Transcription, Translation & Protein Synthesis

Genes are used by the cell to synthesize proteins. In order to produce a protein, genes must be transcribed and

translated by the machinery of the cell.

Gene Expression

The use of genes to produce proteins is called gene expression. Two main processes are involved in gene

expression: transcription and translation.

1. During transcription, DNA in the nucleus of a cell is copied into messenger RNA molecules.

2. The messenger RNA then moves into the cell's cytoplasm and attaches to a ribosome, where it

is translated into a protein.

Central Dogma of Molecular Biology

Once DNA is transcribed into messenger RNA and translated into a protein, the process cannot be

reversed. That is, information cannot be transferred from the protein back to the nucleic acid. This is the

central dogma of molecular biology.

Transcription

The sequence of nitrogenous bases in a gene provides the genetic instructions needed to construct a

protein. Transcription occurs when a series of chemical signals within the cell causes the gene for a specific

protein to "turn on," or become active. During transcription, a segment of DNA is transcribed, or copied, to

produce a complementary strand of messenger RNA (mRNA). Transcription occurs in the nucleus of the

cell.

The three main processes that occur during transcription are described below.

1. Initiation — During initiation, enzymes bind to a DNA sequence and unwind the double helix to expose a

strand of nucleotides.

2. Elongation — As the DNA molecule unwinds, an enzyme called RNA polymerase pairs complementary RNA

nucleotides with the DNA nucleotides on one of the exposed strands. Adenine (A) on DNA pairs with uracil

(U) on RNA, cytosine (C) pairs with guanine (G), and thymine (T) pairs with adenine (A). For example, if the

DNA strand reads 'ACG,' the complementary RNA strand would read 'UGC.'

3. Termination — Once the gene is transcribed, the new RNA molecule breaks away and the DNA strands are

wound back together.

During transcription, a DNA molecule is unwound, and an RNA strand is synthesized using an exposed DNA strand as a template.

Now that transcription is completed, the RNA molecule moves to the cytoplasm of the cell, where it will be

translated into a protein.

Translation

Translation occurs in cell organelles called ribosomes. Ribosomes contain proteins and a type of RNA

called ribosomal RNA (rRNA). It is a major component of cellular ribosomes and it can act as a catalyst in

chemical reactions.

During translation, an mRNA strand is used to synthesize a chain of amino acid residues called a

polypeptide. When mRNA leaves the nucleus, it travels until it reaches a ribosome. Each three-base segment

of the mRNA strand is called a codon. A polypeptide is formed by matching the anticodon of a transfer

RNA (tRNA) molecule—each of which carries a specific amino acid—to the corresponding codon on the

mRNA strand. Later, the polypeptide will fold into a functional protein.

The steps of translation are shown below.

1. A ribosome attaches to the 5' end of the mRNA strand.

2. Transfer RNA (tRNA) molecules carrying amino acids approach the ribosome.

3. The tRNA molecule whose anticodon corresponds to the first codon on the mRNA strand quickly attaches at

the ribosome.

4. A new tRNA molecule carrying another amino acid attaches to the next codon on the mRNA strand.

5. As amino acids are added next to each other, peptide bonds form between them.

6. The previous tRNA molecule detaches from the mRNA strand and departs from the ribosome, leaving its amino

acid behind.

7. The chain of amino acid residues continues to grow until the ribosome reaches a stop codon at the 3' end of the

mRNA strand. This signals that no more amino acids should be added. The result is a polypeptide.

During translation, tRNA molecules bring amino acids to the ribosome, where they are linked together by peptide bonds to form a

polypeptide.

Codons & the Genetic Code

How exactly does a sequence of nucleotides result in a chain of amino acids? The answer lies in the genetic

code (or triplet code), which determines which amino acid corresponds with each three-base codon.

Because codons contain three nitrogenous bases, the genetic code could theoretically produce 64 amino acids

(four possible bases in the first position multiplied by four in the second position and four in the

third). However, most amino acids are coded for by more than one codon. Consequently, the genetic code can

only produce 20 different amino acids. While this phenomenon may seem inefficient at first glance,

such redundancies often allow cells to produce the correct protein even if a gene has been affected by a

mutation. For example, suppose a mutation caused a CUU codon to change to CUC, CUA, or CUG. In all

three cases, the correct amino acid, leucine, would be produced.

The genetic code is shown in the table above. To determine which amino acid corresponds to a codon, find the row matching the first

RNA base, the column matching the second base, and the specific codon containing the third base.

An mRNA transcript must have distinct starting and ending points, which are indicated by a start

codon (AUG, which codes for methionine) and a stop codon (UAA, UAG, or UGA). As with all nucleic acid

sequences, codons are transcribed and translated in the 5'→3' direction.

Gene Sequence Determines Protein Structure

The final structure of a protein is determined by the sequence of its amino acid residues. In turn, the amino

acid sequence is determined by the original gene sequence that was transcribed and translated. Once a chain

of amino acid residues is produced, it undergoes a series of folds, bends, and twists to arrive at its final

structure. There are four distinct levels of protein structure:

Primary Structure — The primary structure of a protein is simply its linear amino acid sequence

(polypeptide). Each type of protein has a unique primary structure that distinguishes it from every other

protein.

Secondary Structure — In certain places, hydrogen bonding causes the polypeptide to twist into structures

called alpha helices or fold into structures called beta sheets. Most proteins contain both of these structures.

Tertiary Structure — Tertiary structure refers to the tightly compacted form of a single protein

molecule. This three-dimensional structure is primarily determined by the hydrophobic (water repelling)

amino acid residues in the polypeptide, which naturally face inward, while hydrophilic amino acid residues

face outward.

Quaternary Structure — Many proteins are actually composed of multiple subunits. When these subunits

(each of which is translated separately) come together, the protein achieves quaternary structure. As shown in

the diagram, the oxygen-carrying protein hemoglobin is made up of four distinct subunits.

Proteins and Life

The process of protein synthesis produces the proteins that carry out the functions needed for life. These

functions include breaking down food, protecting a body from bacteria and viruses, and moving

muscles. This is a small list of the life functions carried out by proteins. These functions are performed by

specialized cells, which synthesize the specific proteins needed to perform a function. The genes that are

expressed by a specialized cell determine the proteins that the cell can synthesize. The structure of a protein

determines its function. Specialized cells and the proteins that they synthesize are essential for life.

Comparing DNA & RNA

Both DNA and RNA play a role in storing and transmitting cellular information, but there are key differences in

the structures and specific functions of the two types of nucleic acids.

DNA Structure vs. RNA Structure

The diagram below shows several key structural differences between DNA and RNA. The nitrogenous bases

found in each molecule are also shown.

DNA and RNA share key similarities, but there are also significant structural differences between them.

Molecular Shape

DNA is composed of two nucleotide chains wound together into a double helix.

RNA is composed of one nucleotide chain in a single helix.

Nitrogenous Bases

DNA contains the nitrogenous bases cytosine (C), guanine (G), adenine (A), and thymine (T).

RNA contains the nitrogenous bases cytosine (C), guanine (G), adenine (A), and uracil (U).

Nucleotide Components

DNA nucleotides each consist of a nitrogenous base, a phosphate group, and a deoxyribose five-carbon sugar.

RNA nucleotides each consist of a nitrogenous base, a phosphate group, and a ribose five-carbon sugar.

DNA Function vs. RNA Function

Together, DNA and RNA contain all the instructions a cell needs to carry out its life functions. However,

they differ in function. Several of these key differences are discussed below.

DNA is the ultimate source of genetic information in the cell. DNA is used as a template to produce RNA

during the process of transcription. Before a cell divides, it copies its DNA so its genetic information can be

passed on to new cells.

RNA has several unique roles in the cell. There are three main types of RNA:

mRNA, or messenger RNA, is used to produce proteins. It carries information from the nucleus to

ribosomes.

tRNA, or transfer RNA, attaches amino acids to the growing polypeptide chain during translation.

rRNA, or ribosomal RNA, is the major structural component of cellular ribosomes.

Base Pairing Rules

DNA and RNA molecules each contain a unique sequence of nucleotides that ultimately determines their

function in the cell. Even though there are only four nitrogenous bases in a strand of DNA or RNA, these

bases can be ordered in innumerable ways. This enables the production of the incredible variety of

substances, such as proteins, enzymes, and RNA structures, that support an organism's life processes.

There are two classes of nitrogenous bases: purines and pyrimidines.

Purines Pyrimidines

Nitrogenous bases are held together by hydrogen bonds that occur only between complementary bases. Like

puzzle pieces, a pyrimidine will pair with only one specific purine, and a purine will pair with only one

specific pyrimidine (with the exception of adenine, which pairs with thymine in DNA and uracil in RNA).

This is the basis of the genetic code that all living things share.

Adenine and thymine bond to one another, and cytosine and guanine bond to one another.

The base pairing rules for DNA and RNA are shown below.

DNA Base Pairing Rules

adenine

thymine

thymine

adenine

cytosine

guanine

guanine

cytosine

RNA Base Pairing Rules

adenine

uracil

uracil

adenine

cytosine

guanine

guanine

cytosi

Gene Expression Essentials

Click on the image below to explore gene expression and the factors that influence protein synthesis.

PhET Interactive Simulations

University of Colorado

https://phet.colorado.edu

Transcription & Translation Question 1 . The picture below shows the process of transcription.

During transcription, enzymes bind to a molecule of DNA. Then, the enzymes unwind and separate the DNA's double helical strands. As the molecule unwinds, complementary nucleotides pair with one of the DNA strands to form

A.

a protein molecule.

B.

an identical strand of DNA.

C.

an RNA molecule.

D.

a DNA polymerase.

Question 2 . The DNA sequences that make up the genetic code of an organism determine which traits the organism will exhibit. How are the instructions coded by DNA translated into an organism's physical traits?

A.

DNA sequences both code genetic instructions within an organism and express an organism's

physical traits.

B.

DNA sequences that code for genetic instructions attach to phosphate groups that express an

organism's physical traits.

C.

Instructions coded by DNA sequences are translated into nucleotides which express an organism's

physical traits.

D.

Instructions coded by DNA sequences are translated into proteins which express an organism's

physical traits.

Question 3 .

Drag the nitrogenous bases to the correct locations on the image. Each base can be used more than once, but not all bases must be used.

Sections of a DNA strand and an incomplete RNA strand are shown in the diagram below.

During RNA transcription, a DNA template strand is used to produce an RNA strand. Simulate the process of transcription by adding the remaining complementary nitrogenous bases to the RNA strand.

Question 4 . The type of RNA that transfers amino acids to growing polypeptide chains during translation is known as

A.

tRNA.

B.

mRNA.

C.

rRNA.

D.

pRNA.

Question 5 .

Which of the following is true regarding the process shown above?

A.

The process shown above is known as transcription and involves the production of proteins from

DNA.

B.

The process shown above is known as translation and involves the production of proteins from RNA.

C.

The process shown above is known as cloning and involves the production of RNA from protein

molecules.

D.

The process shown above is known as replication and involves the production of DNA from RNA.

Question 6 . The model shows transcription and translation occurring in a eukaryotic cell.

What is a limitation of the model in representing the steps of transcription and translation?

A.

It does not demonstrate that DNA polymerase is involved in transcribing DNA into RNA.

B.

It does not show that the ribosome stops translating when it encounters a stop codon.

C.

It does not suggest that transcription and translation are spatially separated in the cell.

D.

It does not indicate that a particular RNA codon corresponds to a specific amino acid.

Question 7 . Proteins play a vital role in all cells. In fact, cells need thousands of proteins in order to function properly. What primarily directs the synthesis of proteins?

A.

the genetic information stored in DNA

B.

the folding patterns of strands of RNA

C.

the energy that is stored in mitochondria

D.

the cytoplasm found throughout a cell

Question 8 . Nadia made a table that describes the function of the three main types of RNA. She labeled the RNA types with 1, 2, and 3. She also labeled the RNA types in an image that shows the structure of each type of RNA and models the process of protein synthesis. She used the labels X, Y, and Z in the image.

Which combination correctly identifies a type of RNA from the table and the image?

A.

X and 1 represent rRNA

B.

Y and 2 represent mRNA

C.

Y and 3 represent tRNA

D.

Z and 2 represent mRNA

Question 9 . Lactase is an enzyme produced by cells in the small intestine. Lactase helps break down lactose, a sugar in milk. In humans, lactase is encoded by a single gene. Transcription of this gene produces a primary transcript. The primary transcript is processed and then exported out of the nucleus. In the cytoplasm, the transcript is translated to synthesize lactase. This process is summarized in the diagram below.

What are the roles of RNA molecules during the step labeled with an X in the diagram?

A.

mRNA rRNA tRNA

produces amino

acids to make

lactase

makes up the

ribosome, which

synthesizes lactase

carries nucleotides

to the mRNA copy

of the lactase gene

B.

mRNA rRNA tRNA

forms peptide bonds

between amino acids

produces amino

acids from an

RNA template

contains instructions

to synthesize lactase

C.

mRNA rRNA tRNA

carries instructions

to build lactase

makes up part of

the ribosome, which

decodes mRNA

carries amino

acids to the growing

lactase molecule

D.

mRNA rRNA tRNA

contains a copy of

the lactase gene

folds the amino acid

chain to produce

the lactase molecule

forms peptide bonds

between amino

acids in lactase

Question 10 . The diagram below shows a polypeptide chain folding to become a protein with a compact, 3-dimensional structure.

The final shape of a protein is determined by the sequence of its amino acid residues. What determines this amino acid sequence?

A.

the sequence of nucleotides in the polypeptide

B.

the sequence of nucleotides in the DNA template

C.

the number of nucleotides in the DNA template

D.

the number of nucleotides in the polypeptide

Answers

1. C

2. D

3. --

4. A

5. B

6. B

7. A

8. C

9. C

10. B

Explanations

1. During transcription, enzymes bind to a molecule of DNA. Then, the enzymes unwind and separate the DNA's double

helical strands. As the molecule unwinds, complementary RNA nucleotides temporarily pair with the nucleotides on

one of the DNA strands to form an RNA molecule. Once base pairing is complete, the new RNA molecule (mRNA)

breaks away from the DNA strands, and the DNA strands reattach to each other.

RNA is very similar to DNA, except RNA contains the nitrogenous base uracil (U) rather than thymine (T), which is

present in DNA.

2. DNA is a nucleic acid that contains the genetic instructions, or blueprint, of an organism's development and

functioning. DNA sequences are copied into RNA during transcription, and then the RNA is copied into proteins

during translation. Instructions coded by DNA sequences are translated into proteins which express an

organism's physical traits. That is, the proteins determine the phenotype of an organism.

3. The completed RNA molecule should look like the one in the bottom half of the image below.

During RNA transcription, nitrogenous bases that are complementary to the DNA template strand are added to the

growing RNA strand. The base uracil (U) participates in RNA transcription instead of thymine (T). The base pairing

rules during RNA transcription are as follows:

adenine (A) on the DNA strand pairs with uracil (U) on the RNA strand

thymine (T) on the DNA strand pairs with adenine (A) on the RNA strand

cytosine (C) pairs with guanine (G) and vice versa

4. The type of RNA that transfers amino acids to growing polypeptide chains during translation is known as tRNA.

rRNA is the major component of cellular ribosomes. mRNA carries information from DNA to the ribosome of a cell.

5. The process shown in the diagram is known as translation, and it involves the production of proteins from RNA.

A codon is a series of three nucleotides that correspond to a specific amino acid. During the process of translation, a

codon on an mRNA molecule attaches to a ribosome. Then, the matching tRNA molecule (anticodon) carries the

appropriate amino acid to the ribosome where it is linked to other amino acids via peptide bonds. Once an amino acid

is attached, the ribosome slides to the next codon on the mRNA molecule and repeats the process.

The chain of amino acids continues to grow until the ribosome reaches a stop codon on the mRNA strand. The stop

codon signals that no more amino acids should be added, and the protein is complete.

6. During translation, the ribosome moves along RNA and attracts the corresponding tRNA molecules, which insert their

amino acids into the growing protein. This process continues until the ribosome encounters a stop codon, like UAA,

which signals that the protein is complete. When the ribosome reaches a stop codon, it stops translating, releases the

protein, and detaches from RNA.

The model shows the ribosome translating RNA into a protein, but it does not show the process through which

translation ends. So, a limitation of the model in representing the steps of transcription and translation is that it does

not show that the ribosome stops translating when it encounters a stop codon.

7. The synthesis of cellular proteins is primarily directed by the genetic information stored in DNA.

In the nuclei of cells, DNA molecules are transcribed into messenger RNA molecules. In turn, these molecules are

transported to the cytoplasm and translated into protein molecules that the cells need.

8. The three main types of RNA are mRNA, tRNA, and rRNA. In the image and the table, Y and 3 represent tRNA. Z

and 1 represent rRNA. X and 2 represent mRNA.

During translation, tRNA interacts with codons (sets of three bases) in mRNA. The amino acid that corresponds to

each codon is released to a peptide chain by tRNA. The peptide chain becomes a protein.

9. In the diagram, the step labeled X corresponds to the transition from the transcript to the amino acid chain. So, this

step shows the process of translation, in which a protein is synthesized from a messenger RNA (mRNA) template.

During translation, mRNA, which contains an RNA copy of the DNA sequence for the gene, provides instructions for

building the protein. The ribosome, a complex of ribosomal RNA (rRNA) and proteins, forms around mRNA and

decodes it by matching its nucleotide sequences to the correct amino acids. These amino acids are carried to the

ribosome by transfer RNA molecules (tRNAs).

So, the roles of the RNA molecules are:

mRNA rRNA tRNA

carries instructions

to build lactase

makes up part of

the ribosome, which

decodes mRNA

carries amino

acids to the growing

lactase molecule

10. DNA structure determines protein structure. That is, the sequence of nucleotides in the DNA template determines

the sequence of amino acids residues added to the protein during translation.

Some amino acids are hydrophobic, while others are hydrophilic. When a polypeptide folds and twists to become a

protein, hydrophobic amino acids naturally become situated in the center of a protein, shielded from the surrounding

water by hydrophilic amino acids. It is the position of these amino acids—determined by DNA— that causes the

protein to fold into a particular shape.