8- DNA and Protein Synthesis

Where does protein synthesis take place within the cell?

The cytoplasm

What is the name given to the RNA that transfers the DNA code from the nucleus to the cytoplasm?

Messenger RNA (mRNA)

How is mRNA able to leave the nucleus and enter the cytoplasm?

It is small enough to leave the nucleus through nuclear pores

What is a codon?

A sequence of three bases on mRNA that codes for a single amino acid

What is the genome?

• The complete set of genes in a cell

• Including those in mitochondria and/or chloroplasts

What is the proteome?

• The full range of proteins produced by the genome

• Under a certain set of conditions at a given time

What does an RNA nucleotide contain?

• Ribose sugar

• Phosphate group

• Organic base

What are the two types of RNA involved in protein synthesis?

• mRNA

• tRNA

What does mRNA stand for?

Messenger RNA

What does tRNA stand for?

Transfer RNA

What does mRNA act as in protein synthesis?

A template

Describe the structure of tRNA.

• Single-stranded chain

• Folded into a clover leaf shape

• Anticodon

• Amino acid binding site

• Base pairs held together by hydrogen bonds

Describe the structure of mRNA.

A single strand of RNA arranged in a helix

What are the features of the anticodon on tRNA?

• A group of three bases

• Complementary to a codon on mRNA

What are the four bases in RNA?

• Guanine

• Cytosine

• Adenine

• Uracil

What are the three stages involved in protein synthesis in eukaryotes?

• Transcription

• Splicing

• Translation

What is transcription?

The process of making pre-mRNA from DNA

Outline the stages involved in transcription. (6 marks)

• RNA polymerase breaks hydrogen bonds between base pairs to unwind the DNA double helix. DNA strands now act as template strands

• RNA polymerase adds free-floating RNA nucleotides to complementary bases on the DNA template strand

• RNA polymerase links adjacent RNA nucleotides with phosphodiester bonds via condensation reactions

• When RNA polymerase reaches the terminator sequence, pre-mRNA strand detaches from the DNA template strand

• RNA polymerase reforms hydrogen bonds between DNA strands

• RNA polymerase detaches from DNA template strand

What happens during splicing?

• Introns are removed from pre-mRNA

• Functional exons are joined together

• Produces mRNA

Why do prokaryotes not undergo splicing?

They do not contain introns

What is an anticodon?

• A set of 3 bases on tRNA

• Complementary to a codon on mRNA

Outline the stages involved in translation. (5 marks)

• Ribosome binds to mRNA at the start codon

• tRNA binds to codon on mRNA complementary to its anticodon, bringing a specific amino acid with it. Another tRNA does the same

• Ribosome joins adjacent amino acids via a condensation reaction, forming a peptide bond between them. This reaction is catalysed by an enzyme and requires ATP

• Ribosome moves one codon along the mRNA strand, and tRNA molecules detach from the amino acids and mRNA strand

• Process repeats until ribosome reaches the stop codon

• Ribosome then detaches from polypeptide chain and mRNA strand