parts of the gene?
intron and exon
Why is the other portion of the gene called exons?
because they are ultimately expressed, only exons result in a protein product
What does primary mRNA contain?
bases that are complementary to both exons and introns but during processing
- one end of the mRNA is modified by the addition of a cap, and the other end is modified by the addition of poly-A tail
- the introns are removed, and the exons are joined to form a mature mRNA molecule consisting of continuous exons
What is a cap?
composed of an altered guanine nucleotide
What is poly-A tail?
a series of adenosine nucleotides
What does the processing of mRNA ordinarily bring together?
all the exons of a gene
in some instances, cells use only certain exons to form a mature RNA transcript
the result can be a different protein product in each cell
Describe mRNA processing.
a cap and tail are added to mRNA, and the introns are removed so that only exons remain
What is the genetic code?
the sequence of bases in DNA is transcribed into mRNA, which ultimately codes for a particular sequence of amino acids to form a polypeptide
What is a codon?
each triplet of nucleotides
What does it mean for the code to be degenerate?
most amino acids are coded for by more than one codon
Give an example of degeneracy.
leucine has 6 codons, serine has 4
this degeneracy offers some protection against possibly harmful mutations that change the sequence of bases
What are stop codons?
codons that do not code for amino acids but instead termination of polypeptide synthesis
ie. of 64 codons, only 61 actually code for amino acids
What does it mean by the genetic coder is almost universal in all living organisms?
ie. a codon in a fruit fly codes for the same amino acids as in a bird, a fern, or a human
What does the universal nature of the genetic code suggest?
that it dates back to the very first organisms on Earth and that all living organisms have a common evolutionary history
What does tRNA do?
bring amino acids to the ribosomes
Where is the site of protein synthesis?
ribosomes
What is the structure of tRNA?
a single-stranded polynucleotide that doubles back on itself such that complementary base pairing creates a boot-like shape
on one end is an amino acid, and on the other is an ANTICODON
What is an anticodon?
a triplet of three bases complementary to a codon of mRNA
How many possible codons are there?
64
How many different tRNA molecules are there?
40
Why are there 64 possible codons, but only 40 different tRNA molecules?
the wobble effect, which states that for some tRNAs, the third nucleotide in the mRNA codon may vary
What is the wobble effect believed to provide?
additional degeneracy to the genetic code, and help protect against mutations that may alter the amino acid sequence of a protein
What happens when a tRNA-amino acid complex comes to the ribosome?
its anticodon pairs with an mRNA codon
What does the order of the codons of the mRNA determine?
the order that tRNA-amino acids come to a ribosome and therefore, the final sequence of amino acids in a protein
Describe tRNA being an amino acid carrier.
- tRNA is a polynucleotide that folds into a boot-like shape because of complementary base pairing
- at one end of the molecule is its specific anticodon, and at the other, an amino acid attaches that corresponds to this anticodon
Describe gene expression that results in a protein product.
- during transcription, the base sequence in DNA is copied into a sequence of bases in mRNA
- during translation, tRNAs bring amino acids to the ribosomes in the order dictated by the base sequence of mRNA
- the sequence of amino acids forms a polypeptide chain, or complete protein
What are ribosomes?
small structural bodies found in the cytoplasm and on the endoplasmic reticulum where translation also occurs
What are ribosomes composed of?
many proteins and several rRNAs
Where is rRNA produced in eukaryotic cells?
in the nucleolus within the nucleus
What happens to rRNA after it is produced?
the rRNA joins with proteins manufactured in and imported from the cytoplasm to form two ribosomal subunits, one large and one small
the subunits leave the nucleus and join together in the cytoplasm to form a ribosome just as protein synthesis begins
What do the binding sites of ribosomes do?
facilitate complementary base pairing between tRNA anticodons and mRNA codons
What happens as the ribosome moves down the mRNA molecule?
new tRNAa arrive and a polypeptide forms and grows longer
When does translation terminate?
once the polypeptide is fully formed and an mRNA stop codon is reached
the ribosome then dissociates into its subunits and falls off the mRNA molecule
What happens as soon as the initial portion of mRNA has been translated?
as soon as the initial portion of mRNA has been translated by one ribosome, and the ribosome has begun to move down the mRNA, another ribosome attaches to the same mRNA
therefore, several ribosomes are often attached to and translating a single mRNA, forming several copies of a polypeptide simultaneously
the entire complex is called a POLYRIBOSOME or POLYSOME
Describe the overview of gene expression.
one strand of DNA acts as a template for mRNA synthesis, and the sequence of bases in mRNA determines the sequence of amino acids in a polypeptide
What happens during translation?
the codons of an mRNA base pair with the anticodons of tRNA molecules carrying specific amino acids
What does the order of the codons during translation determine?
the order of the tRNA molecules at a ribosome and the sequence of amino acids in a polypeptide
Why must the process of translation be extremely orderly?
so that the amino acids of a polypeptide are sequenced correctly
What are the 3 steps of protein synthesis?
initiation
elongation
termination
What is required for each of the steps of translation to function properly?
enzymes
Which steps of translation require energy?
initiation and elongation
What is initiation?
the step that brings all the translation components together
What are initiation factors?
proteins required to assemble the small ribosomal subunit as well as mRNA, initiator tRNA, and the large ribosomal subunit for the start of protein synthesis
What happens during initiation in prokaryotes (bacteria)?
a small ribosomal subunit attaches to the mRNA in the vicinity of the start codon
the initiator tRNA pairs with this codon (AUG) because its anticodon is UAC
then a large ribosomal subunit joins to the small subunit
What happens during initiation in eukaryotes?
similar in many ways to prokaryotes, but much more complicated
What are the 3 binding sites for tRNAs in a ribosome?
P (peptide) site
A (amino acid) site
E site (the exit where tRNA leaves)
What is initiator tRNA capable of doing?
binding to the P site, even though it carries only the amino acid methionine
What is the A site for?
tRNA carrying the next amino acid
What is elongation?
the protein synthesis step in which a polypeptide increases in length one amino acid at a time
What does elongation require?
tRNAs and elongation factors
What do elongation factors do?
facilitate the binding of tRNA anticodons to mRNA codons at a ribosome
What are the 4 steps of elongation?
- a tRNA with an attached peptide is already at the P site and a tRNA carrying the next amino acid in the chain is just arriving at the A site
- once the next tRNA is in place at the A site, the peptide chain will be transferred to this tRNA
- energy and part of the ribosomal subunit are needed to bring about this transfer, the energy contributes to peptide bond formation, which makes the peptide one amino acid longer by adding the peptide from the A site
- translocation occurs: mRNA moves forward one codon length and the peptide-bearing tRNA is now at the ribosome P site, the spent tRNA exits, and the new codon is at the A site and is ready to receive the next complementary tRNA
CYCLE IS REPEATED AT A RAPID RATE
Describe elongation (simple diagram).
- tRNA-amino acid approaches the ribosome and binds at the A site
- two tRNAs can be at a ribosome at one time, the anticodons are paired to the codons
- peptide bond formation attaches the peptide chain to the newly arrived amino acid
- ribosome moves forward, the “empty” tRNA exits from the E site, next amino acid-tRNA complex is approaching the ribosome
What is termination?
the final step in protein synthesis in which the polypeptide and the assembled components that carries out protein synthesis are separated from one another
Where does termination of polypeptide synthesis occur?
at a stop codon
What does termination require?
a protein called a release factor which cleaves the polypeptide from the last tRNA
the polypeptide is set free and begins to take on its 3D shape
the ribosome dissociates into its two subunits
Why are properly functioning proteins of paramount importance to the cell and to the organism?
ie. if an organism inherits a faulty gene, the result can be a genetic disorder (such as Huntington disease) caused by a malfunctioning protein or a propensity toward cancer
What are proteins the link between?
the genetic makeup (genotype) and the physical characteristics (phenotype) of organisms
What distinguishes the different types of organisms?
the DNA sequence underlying these proteins
What do proteins account for?
- the difference between cell types
- the differences between organisms
Describe the review of gene expression.
mRNA is produced and processed in the nucleus during transcription, and protein synthesis occurs at the ribosomes (in cytoplasm and rough ER) during translation
When is a gene expressed?
when its protein product has been synthesized
REVIEW OF GENE EXPRESSION
- protein synthesis requires transcription and translation
- transcription: segment of DNA strand serves as template for formation of mRNA
- bases in mRNA are complementary to those in DNA
- every three mRNA bases is a codon for a certain amino acid
- mRNA is processed before it leaves the nucleus, during which time the introns are removed and the ends are modified
- mRNA carries a sequence of codons to the ribosomes
- translation: tRNAs bring attached amino acids to ribosomes
- because tRNA anticodons pair with codons, amino acids become sequenced in the order originally specified by DNA
What do the genes we receive from our parents determine?
the proteins in our cells and these proteins are responsible for ur inherited traits
What is the central dogma of biology?
describes the two-step process, transcription and translation, by which the information in genes flows into protein
- Within each cell the genetic information flows from DNA to RNA to protein.
- This flow of information is unidirectional and irreversible.
- The information carried within the DNA dictates the end product (protein) that will be synthesized. This information is the genetic code.
- Conversion of DNA encoded information to RNA – is called transcription.
- The information from a mRNA is then translated to an amino acid sequence in the corresponding protein
