1.3 - Gene Expression

Question 1

Gene expression

Answer 1

Gene expression is the process in which sections of DNA are used as a template to make proteins in a cell.

The process involves the transcription and translation of DNA sequences and only a fraction of the genes in a cell are expressed.

Question 2

Differences between DNA and RNA

Answer 2

DNA - Double stranded; deoxyribose sugar; A, T, C and G bases.
RNA - Single stranded; ribose sugar; A, U, C and G bases.

Uracil replaces thymine.

Question 3

mRNA

Answer 3

Also known as messenger RNA. mRNA is a linear molecule which carries a complementary copy of DNA from the nucleus to the ribosome. mRNA is transcribed (copied) from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm.

Question 4

rRNA

Answer 4

Also known as ribosomal RNA. rRNA molecules combine with proteins to form the ribosome. The ribosome is the structure where protein synthesis takes place.

Question 5

tRNA

Answer 5

Also known as transfer RNA. tRNA molecules fold due to complementary base pairing. Each tRNA carries a specific amino acid to the ribosome to build a polypeptide chain.

Question 6

Codon Definition

Answer 6

Every group of three bases (a ‘triplet’) is described as a codon because each one codes for an amino acid.

Question 7

Protein Synthesis - Transcription

Answer 7

1. RNA polymerase unwinds the DNA molecule.
2. Hydrogen bonds between bases are broken so the double helix unzips.
3. RNA polymerase adds complementary RNA nucleotides, through base pairing with DNA, onto the 3’ end of a growing chain of mRNA.
4. This forms a primary transcript of mRNA.
5. The primary transcript of mRNA is processed to produce a mature transcript of mRNA.
6. The mature mRNA transcript is now ready to leave the nucleus and travel to the ribosome.

Question 8

RNA splicing

Answer 8

The primary transcript of mRNA contains copies of both regions of DNA that code for proteins (exons) and non-coding regions that do not code for proteins (introns).

During RNA splicing, introns are removed and exons are joined (spliced) together to form a mature transcript of mRNA.

The order of the exons is unchanged during splicing.

Question 9

Translation

Answer 9

Translation is the synthesis of a polypeptide chain following the code within the mature transcript.

Translation occurs in the ribosome.

Question 10

Steps of translation

Answer 10

1. mRNA attaches to a ribosome. Each triplet of bases on mRNA is known as a codon. One codon codes for one amino acid.
2. Translation begins at a start codon and ends at a stop codon.
3. tRNA anticodons bond with mRNA codons through complementary base pairing to bring a specific amino acid to the ribosome.
4. Peptide bonds form between amino acids to form a polypeptide chain.
5. Each tRNA leaves the ribosome as the polypeptide is formed.

After translation, the polypeptide chain can be changed by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups to the protein.

Question 11

Alternate RNA splicing

Answer 11

The same gene can be used to make different proteins as a result of what segments are treated as introns and exons.

Different mature mRNA transcripts are produced from the same primary transcript depending on which exons are used.

Question 12

Proteins

Answer 12

Amino acids are joined by peptide bonds to form polypeptide chains.

Polypeptides fold to form the 3D shape of a protein. The 3D shape is held together by hydrogen bonds and other interactions between individual amino acids.

Proteins have a variety of shapes/structures which determine their functions. They can speed up reactions as enzymes, send messages as hormones, fight infection as antibodies, and much more.

Phenotype is determined by the proteins produced during gene expression. However, environmental factors can also influence phenotype.