Campbell Biology Chapter 17 Review: From Gene to Protein

17.1 Genes, proteins and the genetic code

pp. 325–331

On the AP exam? Yes

Topics 6.3 and 6.4 cover this: know the DNA → RNA → protein flow, template versus coding strand, reading codons with a codon chart, and why a shared code points to common ancestry. You won't need the historical experiments or any codon except AUG from memory.

In the course: Topic 6.3 Transcription and RNA Processing, Topic 6.4 Translation, Topic 6.1 DNA and RNA Structure (notes, videos and more questions)

Key points

Genes shape traits mainly through the proteins they code for. Early work with mutant microbes showed that breaking one gene could block one step in a chemical pathway, which led to the idea that each gene makes one enzyme.
That idea was refined twice. Not every protein is an enzyme, and many proteins are built from several different polypeptide chains, each coded by its own gene. Some genes make RNAs that are never translated, such as tRNA and rRNA.
Information moves in one main direction, DNA → RNA → protein. Transcription copies a gene into RNA in the same nucleotide 'language'. Translation switches languages, turning a string of bases into a string of amino acids at the ribosome.
Bacteria have no nucleus, so ribosomes can start on an mRNA before it is even finished. In eukaryotes, transcription happens in the nucleus, the pre-mRNA is processed, and translation happens out in the cytoplasm.
Only one DNA strand of a gene, the template strand, is read. The finished mRNA has the same sequence as the other strand (the coding strand), with U in place of T.
The code uses three-base words called codons. Four bases in three positions give 4³ = 64 codons: 61 name amino acids, 3 mean stop, and AUG means both 'start' and methionine. Several codons can share an amino acid, but no codon ever has two meanings.
Codons are read one after another with no gaps or overlaps, so where reading starts sets the reading frame. Nearly every organism uses the same code, which is strong evidence that all life shares a common ancestor.

Key terms (13)

gene expression: The whole process of using a gene's information to make its product, usually a protein, sometimes just an RNA.
transcription: Making an RNA copy of a gene, using one DNA strand as the pattern.
translation: Building a polypeptide by reading the codons of an mRNA, one amino acid per codon.
messenger RNA (mRNA): The RNA copy of a protein-coding gene. It takes the gene's instructions out to the ribosomes, where the protein is built.
primary transcript: The RNA exactly as it first comes off the DNA, before any editing. For a eukaryotic protein-coding gene it's called pre-mRNA.
template strand: The DNA strand RNA polymerase actually reads. The RNA it makes is complementary to it.
coding strand: The DNA strand that is not read. Its sequence matches the mRNA, except it has T wherever the mRNA has U. Also called the nontemplate strand.
codon: A group of three bases in mRNA that stands for one amino acid or for 'stop'.
reading frame: The way a sequence is divided into consecutive groups of three. Start one base off and every codon after that changes.
start codon: AUG, the codon where translation begins. It also codes for methionine, so new chains start with that amino acid.
stop codon: One of three codons (UAA, UAG, UGA) that names no amino acid and tells the ribosome the chain is finished.
one gene–one polypeptide hypothesis: The idea that each gene directs the making of one polypeptide chain. It updated the older 'one gene, one enzyme' idea, though genes for RNAs like tRNA are exceptions.
central dogma: Shorthand for the usual direction of information flow in cells: DNA → RNA → protein.

Check yourself: 17.1 Genes, proteins and the genetic code

4 questions on 17.1 Genes, proteins and the genetic code. Pick an answer to see if you got it, and why.

Question 1 of 4

Suppose a lab-built organism used six different kinds of bases in its nucleic acids instead of four, and its genetic code used two-base codons. How many different codons would be possible, and would that be enough to give each of the 20 amino acids its own codon plus at least one stop signal?

Question 2 of 4

A short stretch of a gene's template strand, written from its 5′ end, reads 5′-TTAAACTGGCAT-3′. Which mRNA is transcribed from it?

Question 3 of 4

Researchers insert a gene from a heat-loving archaeon into yeast cells. The yeast cells make the archaeal enzyme with exactly the same amino acid sequence that the archaeon itself makes. Which conclusion does this result best support?

Question 4 of 4

A soil bacterium makes a purple pigment in three steps: colorless compound W is converted to X, X to Y, and Y to the pigment, each step by a different enzyme. Three white mutant strains each have one defective gene. Strain 1 builds up compound X, strain 2 builds up W, and strain 3 builds up Y. Which strain most likely lacks a working enzyme for the X → Y step?

0 of 4 answered

17.2 Transcription: copying a gene into RNA

pp. 331–334

On the AP exam? Yes

Topic 6.3 covers RNA polymerase building RNA 5′ → 3′ from the template strand, and Topic 6.6 covers promoters and transcription factors. The names of the three eukaryotic polymerases and the details of how transcription ends are background.

In the course: Topic 6.3 Transcription and RNA Processing, Topic 6.6 Gene Expression and Cell Specialization (notes, videos and more questions)

Key points

RNA polymerase pulls the two DNA strands apart and links RNA nucleotides that pair with the template strand: A with U, T with A, G with C and C with G.
Like DNA polymerase, it only adds to the 3′ end, so the RNA grows 5′ → 3′ while the template is read 3′ → 5′. Unlike DNA polymerase, it can start a new chain without a primer.
The promoter is a stretch of DNA at the start of a gene where RNA polymerase attaches. The promoter's position and direction decide where copying starts and which strand is the template, so neighboring genes can use opposite strands.
Bacterial RNA polymerase recognizes promoters by itself. In eukaryotes, transcription factors must bind the promoter first (often at a short sequence called the TATA box) and then guide RNA polymerase into place, forming an initiation complex.
Transcription has three stages: initiation, elongation and termination. Behind the moving enzyme, the finished stretch of RNA separates from the template and the two DNA strands pair up again.
Many polymerases can travel along the same gene one behind another, so a cell can make lots of RNA, and then lots of protein, from one gene quickly.
Bacteria stop at a DNA sequence called a terminator. In eukaryotes, the RNA is cut free a short distance after the polymerase copies a signal sequence, and the pre-mRNA then goes on to be processed.

Key terms (10)

RNA polymerase: The enzyme that builds RNA from a DNA template. It opens the helix as it goes and needs no primer.
promoter: A DNA sequence at the start of a gene where RNA polymerase (with help in eukaryotes) attaches and begins transcription.
terminator: In bacteria, the DNA sequence at the end of a gene that signals RNA polymerase to let go.
transcription unit: The full stretch of DNA that gets copied into one RNA molecule.
upstream and downstream: Directions along a gene. Downstream is the way RNA polymerase travels; upstream is back toward the promoter.
transcription factor: A protein that binds DNA and helps control whether RNA polymerase starts copying a gene. Eukaryotes need several just to begin.
TATA box: A short DNA sequence rich in T and A, found in many eukaryotic promoters, that a transcription factor recognizes.
transcription initiation complex: The group of transcription factors plus RNA polymerase assembled on a promoter, ready to start making RNA.
elongation: The middle stage of transcription, when RNA polymerase moves along the template adding nucleotides to the RNA's 3′ end.
RNA polymerase II: The eukaryotic RNA polymerase that makes pre-mRNA. Other eukaryotic polymerases make RNAs like rRNA and tRNA.

Check yourself: 17.2 Transcription: copying a gene into RNA

4 questions on 17.2 Transcription: copying a gene into RNA. Pick an answer to see if you got it, and why.

Question 1 of 4

Growing cells are given a radioactively labeled nitrogenous base. Soon afterward, the label shows up in newly made RNA but never in newly made DNA. Which base was most likely labeled?

Question 2 of 4

In a drawing of a gene, the template strand runs with its 3′ end on the left and its 5′ end on the right. Which statement correctly describes transcription of this gene?

Question 3 of 4

In a test tube, purified eukaryotic RNA polymerase is mixed with RNA nucleotides and DNA containing a gene and its promoter. Very little RNA is made. When a mix of nuclear proteins that contains no RNA polymerase is added, RNA production rises about 50-fold, and the new RNAs all start at the same position. What does the protein mix most likely provide?

Question 4 of 4

Genes P and Q sit next to each other on the same chromosome. The mRNA of gene P has the same sequence as DNA strand 1 (with U in place of T), while the mRNA of gene Q has the same sequence as strand 2. What best explains this?

0 of 4 answered

17.3 RNA processing: cap, tail and splicing

pp. 334–336

On the AP exam? Yes

Topic 6.3 expects the poly-A tail (stability), the 5′ cap (called the GTP cap in AP materials, for ribosome recognition), intron removal and alternative splicing. Spliceosome parts and exon shuffling are background; ribozymes connect to the RNA world idea in Topic 7.12.

In the course: Topic 6.3 Transcription and RNA Processing, Topic 7.12 Origins of Life on Earth (notes, videos and more questions)

Key points

In eukaryotes, the first RNA copy of a protein-coding gene (pre-mRNA) is edited in the nucleus before it is allowed out. Both ends are changed, and pieces in the middle are removed.
The 5′ end gets a cap made from a modified G nucleotide (AP materials call it the GTP cap). The cap helps ribosomes recognize the mRNA and attach to it.
The 3′ end gets a poly-A tail, a long run of A nucleotides. The tail makes the mRNA more stable, so it lasts longer before enzymes break it down. Cap and tail also help the mRNA leave the nucleus.
Most eukaryotic genes are split. Coding pieces called exons are separated by noncoding pieces called introns. Both are transcribed, but splicing cuts out the introns and joins the exons into one continuous message.
Splicing is carried out by a spliceosome, a large complex of proteins and small RNAs. Its RNAs pair with sequences at the ends of each intron and help catalyze the cuts. Some introns can even cut themselves out with no protein at all.
RNAs that speed up reactions are called ribozymes. They work because a single RNA strand can fold into a precise shape. They also support the idea that early life used RNA both to store information and to catalyze reactions.
In alternative splicing, one gene's pre-mRNA is spliced in more than one way, so one gene can code for several related proteins. Exons often match separate working parts of a protein, so mixing exons can mix those parts.

Key terms (13)

RNA processing: The editing a eukaryotic pre-mRNA gets in the nucleus: adding a cap and tail and splicing out introns.
pre-mRNA: The unedited RNA copy of a eukaryotic protein-coding gene, still containing its introns.
5′ cap: A modified G nucleotide added to the front (5′ end) of an mRNA. It helps ribosomes recognize the mRNA. AP materials call it the GTP cap.
poly-A tail: A long string of A nucleotides tacked onto an mRNA's 3′ end. It makes the mRNA last longer in the cytoplasm.
untranslated region (UTR): A stretch at either end of a mature mRNA that stays in the message but isn't turned into amino acids.
intron: A noncoding piece of a gene that is copied into pre-mRNA but cut out before the mRNA leaves the nucleus.
exon: A piece of a gene that stays in the mature mRNA. Most of it is translated into protein.
RNA splicing: Cutting the introns out of pre-mRNA and joining the exons together.
spliceosome: A large complex of proteins and small RNAs that finds intron boundaries and splices pre-mRNA.
ribozyme: An RNA molecule that acts as a catalyst, speeding up a specific reaction the way an enzyme does.
alternative splicing: Joining a gene's exons in different combinations, so one gene can give several related proteins.
domain: A part of a protein that folds on its own and does a particular job, such as binding DNA. Different exons often code for different domains.
exon shuffling: The mixing of exons within or between genes over evolutionary time, which can create proteins with new combinations of domains.

Check yourself: 17.3 RNA processing: cap, tail and splicing

4 questions on 17.3 RNA processing: cap, tail and splicing. Pick an answer to see if you got it, and why.

Question 1 of 4

Researchers make two versions of the same mRNA that are identical except that one has a poly-A tail and the other does not. They inject equal amounts of each into the cytoplasm of cultured human cells and measure how much of each remains (invented data). Time after injection (h) | mRNA with tail remaining (%) | mRNA without tail remaining (%) 0 | 100 | 100 2 | 94 | 51 4 | 88 | 26 6 | 83 | 13 Which conclusion do the data best support?

Question 2 of 4Calculator allowed

A eukaryotic gene's pre-mRNA is 8,400 nucleotides long (not counting the cap and tail). It has four exons, of 310, 420, 275 and 395 nucleotides, with the untranslated regions counted as part of the first and last exons. About what percentage of the pre-mRNA is removed by splicing?

Question 3 of 4

A gene has five exons. In liver cells its mature mRNA contains exons 1, 2, 3 and 5. In muscle cells its mature mRNA contains exons 1, 2, 4 and 5. Which statement best explains this?

Question 4 of 4

A mutation changes the bases at the end of intron 2 of a gene, so the spliceosome no longer recognizes that intron, and it stays in the mature mRNA. The intron is 140 nucleotides long. What is the most likely effect on the protein?

0 of 4 answered

17.4 Translation: tRNA, ribosomes and building the chain

pp. 337–344

On the AP exam? Yes

Topic 6.4 is the core: start at AUG, read codons, tRNAs bring amino acids, stop at a stop codon, and ribosomes work in the cytoplasm and on rough ER (Topic 2.1). The exam leaves out the names of translation enzymes and factors; the ribosome's A, P and E sites, wobble and the SRP are useful background.

In the course: Topic 6.4 Translation, Topic 6.3 Transcription and RNA Processing, Topic 2.1 Cell Structure and Function (notes, videos and more questions)

Key points

Translation needs an mRNA, ribosomes, tRNAs loaded with amino acids, energy (from GTP) and helper proteins. The ribosome reads the mRNA one codon at a time, and tRNAs bring the matching amino acids.
A tRNA is a short RNA folded into an L shape. One end has an anticodon that pairs with a codon; the other end holds one specific amino acid. Matching enzymes load each tRNA with the right amino acid, using energy from ATP.
Base pairing at the third position of a codon is looser than at the first two (wobble), so some tRNAs can read more than one codon. That's why a cell needs fewer than 61 kinds of tRNA.
A ribosome has a small and a large subunit, each built of rRNA and proteins. It has a site for the mRNA and three sites for tRNAs: A (the incoming tRNA), P (the tRNA holding the growing chain) and E (the exit). rRNA, not protein, forms the peptide bonds.
Initiation: the small subunit, the mRNA and a tRNA carrying methionine meet at the start codon AUG, which sets the reading frame, and then the large subunit joins. Elongation: amino acids are added one at a time as the mRNA moves along by one codon. Termination: a stop codon brings in a release factor instead of a tRNA, and the chain is let go.
Chains are built from the amino end to the carboxyl end. Several ribosomes can read one mRNA at the same time (a polyribosome), turning out many copies quickly.
Afterward, chains fold (sometimes with help from chaperone proteins) and may be modified, trimmed or joined to other chains. Proteins headed for secretion, membranes or the endomembrane system begin with a signal peptide that moves their ribosome onto the rough ER; most others are finished by free ribosomes in the cytosol.

Key terms (13)

transfer RNA (tRNA): A small folded RNA that carries one kind of amino acid to the ribosome and matches it to the right codon.
anticodon: The three bases on a tRNA's loop that match up with an mRNA codon by base pairing.
aminoacyl-tRNA synthetase: An enzyme that attaches the correct amino acid to its tRNAs, using ATP. There is one for each amino acid.
wobble: Relaxed pairing at a codon's third position, which lets one tRNA read more than one codon.
ribosomal RNA (rRNA): The RNA that forms most of a ribosome's structure and catalyzes peptide bond formation. It's the most abundant RNA in a cell.
A, P and E sites: The three tRNA spots on a ribosome: A takes the incoming loaded tRNA, P keeps hold of the tRNA carrying the chain built so far, and E is where empty tRNAs leave.
peptide bond: The covalent bond joining one amino acid's carboxyl group to the next amino acid's amino group.
release factor: A protein that fills the A site when a stop codon arrives and triggers release of the finished chain.
polyribosome: Several ribosomes reading the same mRNA at the same time, each making its own copy of the polypeptide.
post-translational modification: A change made to a protein after it's built, such as adding sugars or phosphates or cutting the chain.
signal peptide: A short stretch of amino acids at the start of a new chain that sends it, and its ribosome, to the ER.
signal-recognition particle (SRP): A protein–RNA complex that grabs a signal peptide as it comes out of the ribosome and guides the ribosome to the ER membrane.
free and bound ribosomes: Free ribosomes float in the cytosol and mostly make cytosolic proteins. Bound ribosomes sit on the rough ER and make secreted, membrane and endomembrane proteins. They're the same ribosomes and can switch roles.

Check yourself: 17.4 Translation: tRNA, ribosomes and building the chain

4 questions on 17.4 Translation: tRNA, ribosomes and building the chain. Pick an answer to see if you got it, and why.

Question 1 of 4

An mRNA codon reads 5′-CAU-3′. Which tRNA anticodon pairs with it?

Question 2 of 4

An mRNA has the sequence 5′-GAAUGCACGAUAAAUGACCG-3′. Translation begins at the first AUG. Use these codon meanings: AAA = Lys, AAU = Asn, ACC = Thr, ACG = Thr, AUA = Ile, AUG = Met (start), CAC = His, CCG = Pro, GAA = Glu, GAC = Asp, GAU = Asp, UGA = stop, UGC = Cys. Which polypeptide is made?

Question 3 of 4

A newly discovered compound binds the large subunit of bacterial ribosomes at the spot where peptide bonds are formed. It does not bind eukaryotic ribosomes. What is the most likely effect on a bacterium treated with it?

Question 4 of 4

A species uses all 61 codons that specify amino acids, but its cells make only 41 different kinds of tRNA. How can every codon still be read?

0 of 4 answered

17.5 Small mutations and their effects on proteins

pp. 344–346

On the AP exam? Yes

Topic 6.7 covers point mutations, silent and nonsense changes, frameshifts from insertions or deletions, and mutations caused by copying errors, radiation and chemicals. 'Missense' isn't a CED term but is handy, and you won't need to know specific disorders.

In the course: Topic 6.7 Mutations (notes, videos and more questions)

Key points

A mutation is a change in a cell's DNA sequence. Mutations are the original source of new alleles, so they supply the variation evolution works with. In animals, only mutations in the cells that make eggs or sperm can be passed to offspring.
Small-scale mutations change one or a few base pairs. A point mutation, or substitution, swaps one base pair for another.
A substitution can be silent (the new codon means the same amino acid, thanks to the code's redundancy), missense (a different amino acid) or nonsense (the codon becomes a stop codon, so the protein ends early). A nonsense change usually leaves the protein unable to work.
A missense change can be harmless if the new amino acid is similar or sits in an unimportant spot, but it can wreck a protein if it hits an active site or a key fold.
Inserting or deleting bases in a number that isn't a multiple of three causes a frameshift: every codon after the change is misread, usually until an early stop codon. Adding or removing exactly three bases adds or removes one amino acid instead.
Mutations come from uncorrected mistakes in DNA copying or repair, and from mutagens. Physical mutagens include X-rays and UV light. Chemical mutagens may change a base so it pairs wrongly, slip in as a fake base during copying, or squeeze between base pairs. Most cancer-causing chemicals are mutagens.
Whether a mutation helps, hurts or does nothing depends on how it changes the protein and on the environment the organism lives in.

Key terms (12)

mutation: A change in the base sequence of an organism's DNA (or of a virus's genetic material). It may be harmful, helpful or neutral.
point mutation: A change in a single base pair of a gene.
substitution: Replacing one base pair with a different one. It doesn't change the length of the gene.
silent mutation: A substitution that turns a codon into a synonym, another codon meaning that amino acid, so the protein doesn't change.
missense mutation: A substitution that swaps one amino acid for another in the protein.
nonsense mutation: A substitution that turns an amino acid codon into a stop codon, so the protein comes out too short.
insertion: Extra base pairs added into a gene.
deletion: Base pairs lost from a gene.
frameshift mutation: An insertion or deletion of a number of bases that isn't a multiple of three, which changes how every later codon is grouped.
spontaneous mutation: A mutation that happens without any outside cause, such as an uncorrected copying mistake during DNA replication.
mutagen: Anything that raises the rate of mutation, such as UV light, X-rays or certain chemicals.
carcinogen: Something that causes cancer. Most carcinogens work by damaging DNA, so most are also mutagens.

Check yourself: 17.5 Small mutations and their effects on proteins

4 questions on 17.5 Small mutations and their effects on proteins. Pick an answer to see if you got it, and why.

Question 1 of 4

In a gene's mRNA, the codon GCA codes for the amino acid alanine. Which single-base substitution in this codon is most likely to be a silent mutation?

Question 2 of 4

A gene codes for an enzyme 350 amino acids long. Researchers study four mutant alleles, each caused by one mutation (invented data). Allele | Protein length (amino acids) | Change in amino acid sequence | Enzyme activity (% of normal) 1 | 350 | none | 100 2 | 350 | one amino acid replaced | 14 3 | 349 | one amino acid missing | 88 4 | 121 | none before the end | 0 Which allele most likely carries a nonsense mutation?

Question 3 of 4

The coding part of an mRNA reads 5′-AUG CCA GUA AGC UUC GGA UAA-3′, which codes for Met–Pro–Val–Ser–Phe–Gly. A mutation deletes the G at the start of the third codon. Use these codon meanings: AUG = Met, CCA = Pro, GUA = Val, AGC = Ser, UUC = Phe, GGA = Gly, GCU = Ala, UCG = Ser, GAU = Asp, UAA = stop. What polypeptide does the mutant mRNA code for?

Question 4 of 4

Bacteria are spread on plates containing different amounts of a chemical, and the number of mutant colonies is counted (invented data). Chemical per plate (μg) | Mutant colonies per 10⁸ cells 0 | 3 1 | 18 2 | 35 4 | 71 Which conclusion is best supported?

0 of 4 answered

17.6 Gene expression across the domains, and what a gene is

pp. 346–348

On the AP exam? Yes

Topic 6.4 tests that prokaryotes translate mRNA while it's still being transcribed, and Topics 2.9 and 2.10 cover the nucleus as a compartment and how prokaryotes differ. The details of how archaea compare are background, and Topic 6.6 picks up the idea that cells express only some of their genes.

In the course: Topic 6.4 Translation, Topic 2.9 Cell Compartmentalization, Topic 2.10 Origins of Cell Compartmentalization, Topic 6.6 Gene Expression and Cell Specialization, Topic 7.9 Phylogeny (notes, videos and more questions)

Key points

Bacteria, archaea and eukaryotes all transcribe and translate genes in the same basic way, but their machinery differs in the details.
Archaea sit in between. Like bacteria, they have no nucleus, yet their RNA polymerase and the proteins that help start transcription look much more like the eukaryotic versions. Their ribosomes and translation show a mix of bacterial and eukaryotic features.
The biggest difference is the nucleus. Without one, bacteria (and most likely archaea) can translate an mRNA while it's still being transcribed, so a new protein can appear very quickly.
In eukaryotes the nuclear envelope keeps transcription and translation apart. That leaves room for RNA processing, which is one more place where cells can control which genes make protein.
The meaning of 'gene' has grown over time: a unit of inheritance, then a place on a chromosome, then a DNA sequence, then a sequence coding for a polypeptide. Today biologists treat a gene as any stretch of DNA the cell can use to make a working product, whether that's a polypeptide or an RNA that does a job itself.
Each cell expresses only some of its genes, which is how cells with the same DNA end up so different. The next chapter is about how that is controlled.

Key terms (8)

bacteria: One of the two domains of prokaryotes. They have no nucleus, and they transcribe and translate in the same compartment.
archaea: The other domain of prokaryotes. They have no nucleus, but their transcription machinery looks a lot like that of eukaryotes.
eukaryote: An organism whose cells have a nucleus and other membrane-bound organelles. It's the third domain of life.
coupled transcription and translation: In prokaryotes, ribosomes start translating an mRNA while RNA polymerase is still making it.
nuclear envelope: The double membrane around the nucleus. It separates transcription and RNA processing from translation.
gene (modern definition): Any stretch of DNA the cell can use to make a working product, whether that's a polypeptide or an RNA such as tRNA or rRNA.
regulatory sequence: DNA that isn't turned into the product but controls when or how much a gene is used, like a promoter.
functional RNA: An RNA whose job is done by the RNA itself, without being translated, such as rRNA, tRNA or snRNA.

Check yourself: 17.6 Gene expression across the domains, and what a gene is

4 questions on 17.6 Gene expression across the domains, and what a gene is. Pick an answer to see if you got it, and why.

Question 1 of 4

Which of these can happen in a bacterial cell but not in a eukaryotic cell?

Question 2 of 4

Researchers switch on a similar stress-response gene in a bacterium and in yeast and record when its mRNA and its protein are first detected (invented data). Organism | First mRNA detected (min) | First protein detected (min) Bacterium | 0.5 | 1 Yeast | 1 | 9 What best explains the much longer gap between mRNA and protein in yeast?

Question 3 of 4

When gene sequences are compared, an archaeal RNA polymerase turns out to be more similar to a human RNA polymerase than to a bacterial one. Which conclusion does this most directly support?

Question 4 of 4

Which DNA segment counts as a gene under today's definition, but not under the older idea that a gene is a sequence that codes for one polypeptide?

0 of 4 answered