gene expression

Question 1

DNA –> (____) –> RNA –> (____) –> protein/polypeptide

Answer 1

DNA –> (transcription) –> RNA –> (translation) –> protein/polypeptide

Question 2

DNA occurs in the _____ of the ______. While some RNA also occur in the ____, most is found in the _____, particularly in the _____.

Answer 2

DNA occurs in the chromosomes of the nucleus. While some RNA also occur in the nucleus, most is found in the cytoplasm, particularly in the ribosomes.

Question 3

3 types of RNA?

Answer 3

Messenger RNA (mRNA) - transfer information from nucleus to cytoplasm
Transfer RNA (tRNA) - construction of proteins at ribosomes in the cytoplasm
Ribosomal RNA (rRNA) - forms structural and functional components of ribosomes

Question 4

The role of DNA is to instruct the cell to make specific ________.

Answer 4

The role of DNA is to instruct the cell to make specific polypeptides and proteins.

Question 5

How is DNA, genes, chromosomes related?

Answer 5

The huge length of DNA molecule is a single chromosome that carries the codes for a very large number of polypeptides. Within this extremely long molecule, the relatively short length of DNA that codes for a single polypeptide is called a gene.

Question 6

The DNA triplet code is a ___ base / triplet code. Each triplet code codes for one ____ ____.

Answer 6

The DNA triplet code is a three base / triplet code. Each triplet code codes for one amino acid.

Question 7

What determines the unique properties of each protein?

Answer 7

• Which amino acids are involved in its construction
• The sequence in which these amino acids are joined

Question 8

What are the three stages of protein synthesis?

Answer 8

1. Transcription - transfer of genetic information from DNA into an RNA molecule
2. Amino acid activation (nothing much)
3. Translation - transfer of information from RNA into a protein

Question 9

Describe transcription.

Answer 9

• Transcription occurs in the nucleus
• A complementary copy of the code is made by building a molecule of mRNA
• DNA triplet codes are transcribed into codons in mRNA
• Catalysed by RNA polymerase: stupid enzyme (5’ –> 3’)

Question 10

Name the first step of transcription and describe the process.

Answer 10

Initiation.
- Double helix unwinds and the hydrogen bonds are broken at the site of the gene being transcribed
- RNA polymerase recognises and binds to a “start” signal”, a nucleotide sequence called a promoter located at the beginning of a gene

Question 11

Name the second step of transcription and describe the process.

Answer 11

Elongation.
- The template strand (3’ –> 5’) is used as the template for transcription
- RNA polymerase matches free nucleotides by complementary base pairing, working in the 5’ –> 3’ direction
- Hydrogen bonds then form between complementary bases. Each free nucleotide is then joined by a condensation reaction between the sugar + phosphate groups of adjacent nucleotides on the DNA strand

Question 12

Name the third step of transcription and describe the process.

Answer 12

Termination.
- When RNA polymerase reaches a special sequence called a terminator, on the template strand, transcription is terminated and the mRNA is released.
- It leaves the nucleus through pores in the nuclear envelope and passes to ribosomes in the cytoplasm.

Question 13

If you are given the coding strand how do you figure out the mRNA sequence?

Answer 13

COPY! But change the “T” to “U”

Question 14

Describe amino acid activation.

Answer 14

• Amino acid activation occurs in the cytoplasm.
• Amino acids are activated by combining with transfer RNA (tRNA)
• At one end of each tRNA is a site where an amino acid is attached. The another end is folded into a loop with an anticodon (complementary to the codon of mRNA that codes for the specific amino acid)

Question 15

Name and describe the first step of translation

Answer 15

Initiation.
- On arrival at the ribosome, the mRNA binds to the small subunit at an attachment site. There are 6 bases (2 codons) of the mRNA exposed to the large subunit at any time.
- The first three exposed bases are always start codon AUG. A molecule of tRNA with the complementary anticodon UAC forms hydrogen bonds with this codon. Amino acid is attached to this tRNA molecule.

Question 16

Name and describe the second step of translation

Answer 16

Elongation.
- A second tRNA bonds with the next three bases of the mRNA, bringing another amino acid
- Ribosome holds the two amino acids close together and a condensation reaction forms a peptide bond (catalysed by an enzyme found in large subunit). A dipeptide is formed
- The ribosome moves along the mRNA in the 5’ –> 3’ direction and the next codon is read. The first tRNA (WITHOUT the amino acid) leaves the ribosome.
- By these steps constantly repeated, a polypeptide is formed and emerges from the large subunit.

Question 17

Name and describe the third step of translation

Answer 17

Termination.
- Eventually, a “stop” codon is reached. At this point, the completed polypeptide is released from the ribosome into the cytoplasm.
- It is common for several ribosomes to move along the mRNA at one time. The structure is called a a polysome (mRNA, ribosomes and their growing protein chains)

Question 18

What are the three types of mutagens? How do they affect DNA?

Answer 18

Mutagens alter the DNA sequence of the cell.
Chemical mutagens (e.g. ethidium bromide): block DNA replication, resulting in a shorter DNA synthesised
Physical mutagens (e.g. X-rays, UV rays): damage DNA by breaking it into smaller fragments
Biological mutagens (e.g. HPV): inserts DNA into chromosomes of human cell, thereby altering the DNA sequence and disrupting gene functions

Question 19

What are the types of gene mutations?

Answer 19

Point mutation/nucleotide substitution (replacement of 1 nucleotide)
Nucleotide deletions/insertions (loss or addition of one or more nucleotides)

Question 20

How to answer questions for missense mutation?

Answer 20

EXAMPLE: There is a point mutation at the 13th nucleotide from C to T. This missense mutation alters the 5th codon CGC to UGC, hence producing an altered amino acid CYS instead of ARG. Hence, a non-functional protein is produced, human cationic trypsinogen gene.

Question 21

How to answer questions for nonsense mutation?

Answer 21

EXAMPLE: There is a point mutation at the 7th nucleotide from C to T. This nonsense mutation alters the 3rd codon CGA to UGA, which results in a premature “stop” codon UGA instead of ARG and an altered amino acid is produced. Translation of the messenger RNA transcribed from this mutant gene will stop prematurely, resulting in a truncated non-functional neurofibrumin.

Question 22

How to answer questions for insertion?

Answer 22

There is an insertion of 345 base pairs. Within these extra 345 base pairs is a premature stop codon UGA at 688th-690th nucleotides. Therefore, translation of the messenger RNA transcribed from this mutant gene will stop prematurely, resulting in a truncated non-functional dihydroflavonol 4-reductase.

Question 23

What are silent mutations?

Answer 23

They are a result of nucleotide substitution. Since genetic code is degenerate, even when the base of a codon is changed, the same amino acid can still be encoded, such mutations are said to be silent as they do not cause a change in their resulting polypeptide sequence and proper folding of the protein can still occur.