Chapter 12 Gene Expression and Regulation
How Is the Information in DNA Used in a Cell?
The link between DNA and protein DNA contains the “molecular blueprint” of
every cell Proteins are the construction workers of the
cell Proteins control cell shape, function,
reproduction, and synthesis of biomolecules Therefore, there must be a flow of information
from DNA to protein
How Is the Information in DNA Used in a Cell?
DNA provides instructions for protein synthesis via RNA intermediaries DNA in eukaryotes is kept in the nucleus Protein synthesis occurs at ribosomes in the
cytoplasm RNA differs structurally from DNA in three
ways RNA has the sugar ribose RNA is usually single-stranded RNA contains the nitrogenous base uracil (U)
instead of thymine (T)
How Is the Information in DNA Used in a Cell?
DNA provides instructions for protein synthesis via RNA intermediaries There are three types of RNA involved in
protein synthesis Messenger RNA (mRNA) carries a copy of
DNA gene information to the ribosome in the cytoplasm
Ribosomal RNA (rRNA) plus proteins make up the structure of ribosomes
Transfer RNA (tRNA) brings amino acids to the ribosome
How Is the Information in DNA Used in a Cell?
DNA provides instructions for protein synthesis via RNA intermediaries RNA occurs in many other roles besides
protein synthesis RNA is used as the genetic material in some
viruses, such as HIV Ribozymes – enzymatic RNA “Regulatory” RNA MicroRNA
How Is the Information in DNA Used in a Cell?
Overview : Genetic information is transcribed into RNA and then translated into protein mRNA carries the code for protein synthesis
from DNA to the ribosomes Ribosomal rRNA and proteins form ribosomes Transfer tRNA carries amino acids to the
ribosomes for addition to the growing protein
How Is the Information in DNA Used in a Cell?
Overview: Genetic information is transcribed into RNA and then translated into protein DNA directs protein synthesis in a two-step
process1.Transcription - Information in a DNA gene
is copied into RNA (like a court transcription – same language just a copy of information)
2.Translation - the genetic information contained in the mRNA is converted to another language by Messenger RNA, tRNA, amino acids, and a ribosome, to synthesize a protein
Genetic Information Flows from DNA to RNA to Protein
Fig. 12-2
(a) Transcription
Translation of the mRNA produces a protein molecule with an amino acid sequence determined by the nucleotide sequence in the mRNA
(b) Translation
Transcription of thegene produces anmRNA with anucleotide sequencecomplementary to oneof the DNA strands
DNA
messenger RNA
protein
ribosome
(cytoplasm)(nucleus)
gene
How Is the Information in DNA Used in a Cell?
The genetic code uses three bases to specify an amino acid The genetic code provides the rules Given that there are 20 amino acids but only
four bases, statistically, the smallest number of bases that could combine to yield a different sequence for each of the 20 amino acids is three A two-base code could produce only 16
combinations The three-base code has the potential to
create 64 combinations
How Is the Information in DNA Used in a Cell?
The genetic code uses three bases to specify an amino acid Marshall Nirenberg and Heinrich Matthaei
cracked the genetic code by creating artificial mRNAs of known sequence and observing what proteins they produced For example, an mRNA strand composed
entirely of uracil (UUUUUUUU…) produced a protein consisting entirely of the amino acid phenylalanine
Therefore, they concluded that the triplet UUU is the codon for phenylalanine
How Is the Information in DNA Used in a Cell?
The genetic code uses three bases to specify an amino acid Base triplets in DNA (sequence of 3
nucleotides) Codons in mRNA specifies a unique amino
acid in the genetic code Each mRNA also has a start codon (AUG) and
one of three stop codons (UAG, UAA, and UGA)
Some amino acids are specified by as many as six different codons
How Is the Information in DNA Used in a Cell?
The genetic code uses three bases to specify an amino acid Decoding the codons of mRNA is the job of
tRNA and ribosomes Each unique tRNA has three exposed bases,
called an anticodon, which are complementary to codon bases in mRNA
How Is the Information in a Gene Transcribed into RNA?
Overview of transcription Transcription of a DNA gene into RNA has three
stages1. Initiation - A promoter region at the
beginning of the gene marks where transcription is to be initiated
2. Elongation - The “body” of the gene corresponds with where elongation of the RNA strand occurs
3. Termination - A termination signal at the end of the gene marks where RNA synthesis is to terminate
Initiation
Fig. 12-3 (1 of 4)
direction oftranscription
promoter
beginning ofgene (3´ end)
gene 1 gene 2gene 3
Initiation: RNA polymerase binds to the promoter region of DNA near the beginning of a gene, separating the double helix near the promoter.
DNA
RNApolymerase
DNA
1
Elongation
Fig. 12-3 (2 of 4)
RNA
DNA template strand
Elongation: RNA polymerase travels along the DNA template strand (blue),
unwinding the DNA double helix and synthesizing RNA by catalyzing the addition of ribose
nucleotides into an RNA molecule (red). The nucleotides in the RNA are complementary to
the template strand of the DNA.
2
DNA RNAC - GG - CT - AA - U
Termination and Conclusion of Transcription
termination signal
4
Termination: At the end of the gene, RNA polymerase encounters a DNA sequence called a termination signal. RNA polymerase detaches from the DNA and releases the RNA molecule.
Conclusion of transcription: After termination, the DNA completely rewinds into a double helix. The RNA molecule is free to move from the nucleus to the cytoplasm for translation, and RNA polymerase may move to another gene and begin transcription once again.
DNA
RNA
3
Fig. 12-3 (3 & 4 of 4)
RNA Transcription in Action
Fig. 12-4
growingRNAmolecules
end ofgene
beginningof gene
gene
DNA
How Is the Base Sequence of Messenger RNA Translated
into Protein? Messenger RNA synthesis differs between
prokaryotes and eukaryotes Messenger RNA synthesis in prokaryotes
Genes for related functions are adjacent and are transcribed together
Because prokaryotes have no nuclear membrane, translation and transcription are not separated in space or time
As the mRNA molecule separates from the DNA, ribosomes immediately begin translating it to protein
(a) Gene organization on a prokaryotic chromosome
ribosome
protein
mRNA
DNARNApolymerase
direction of transcription
genes coding enzymes in asingle metabolic pathway
gene 1
gene regulatingDNA sequences gene 2 gene 3
(b) Simultaneous transcription and translation in prokaryotes
DNA
mRNA
ribosome
Messenger RNA Synthesis in Prokaryotic Cells Fig. 12-5
How Is the Base Sequence of Messenger RNA Translated
into Protein? Messenger RNA synthesis in eukaryotes
In eukaryotes, the DNA is in the nucleus and the ribosomes are in the cytoplasm
The genes that encode the proteins for a metabolic pathway are not clustered together on the same chromosome
Each gene consists of two or more segments of DNA that encode for protein, called exons, that are interrupted by other segments that are not translated, called introns
(a) Eukaryotic gene structure
DNA
promoter
exons
introns
(b) RNA synthesis and processing in eukaryotes
DNA
Transcription
finished mRNA
pre-mRNA
An RNA cap and tail are added
RNA splicing
Finished mRNA is moved to the cytoplasm for translation
cap tail
introns are cut out andbrokendown
1
3
4
2
Messenger RNA Synthesis in Eukaryotic Cells Fig. 12-6
How Is the Base Sequence of Messenger RNA Translated into Protein?
Possible functions of intron-exon gene structure1. Through alternative splicing of the exons in a
gene, a cell can make multiple proteins from a single gene
2. Fragmented genes may provide a quick and efficient way for eukaryotes to evolve new proteins with new functions
If breaks in chromosomes occur in introns, exons may remain intact and be spliced to other chromosomes in ways that produce new, useful proteins
How Is the Base Sequence of Messenger RNA Translated
into Protein? During translation, mRNA, tRNA, and
ribosomes cooperate to synthesize proteins Like transcription, translation has three steps
1. Initiation2. Elongation3. Termination
Translation Is the Process of Protein Synthesis: Initiation
Fig. 12-7 (1-3 of 9)
small ribosomal subunit
tRNA anticodon
methioninetRNA
amino acid
first tRNAbindingsite
catalytic sitesecond tRNA binding site
Initiation:
preinitiationcomplex
metmet
A tRNA with an attached methionine amino acid binds to a small ribosomal subunit, forming a preinitiation complex.
The large ribosomal subunit binds to the small subunit. The methionine tRNA binds to the first tRNA site on the large subunit.
The preinitiation complex binds to an mRNA molecule. The methionine (met) tRNA anticodon (UAC) base-pairs with the start codon (AUG) of the mRNA.
C
A ACC G GG U U U
A
A
ACC G GG U U U
CAU
CAUmRNA
large ribosomalsubunit
U
start codon
1 2 3
met
Translation Is the Process of Protein Synthesis: Elongation
Fig. 12-7 (4-6 of 9)
catalytic site
peptide bond
ribosome moves one codon to the right
Elongation:
The "empty" tRNA is released and theribosome moves down the mRNA, onecodon to the right. The tRNA that is attached to the two amino acids is now inthe first tRNA binding site and the secondtRNA binding site is empty.
The catalytic site on the large subunit catalyzes the formation of a peptide bond linking the amino acids methionine and valine. The two amino acids arenow attached to the tRNA in the second binding site.
The second codon of mRNA (GUU) base-pairs with the anticodon (CAA) of a second tRNA carrying the amino acid valine (val). This tRNA binds to the second tRNA site on the large subunit.
AUA ACC G GG U U U A ACC G GG GU U U
C A A A ACAU
A ACC G GG U U U
C A ACAU
initiator tRNA detaches
C
4 5 6
met
met met
valvalval
CA
U
Translation Is the Process of Protein Synthesis: Elongation and Termination
Fig. 12-7 (7-9 of 9)
Termination:
stop codon
The catalytic site forms a peptide bond between the aminoacids, leaving them attached to the tRNA in the second binding site. The tRNA in the first site leaves, and the ribosome moves one codon over on the mRNA.
This process repeats until a stop codon is reached; the mRNA and the completed peptide are released from the ribosome, and the subunits separate.
A A A A ACC GG U U U
AUA A
A
CC G G
G
GU U
U
U
C A A
completedpeptide
The third codon of mRNA (CAU) base-pairs with the anticodon (GUA) of a tRNA carrying the amino acid histidine (his). This tRNA enters the second tRNA binding site on the large subunit.
AUA A
A
CC G G G
G
G U U
U
U
C A A
7 8 9
met
val his his
val
met
val
met
his
arg
arg
ile
Complementary Base-Pairing Is Critical to the Process of Decoding Genetic Information
Fig. 12-8
template DNA strand
complementaryDNA strand
(a) DNA
gene
codons
anticodons
amino acids
etc.
etc.
etc.
etc.
(b) mRNA
(c) tRNA
methionine glycine valine(d) protein
T A C C C T C A A
A U G G G A G U U
U A C C C U C A A
etc.A T G G G A G T T
Review
1. How is genetic material encoded in DNA and RNA?
2. Distinguish between transcription and translation. (define and locate)
How Do Mutations Affect Protein Function?
Mutations are changes in the base sequence of DNA caused by mistakes during replication or by various environmental factors
Mutations take many forms and can affect protein function in many ways Mutations fall into five categories
Inversions Translocations Deletions Insertions Substitutions
How Do Mutations Affect Protein Function?
Inversions and translocations These mutations may be relatively benign if
entire genes, including their promoter, are merely moved from one place to another
However, if a gene is split in two, it will no longer code for a complete, functional protein Severe hemophilia is often caused by an
inversion in the gene that encodes a protein required for blood clotting
How Do Mutations Affect Protein Function?
Deletions and insertions
Depending on how many nucleotides are involved, deletions and insertions can cause a misreading of a gene’s codons during transcription or replication
The codons in THEDOGSAWTHECAT is changed by deletion of the letter “E” to THD OGS AWT HEC AT
Such mutations are called frameshift mutations
How Do Mutations Affect Protein Function?
Deletions and insertions Proteins that result from deletions and
insertions have a very different amino acid sequence and almost always are nonfunctional
Deletions and insertions of three nucleotides (or a multiple of three) do not cause a shift of the reading frame and, so, may simply subtract or add a harmless amino acid to the protein
How Do Mutations Affect Protein Function?
Point mutation (nucleotide substitution) A point mutation sometimes does not change
the amino acid sequence of the protein Because many amino acids are encoded by
more than one codon, the mutation may cause the same amino acid to be added
A known point mutation in the beta-globin gene for hemoglobin causes CTC to change to CTT, but since both codons code for glutamic acid, the protein is unchanged
How Do Mutations Affect Protein Function?
Point mutation (nucleotide substitution) A mutated protein may function normally
In beta-globin, a point mutation of the CTC codon to GTC causes glutamic acid (hydrophilic) to be replaced with glutamine (also hydrophilic), but the resulting protein functions well
How Do Mutations Affect Protein Function?
Point mutation (nucleotide substitution) Some substitutions cause an altered amino
acid sequence that change protein function dramatically, usually for the worse The substitution of an adenine for a thymine
in the CTC CAC mutation in a hemoglobin gene causes valine (hydrophobic) to replace glutamic acid (hydrophilic)
Placing this hydrophobic amino acid on the outside of the hemoglobin molecule leads to the clumping of hemoglobin and distortion of the red blood cell seen in sickle cell anemia
How Do Mutations Affect Protein Function?
Point mutation (nucleotide substitution) The point mutation may introduce a premature
stop codon, leading to an mRNA that produces an incomplete protein
Such a mutation in the beta-globin gene prevents production of functional beta-globin protein This leads to beta-thalassemia People with this mutation have only alpha-
globin subunits and require frequent blood transfusions to survive because it doesn’t bind O2 as well.
How Are Genes Regulated?
The human genome contains 20,000 to 30,000 genes
A given cell “expresses” (transcribes) only a small number of genes
Some genes are expressed in all cells, such as genes coding for RNAs, since all cells require proteins
How Are Genes Regulated?
Other genes are expressed only in certain types of cells, at certain times in an organism’s life, or under specific environmental conditions
For example, even though every cell in your body contains the gene for casein, the major protein in milk, this gene is expressed only in certain cells in the breast, only in mature women, and only when a woman is breast-feeding
How Are Genes Regulated?
Regulation of gene expression may occur at three different levels1. Rate of transcription, regulation determines which
genes in a cell are expressed2. Rate of translation, regulation determines how much
protein is made from a particular type of mRNA3. At the level of protein activity, regulation determines
how long the protein lasts in a cell and how rapidly protein enzymes catalyze specific reactions
Although these general principles apply to both prokaryotic and eukaryotic organisms, there are some differences as well
12.5 How Are Genes Regulated?
In eukaryotic cells, transcriptional regulation occurs on at least three levels The individual gene – promoters have several
binding sites Regions of chromosomes – too tightly wound Entire chromosomes
In female mammals, one entire X chromosome is condensed (Barr bodies)
How Are Genes Regulated?
In female mammals, one entire X chromosome is condensed This effect can be observed
in the fur patterns of calico cats The X chromosome of a cat
contains a gene for fur pigmentation
Different patches of skin cells in a cat inactivate different X chromosomes