DNA and Protein Synthesis

Proteins and DNA

Why is protein synthesis needed?

Cells need to produce specific proteins, such as enzymes, using the genetic information stored in DNA.

Where is DNA found in a eukaryotic cell?

In the nucleus.

Where are proteins synthesised?

On ribosomes in the cytoplasm.

Why can't DNA carry its genetic code directly to ribosomes?

DNA remains in the nucleus and is too large to pass through the nuclear pores, so RNA carries the information to the ribosomes.

What are the two main stages of protein synthesis?

Transcription and translation.

What happens during transcription?

The base sequence of a gene in DNA is copied into a complementary mRNA molecule.

Where does transcription occur?

In the nucleus.

What happens during translation?

The sequence of codons on mRNA is used to assemble amino acids in the correct order to form a polypeptide.

Where does translation occur?

At ribosomes in the cytoplasm.

What are the three main types of RNA involved in protein synthesis?

Messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).

How is RNA different from DNA?

RNA contains ribose instead of deoxyribose, uracil instead of thymine, and is usually single-stranded.

What are the three main functions of RNA in protein synthesis?

It carries genetic instructions from DNA to ribosomes, carries specific amino acids to ribosomes, and forms part of the structure of ribosomes.

Define messenger RNA (mRNA).

A single-stranded RNA molecule that carries a copy of the genetic information from DNA in the nucleus to ribosomes in the cytoplasm.

Where is mRNA formed?

In the nucleus.

What information does an mRNA molecule usually carry?

The instructions needed to produce one polypeptide.

What is the sense strand of DNA?

The DNA strand that carries the code for the protein to be produced.

What is another name for the sense strand?

The coding strand.

What is the antisense strand of DNA?

The DNA strand used as the template to make mRNA during transcription.

What is another name for the antisense strand?

The template strand.

Which DNA strand is used to make mRNA?

The antisense or template strand.

Why does the mRNA base sequence resemble the DNA sense strand?

Because mRNA is complementary to the antisense strand, so its sequence matches the sense strand except that uracil replaces thymine.

Define transcription.

The process in which the base sequence of a DNA gene is used to produce a complementary strand of mRNA in the nucleus.

What happens to DNA at the beginning of transcription?

A section of the DNA double helix unwinds and unzips as hydrogen bonds between complementary bases break.

Why does only part of the DNA molecule unzip during transcription?

Only the gene containing the information for the required polypeptide needs to be transcribed.

What acts as the template during transcription?

The antisense strand of DNA.

What happens to free RNA nucleotides during transcription?

They align with exposed bases on the DNA template strand by complementary base pairing.

What are the complementary base-pairing rules during transcription?

DNA A pairs with RNA U, DNA T with RNA A, DNA C with RNA G, and DNA G with RNA C.

Which enzyme joins RNA nucleotides together during transcription?

RNA polymerase.

What is the role of RNA polymerase?

It joins RNA nucleotides together to form the mRNA strand.

Where does transcription begin?

At a start codon.

What does a start codon indicate?

The beginning of the sequence to be translated into a polypeptide.

Where does transcription end according to this model?

When the RNA chain reaches a stop codon.

What happens to mRNA after transcription?

It separates from the DNA template, leaves the nucleus through a nuclear pore and travels to a ribosome in the cytoplasm.

What happens to the DNA strands after transcription?

They rejoin as hydrogen bonds reform between complementary bases and the DNA returns to its double helix.

How does mRNA leave the nucleus?

Through nuclear pores.

Where does mRNA go after leaving the nucleus?

To a ribosome in the cytoplasm.

Define transfer RNA (tRNA).

A small RNA molecule that carries a specific amino acid to a ribosome during translation.

Where is tRNA found?

In the cytoplasm.

What is the general shape of a tRNA molecule?

A folded cloverleaf shape produced by hydrogen bonding between bases.

What are the two important functional regions of a tRNA molecule?

An amino acid binding site and an anticodon.

What is the amino acid binding site on tRNA?

The region where a specific amino acid attaches to the tRNA molecule.

What is an anticodon?

A sequence of three bases on tRNA that is complementary to a specific codon on mRNA.

What determines which amino acid a tRNA carries?

Its structure and anticodon are associated with one specific amino acid.

Does each tRNA carry any amino acid?

No. Each type of tRNA carries one specific amino acid.

What happens to tRNA before translation?

It binds to its specific amino acid in the cytoplasm.

What provides the energy needed to attach an amino acid to tRNA?

ATP.

How does tRNA identify the correct position on mRNA?

Its anticodon forms complementary base pairs with the corresponding mRNA codon.

What holds an mRNA codon and tRNA anticodon together?

Hydrogen bonds between complementary bases.

What are ribosomes made from?

Ribosomal RNA (rRNA) and proteins.

What is the function of ribosomes?

They are the site of translation and hold mRNA and tRNA in the correct positions for protein synthesis.

What happens to mRNA when it reaches a ribosome?

It binds to the ribosome, which moves along the mRNA reading its codons.

Define translation.

The process in which the sequence of codons on mRNA is used to produce a specific sequence of amino acids in a polypeptide.

What happens first during translation?

mRNA binds to a ribosome and a tRNA carrying a specific amino acid binds to the appropriate mRNA codon.

How does the correct tRNA bind to an mRNA codon?

Its complementary anticodon binds to the codon by hydrogen bonding.

What happens when two amino acids are positioned next to each other at the ribosome?

A peptide bond forms between them.

What type of bond joins amino acids during protein synthesis?

A peptide bond.

What happens to a tRNA after its amino acid has been added to the growing polypeptide?

The tRNA detaches from the ribosome and returns to the cytoplasm to collect another amino acid.

What happens as the ribosome moves along the mRNA?

It reads successive codons and allows matching tRNAs to bring the correct amino acids in sequence.

What determines the order of amino acids in the polypeptide?

The sequence of codons on the mRNA.

What determines the sequence of codons on mRNA?

The base sequence of the DNA template strand.

What is the relationship between DNA and the amino acid sequence?

DNA base sequence determines the mRNA codon sequence, which determines the amino acid sequence of the polypeptide.

When does translation stop?

When the ribosome reaches a stop codon on the mRNA.

What happens when a stop codon is reached?

Translation ends and the completed polypeptide is released.

What happens to a polypeptide after translation?

It folds and may be modified to form a functional protein with a specific three-dimensional shape.

Why is the amino acid sequence of a protein important?

It determines how the polypeptide folds and therefore the protein's three-dimensional shape and function.

What could happen if the DNA base sequence changes?

The mRNA codon sequence may change, which can alter the amino acid sequence and therefore the structure and function of the protein.

What is the complete information pathway during protein synthesis?

DNA → transcription → mRNA → translation → polypeptide → functional protein.

What is the relationship between a DNA triplet, mRNA codon and tRNA anticodon?

The DNA template triplet is complementary to the mRNA codon, and the tRNA anticodon is complementary to the mRNA codon.

If the DNA coding/sense strand triplet is CGT, what is the DNA template/antisense triplet?

GCA.

If the DNA template triplet is GCA, what is the mRNA codon?

CGU.

If the mRNA codon is CGU, what is the tRNA anticodon?

GCA.

What amino acid is coded for by mRNA codon CGU?

Arginine.

For arginine, what sequence is shown from DNA sense strand to tRNA anticodon?

DNA sense: CGT → DNA template: GCA → mRNA codon: CGU → tRNA anticodon: GCA.

Why is complementary base pairing important during transcription?

It ensures that the mRNA carries an accurate copy of the genetic information in the DNA template strand.

Why is complementary base pairing important during translation?

It ensures that the correct tRNA, and therefore the correct amino acid, is positioned for each mRNA codon.

What is a polysome?

A group of ribosomes attached to and translating the same mRNA molecule.

Why are polysomes useful?

Several ribosomes can translate the same mRNA simultaneously, allowing many identical polypeptides to be produced rapidly.

How do polysomes allow mass production of proteins?

Multiple ribosomes move along the same mRNA at the same time, each producing a copy of the polypeptide.

What ultimately determines the structure of a protein?

The sequence of bases in DNA, because it determines the sequence of amino acids in the polypeptide.