Chapter 7 Flashcards
the building blocks of proteins are _____ amino acids
20
- Some are acquired from your diet, while others are synthesized in your cells.
- The # of amino acids and their sequence differentiate proteins from each other.
The human genome contains more than _____________ genes
20 000
- these 20 000 genes code for 100 000 different
proteins
- genes are diff combo of the 4 nucleotide bases along the length of the DNA strands.
- Messages from the genes, in the form of RNA molecules, travel to the ribosomes to direct the
assembly of the proteins.
- Variations in the copying & editing of the messages
culminate in the assembly of the many diff proteins by the ribosomes
2 key pieces of research, involving defects in metabolism, led scientists to the discovery of how genes encode for proteins
1) Archibald Garrod’s Research (1896-1908)
- Studied alkaptonuria, a human disease causing urine to turn black in air.
- Observed that alkaptonuria is an inherited trait.
- Concluded by 1908 that alkaptonuria is an “inborn error of metabolism”, caused by a defective enzyme encoded by a mutated gene.
- Later research found that affected individuals excrete alkapton, a chemical that accumulates due to an inability to fully break down tyrosine.
- Established the first link between genes and metabolism.
2) George Beadle & Edward Tatum’s Research (1940s)
- Worked with Neurospora crassa (orange bread mold)
- Normal Neurospora grows readily on a minimal medium (MM): a medium that contains several salts, sucrose, and a vitamin, but none of the other, more complex chemicals required by cells.
- Hypothesized that Neurospora suses the simple chemicals in the medium to synthesize all
of the more complex molecules it needs for growth and reproduction.
- Exposed its spores to X-rays, causing random genetic mutations.
- Found some mutated spores could not grow on MM without added nutrients like amino acids or vitamins.
- Demonstrated a direct relationship between genes & enzymes, supporting the gene-protein connection.
Beadle & Tatum’s Conclusion
- they hypothesized that each mutated strain had a defect in a gene that coded for one of the enzymes needed to synthesize a particular nutrient that was not in the MM. –> Ex, Mutants requiring added arginine had a defective gene coding for an enzyme in the arginine synthesis pathway. –> assembly of arginine from raw materials is a multistep process, with diff enzymes responsible for each step.–> thus, diff “arg” mutants might differ in the particular enzyme that is defective & thus in which step of the synthesis of arginine is blocked.
- they deduced that the biosynthesis of arginine required a # of steps, and each step was controlled by a gene that coded for the enzyme for this step
–> Beadle &Tatum had shown the direct relationship between genes & enzymes, which they put forward as the one gene–one enzyme hypothesis. - Impact laid the foundation for molecular biology., Earned them the 1958 Nobel Prize.
- Later scientists recognized not all proteins are enzymes & many proteins consist of more than
one subunit. –> Since this subunit is called a polypeptide, Beadle and Tatum’s hypothesis was restated as the one gene–one polypeptide hypothesis.
one gene–one enzyme hypothesis is
the hypothesis, proposed by Beadle and Tatum, that each gene is unique and codes for the synthesis of a single enzyme
one gene–one polypeptide hypothesis is
the hypothesis that each gene is unique and codes for the synthesis of a single polypeptide; the restated version of the one gene–one enzyme hypothesis
central dogma is
the fundamental principle of molecular genetics, which states that genetic info flows from DNA to RNA to proteins
- In 1956, Francis Crick gave the name central dogma to the flow of information from DNA to RNA to protein
The Central Dogma idea has 2 major processes: List them
1) transcription
2) translation.
Transcription is
the mechanism by which the information encoded in DNA is transcribed into a complementary RNA copy.
-essentially the info in one type of nucleic acid, DNA, is copied onto another type of nucleic acid, RNA.
- occurs in the nucleus of a eukaryotic cell.
- Unlike DNA, RNA is able to exit the nucleus and enter the cytosol
Translation is
the mechanism by which the information coded in the nucleic acids of RNA is copied into the amino acids of proteins
- It takes place on the ribosomes in the cytosol.-
- RNA contains the information for a polypeptide in the language of bases, but this information must be translated into the language of amino acids.
Differences between DNA & RNA
Both are carriers of genetic info but differ in many ways
1) RNA contains a ribose sugar rather than a deoxyribose sugar. –> A ribose sugar has a hydroxyl
group on its 2’ carbon.
2) instead of thymine, RNA contains the base uracil. –> similar in structure to thymine, except thymine has a methyl group on its 1’ carbon. –> Uracil in the RNA pairs with adenine in the DNA strand.
3) DNA is double stranded, whereas RNA is single stranded. –> When a gene is transcribed into RNA, only a single-stranded complementary copy is made. –> In the complementary copy, uracil is substituted for thymine.
4) DNA is longer than RNA (Ms. Akhtar)
3 major types of RNA molecules are involved in protein synthesis: List them
messenger RNA (mRNA)
transfer RNA (tRNA)
ribosomal RNA (rRNA)
Messenger RNA (mRNA) is
the end product of the transcription of a gene; mRNA is translated by ribosomes into a protein
- acts as the intermediary between DNA and the ribosome
- mRNA is the RNA version of the gene encoded by DNA.
- It varies in length, depending on the gene that has been transcribed; the longer the gene, the longer the mRNA is.
Transfer RNA (tRNA) is
a carrier molecule that binds to a specific amino acid & adds the amino acid to the growing polypeptide chain
- role: to transfer the appropriate amino acid to the ribosome to build a protein, as dictated by the mRNA template.
-relatively short in length, averaging 70 to 90 ribonucleotides
- The single-stranded RNA molecule loops in on itself, forming antiparallel double strands, which r
complementary to each other (SEE PAGE 315 FOR DIRAGRAM)
Ribosomal RNA (rRNA) is
an RNA molecule within the ribosome that bonds the correct amino acid to the polypeptide chain
- Along with proteins, it’s a structural component that forms the ribosome, which is the construction site for the assembly of polypeptides
An Overview of Transcription & Translation
Transcription (Step 1 of Protein Synthesis):
- Enzyme RNA polymerase makes an RNA molecule complementary to the DNA sequence of a given gene.
- Since DNA template strand is read in the 3’ to 5’ direction, RNA is formed in the 5’ to 3’ direction.
- Initially formed RNA is a precursor mRNA molecule cuz it can’t produce a protein directly and undergoes modifications to become functional mRNA.
- The mRNA can now exit the nucleus and enters the cytosol, where ribosomes are located.
Translation (Step 2 of Protein Synthesis):
- The mRNA associates with a ribosome in the cytosol.
- tRNA delivers amino acids to the ribosome as it moves along the mRNA.
- Amino acids are joined one by one to form the polypeptide encoded by the gene
The Genetic Code
- The specific amino acid coded for by particular DNA (or complementary RNA) bases is determined by the genetic code.
- while there r only 4 RNA bases, there r 20 amino acids. How is nucleotide info in an mRNA molecule translated into the amino acid sequence of a polypeptide?
- Scientists realized that the 4 bases in an mRNA must be used in combos of at least 3 to provide the capacity to code for 20 amino acids –> if the code used 3-letter combos, 64 different amino acids could be specified (AAA, AAT, AAC, . . . , or 4^3)—more than enough to code for 20 amino acids.
genetic code is
the specific coding relationship between bases & the amino acids they specify; the genetic code can be expressed in terms of either DNA or RNA bases
The Genetic Code: Codons
- Each 3-letter combo is= a codon.
-The codons are in the 59 to 39 order in the mRNA. - Of the 64 codons, 61 specify amino acids = “sense codons.”
–> ex, 1 of these codons, AUG, specifies the amino acid methionine. It is usually the first codon translated in any mRNA in prokaryotes & eukaryotes. Thus, AUG = a start codon, or initiator codon. - The 3 codons that don’t specify amino acids—UAA, UAG, & UGA= stop codons (AKA “nonsense codons” or “termination codons”). –> They act as “periods,” indicating the end of a polypeptide-encoding sentence.
–> When a ribosome reaches a stop codon, polypeptide synthesis stops & the newly synthesized polypeptide chain is released from the ribosome.
A codon is
a group of three base pairs that code for an individual amino acid
A start codon is
AKA initiator codon
- the codon that signals the start of a polypeptide
chain and initiates translation
A stop codon is
a codon that signals the end of a polypeptide chain and causes the ribosome to terminate translation
Redundancy & wobble hypothesis
- there are many synonyms in the genetic code
- Ex, UGU &UGC both specify cysteine, whereas CCU, CCC, CCA, &CCG all specify proline.
–> This feature is known as redundancy & is called the wobble hypothesis. - The presence of this redundancy allows the third base in a codon to change (wobble), while still allowing the codon to code for the same amino acid. –> Notice how both cysteine codons follow the pattern UG_ & all proline codons follow the pattern CC_.
The genetic code is ________________.
universal
- With few exceptions, the same codons specify the
same amino acids in all living organisms, & also in all viruses.
- The universality of the genetic code indicates that it was established, very early in the evolution of life & has remained virtually unchanged through billions of years of evolutionary history.
- Minor exceptions to the universality of the genetic code have been found in a few organisms, including yeast, some protozoans, and a prokaryote, & in DNA of mitochondria &chloroplasts
Transcription is divided into three sequential processes: List Them
1) initiation
2) elongation
3) termination
TRANSCRIPTION: Initiation
1st step of Transcription
- transcription begins when the enzyme RNA polymerase binds to the DNA & unwinds it near the beginning of a gene. –> The binding occurs at a promoter
- Since less E is needed to break 2H-bonds as opposed to 3, the RNA polymerase expends less E opening up the DNA helix if it possesses a high conc of A & T base pairs.
- The part of the gene that is to be transcribed into
RNA = the transcription unit.
A promoter is
a nucleotide sequence that lies just before a gene and allows for the binding of RNA polymerase
TATA box
- A key element of the promoter in eukaryotes is the TATA box –> a section of DNA with a high percentage
of T & A bases, which is recognized by RNA polymerase & enables the binding of RNA polymerase - Prokaryotes have a TATAAT sequence instead of a TATA box for this purpose
- rmr since less E is needed to break 2H-bonds as opposed to 3, the RNA polymerase expends less E opening up the DNA helix if it possesses a high conc of A & T base pairs.
TRANSCRIPTION: Elongation
2nd step of Transcription
- when RNA polymerase binds to the promoter & opens the DNA double helix, it starts to build the single-stranded RNA molecule
- Unlike DNA polymerase, RNA polymerase does not need a primer to start synthesizing RNA. –> RNA is synthesized in the 5’→3’ direction, using the 3’→5’ DNA strand as the template strand.
- rmr that the template strand contains the sequence that is complementary to the sequence that is going to be transcribed.
- As RNA pol moves along the DNA, it unwinds the DNA at the forward end of the enzyme.
- RNA polymerase adds nucleotides one at a time to elongate the RNA strand.
- The newly formed RNA pairs temporarily with the template strand, creating a hybrid RNA-DNA double helix.
- Beyond the hybrid region, the RNA detaches from the DNA, and the DNA reforms its double helix.
- when an RNA pol has started transcription & progressed past the beginning of a gene, another one may start producing another RNA molecule if there is room at the promoter.
–> Most genes undergoing transcription have many RNA polymerase molecules spaced closely along them, and each molecule makes an RNA transcription cuz of high demand. –> ex, single red blood cell contains 375 million hemoglobin molecules. The process of making hemoglobin would be very slow if the gene had only one RNA polymerase enzyme
making one mRNA molecule at a time.
The coding strand is
The opposite strand of DNA—the strand that is not being copied in RNA transcription = coding strand, since it contains the same base-pair sequence as the new RNA molecule, except uracil replaces thymine
TRANSCRIPTION: Termination
3rd step of transcription
- Transcription ends when RNA polymerase encounters a termination sequence.
2 TERMINATION MECHANISMS IN PROKARYOTES
1) termination mechanism involves a protein binding to the mRNA and stopping transcription.
2) the mRNA binding with itself in a hairpin loop and stopping transcription
EUKARYOTE TERMINATION
- 1 termination sequence is a string of As, which r transcribed as a string of Us on the RNA.
- Nuclear proteins bind to the polyuracil site & stop transcription.
- The newly made RNA then dissociates from the DNA template strand. –> Transcription stops, & the RNA
polymerase is free to bind to another promoter region & transcribe another gene
Post-transcriptional Modifications: Capping & Tailing
newly transcribed eukaryotic RNA, AKA primary transcript or precursor mRNA (pre-mRNA), is vulnerable to the enzymes & conditions outside the cell nucleus.
- The pre-mRNA must undergo additional modifications before it can exit the nucleus & reach the ribosome.
- 1 modification is A poly(A) tail (50–250 adenine nucleotides) is added to the 3’ end by poly-A polymerase one nucleotide at a time. –> This tail improves mRNA stability & efficiency of translation while protecting it from cytosolic RNA-digesting enzymes.
- Another modification is at the beginning of the pre-mRNA transcript, where a 5’ cap, consisting of 7 Gs, is added by a different enzyme complex. The 5’ cap functions as the initial attachment site for mRNAs to ribosomes, to allow for translation.
- This whole process= capping and tailing
A poly(A) tail is
a chain of adenine nucleotides that are added to the 3’ end of the pre-mRNA molecule to protect it from
enzymes in the cytosol
A 5’ cap is
a sequence of 7 Gs that is added to the start of a pre-mRNA molecule; ribosomes recognize this site & use it as the site of initial attachment
Post-transcriptional Modifications: Exons & Introns
- The DNA of a eukaryotic gene is composed of coding regions known as exons and non-coding regions known as introns.
- introns r interspersed among exons and are transcribed into pre-mRNAs –> introns do not code
for part of the protein.
–> If left in the mRNA, they would alter the sequence of the amino acids that r used to build the protein. - This would result in additional amino acids & a protein that would not fold as it should & thus would not function correctly –> thus, introns r deleted & the exons r kept in fully processed mRNA.
- The majority of known eukaryotic genes contain at least one intron, and some contain >60.
- Prokaryote DNA doesn’t contain any introns
An exon is
a sequence of DNA or RNA that codes for part of a gene
An Intron is
a non-coding sequence of DNA or RNA
How are introns removed?
- In nucleus, process called mRNA splicing removes introns from pre-mRNAs & joins the exons together.
- mRNA splicing occurs in a spliceosome: a complex formed between pre-mRNA & handful of small ribonucleoproteins called snRNPs.
- snRNPs bind in a particular order to an intron
in the pre-mRNA - The first snRNPs are those that recognize & form
complementary base pairs with mRNA sequences at the junctions of the intron & adjacent exons. - Other snRNPs r recruited, causing the intron to loop out & bring the 2 exon ends close together –> At this point, an active spliceosome has been formed, releasing the intron & joining together the 2 exons.
- cutting & splicing r so exact that not a single base of an intron remains in the finished mRNA, & not a single base is removed from the exons.
Alternative splicing is
a process that makes diff mRNAs from pre-mRNA
(exons and introns), allowing more than 1 possible polypeptide to be made from a single gene
- Exons can be joined in different combinations, producing various mRNAs from a single gene.
- Alternative splicing mechanism increases the
# & variety of proteins encoded by a single gene.
- 3/4 of all human pre-mRNAs are subjected to alternative splicing.
- In each case, the diff mRNAs that are produced from the parent pre-mRNA are translated to make a family of related proteins with various combos of amino acid sequences.
- Each protein in the family, then, varies in its function. Alternative splicing helps us understand why humans with only about 20 000 genes can make about 100 000 proteins.
- After the final mRNA has been produced, it is ready to leave the nucleus & be translated by a ribosome.
- https://www.youtube.com/watch?v=FAsLzgVHmjQ
- https://docs.google.com/document/d/1fhzDwn1LDRvX5rNqi32z-7VNQDNqxFWYtfkGVp5QY2s/edit?usp=sharing
Transcription: Eukaryotes Vs Prokaryotes
LOCATION
Pro: Transcription occurs throughout the cell.
Euk: Transcription takes place in the nucleus.
ENZYMES
Pro: single type of RNA polymerase transcribes all types of genes ( to make all mRNA, rRNA, tRNA)
Euk: Diff RNA polymerases are used to transcribe genes that encode protein (RNA polymerase II) and genes that do not encode protein like rRNA & tRNA (RNA polymerase I, III).
ELONGATION
Pro: Bases are added quickly (15 to 20 nucleotides per second)
Euk: Bases are added slowly (5 to 8 nucleotides
per second)
PROMOTERS
Pro: The promoters are less complex than those in eukaryotes.
Euk: The promoters are immediately upstream of protein-coding genes, & r more complex
TERMINATION
Pro: A protein binds to the mRNA & cleaves it, or the mRNA binds with itself.
Euk: Nuclear proteins bind to the polyuracil site &
terminate transcription.
INTRONS & EXTRONS
Pro: There are no introns.
Euk: There are both introns and exons
PRODUCT
Pro: Transcription results in mRNA ready to be translated into protein by ribosomes.
Euk: Transcription results in pre-mRNA, which
must be modified to protect the final mRNA
from degradation in the cytosol & to remove introns.
tRNA
- tRNAs r short RNAs (70–90 nucleotides) with a distinctive cloverleaf structure formed by base-pairing within the molecule. (mRNA is 100s of nucleotides long)
- All tRNAs have regions that base pair with themselves, winding into 4 double-helical segments to form a cloverleaf pattern
- Anticodon: A 3-nucleotide sequence on tRNA that pairs with a codon on mRNA, located at the tip of 1 of the double-helical segments –> Ex, a tRNA that is linked to serine (Ser) pairs with the codon 5’-AGU-3’ in mRNA. The anticodon of the tRNA that pairs with this codon is 3’-UCA-5’.
- At other end of the cloverleaf is a region that carries the amino acid that corresponds to the anticodon. (Amino Acid Attachment Site CHATGPT)
Recall that 61 of the 64 codons of the genetic code specify an amino acid. Does this mean that we need 61 diff tRNAs to read the different codons?
No
- Francis Crick’s wobble hypothesis proposed that the complete set of 61 codons specify amino acids, but fewer than 61 tRNAs are needed due to the wobble hypothesis:
- Precise pairing occurs at the first two codon nucleotides. –> Ex, UAU & UAC both code for tyrosine. –> If the tRNA’s anticodon is AUA, it can still bind to the codon UAC, despite its complementarity being UAU. Either way, tyrosine is added on.
- Flexibility at the third position allows some tRNAs to recognize multiple codons coding for the same amino acid.
Aminoacylation is
AKA “charging” the tRNA
the process by which a tRNA molecule is bound to its
corresponding amino acid
- the amino acid attaches to the 3’ end of the tRNA molecule at the amino acid attachment site.
- The finished product, a tRNA linked to its correct amino acid= an aminoacyl–tRNA.
- Aminoacylation is catalyzed by 20 different aminoacyl–tRNA synthetase enzymes, 1 for each of the 20 amino acids.
- The E in the aminoacyl–tRNA eventually drives the formation of the peptide bond that links the amino acids during translation.
Ribosomes in protein synthesis
- Ribosomes translate mRNA into polypeptide chains by joining amino acids in a specific sequence dictated by the mRNA.
- A ribosome is made up of 2 diff-sized parts: the large & small ribosomal subunits. Each subunit is made up of a combo of ribosomal RNA (rRNA) & ribosomal proteins
- to fulfill its purpose, it has special binding sites that actively bring together mRNA with aminoacyl-tRNAs
Binding Sites on Ribosomes:
- mRNA Binding Site: is where the mRNA threads through the ribosome.
- A (Aminoacyl) Site: Binds the incoming aminoacyl–tRNA carrying the next amino acid.
- P (Peptidyl) Site: Holds the tRNA carrying the growing polypeptide chain.
- E (Exit) Site: Releases the tRNA after its amino acid has been added to the chain.
There are three major stages of translation: List them
1) initiation
2) elongation
3) termination
TRANSLATION: Initiating Translation
1st step of Translation
-first stage of protein assembly, where translation begins using mRNA as a template.
- starts when the small & large ribosomal subunits associate with the mRNA.
- A specialized initiator methionine–tRNA (Met–tRNA) pairs with the AUG start codon on the mRNA.
Step 1: The initiator Met–tRNA forms a complex with the small ribosomal subunit.
Step 2: The complex binds to the 5’ cap of the mRNA & scans until it finds the first AUG start codon.
Step 3: The large ribosomal subunit binds, completing the ribosome. At the end of initiation, the Met–tRNA is positioned in the P site.
- After the initiator tRNA pairs with the AUG initiator codon, the following stages of translation simply read the nucleotide bases, 3 at a time, on the mRNA.
- cuz each codon consists of 3 bases, a sequence could potentially be read in 3 diff ways, depending on where the ribosome begins. –> The correct pairing of the initiator tRNA with the AUG start codon sets the reading frame, ensuring codons are read in the correct sequence during translation.
A reading frame is
a particular system for separating a base pair sequence into readable codons
TRANSLATION: Elongating the Polypeptide Chain
- Elongation sequentially adds amino acids to the growing polypeptide chain through a cycle of four steps.
- Step 1: Elongation begins when an initiator tRNA, with its attached methionine, is bound to the P site. The A site is empty.
- Step 2: A second tRNA, with a complementary anticodon and an attached amino acid, binds to the codon in the A site. –> E for this step is provided by GTP hydrolysis.
- the amino acid (Met) is cleaved from the tRNA in the P site and forms a peptide bond with the amino acid on the tRNA in the A site. This bond formation is catalyzed by peptidyl transferase, a ribosomal enzyme. At the end of Step 2, the new polypeptide chain is attached to the tRNA in the A site and an empty tRNA remains at the P site.
- Step 3: the ribosome moves along the mRNA to the next codon. The two tRNAs remain bound to their respective codons, thereby bringing the tRNA with the growing polypeptide to the P site and moving the
empty tRNA to the E site. An appropriate tRNA moves into the A site, and Steps 2 and 3 are repeated. After each repeat, the empty tRNA that was in the P site moves to the E site. - Step 4: the empty tRNA is released from the ribosome.
TRANSLATION: Termination of Protein Synthesis
- Translation switches from the elongation to the termination stage when the A site of a ribosome arrives at one of the stop codons (UAA, UAG, or UGA) on the mRNA.
- A protein release factor will bind to the A site instead of an aminoacyl–tRNA.
- In response, the polypeptide chain is released from
the tRNA at the P site as usual. –> but cuz no amino acid is present at the A site, the freed polypeptide chain is detached from the ribosome - At the same time, the ribosomal subunits separate & detach from the mRNA. The empty tRNA & the release factor r also released
A Polysome is
a complex that is formed when multiple ribosomes attach to the same mRNA molecule in order to facilitate rapid translation
- In both prokaryotes & eukaryotes, multiple ribosomes can translate an mRNA molecule at the same time, thereby increasing the production of crucial proteins –> The complex that is formed is called a polysome
- In eukaryotes, polysomes only form outside the nucleus, in the cytosol.
- In prokaryotes, transcription & translation both occur in the cytosol –> ribosomes have access to mRNA even as it is being made.
–> As a result, protein synthesis can occur at a much higher rate in prokaryotes than in eukaryotes.
–> this also allows prokaryotes to rapidly synthesize proteins in response to changing environmental conditions.
Translation in Prokaryotes VS Eukaryotes
LOCATION
Pro: mRNA is translated by ribosomes in the cytosol as it is being transcribed from DNA
Euk: mRNA can only be translated after exiting the nucleus to interact with ribosomes in the cytosol –> some translation occurs in mitochondria & chloroplasts
INITIATION
Pro: mRNA bases pair directly with a ribosomal binding site, just upstream of the start codon –> mRNA 5’ cap is involved
Euk: complex of Met–tRNA, with small ribosomal subunits, binds to an mRNA 5’ cap and scans until it
encounters the start codon
ELONGATION
Pro: 15 to 20 elongation cycles per second
Euk:1 to 3 elongation cycles per second
TERMINATION
BOTH: stop codon appears & a release factor binds so that the polypeptide is released
POLYSOMES
Pro: mRNA strand can be translated by multiple ribosomes simultaneously, even as it is being transcribed from DNA
Euk: mRNA strand can be translated by multiple ribosomes simultaneously, but only in the cytosol
Polypeptide to Protein
- The newly assembled polypeptide is inactive and non-functional immediately after translation.
- The polypeptide chain must fold into the correct three-dimensional shape, which defines its function.
- Multiple processing reactions, carried out by specific enzymes, remove amino acids from the ends or
interior of the chain & may add additional molecules, such as sugars, to the chain. - These reactions activate the polypeptide, which then folds into its functional shape.
- Also, many proteins are composed of 2 or > polypeptide chains. –> the polypeptides made from a # of diff translation events are processed and then assembled together to form a single functioning protein.
- This process of protein processing, from an inactive to an active state, is one of the many mechanisms that a cell uses to control the expression of its genes
Not all proteins are required by all cells at all times. It would be inefficient for a cell to transcribe & translate all its genes at all times. Elaborate
- both prokaryotic & eukaryotic cells regulate gene expression in response to their own life cycles and environmental conditions.
- Ex, human insulin is only required when the glucose
level in the blood is high. - Similarly, the E. coli enzyme that facilitates the breakdown of lactose is only transcribed & translated when the E. coli bacteria are exposed to lactose.
- The optimal functioning of an organism requires that genes be turned on and off as they are needed. Even though each cell contains the entire genome of an organism, cells know which genes to express & when.
- Intricate systems have evolved to fine-tune gene expression in both prokaryotes & eukaryotes.
insulin is
a hormone produced in the pancreas that lowers the blood glucose level by promoting the uptake of glucose by the body cells
