D1.2 Protein Synthesis

What is transcription?

Copying a DNA gene sequence into RNA

What is the first step of transcription?

RNA polymerase binds to the promoter region of a gene/template strand, unwinding and separating DNA strands

What is the second step of transcription?

RNA polymerase adds RNA nucleotides according to the template to form mRNA by complementary base pairing

What is the third step of transcription?

mRNA strand detaches from the template after synthesis

What is the last step of transcription?

Original template DNA strand recoils

What are somatic cells?

Non-reproductive cells in an organism

How do DNA template strands remain chemically stable despite repeated transcription?

• Hydrogen bonding between bases maintains pairing

• Repair enzymes correct damage to preserve sequence

• Double helix reforms quickly after RNA polymerase passes

• DNA bases and sugar-phosphate backbone are resistant to breakdown

What is gene expression?

Process by which genetic information in a gene is used to synthesis a functional polypeptide

When are genes expressed?

• Only when transcribed to RNA

• Can be switched on/off depending on cell type and environment

• Only transcribed genes are expressed as proteins

What is translation?

Process by which information carried by mRNA is decoded and used to build the sequence of amino acids that eventually forms a protein molecule

What is the first step of translation?

In the cytoplasm, mRNA (series of codons) attaches to a ribosome (small subunit)

What is the second step of translation?

tRNA carries specific amino acids which are deposited in the large subunit

What is the third step of translation?

tRNA contains anti-codon complementary to codon (3 bases) on mRNA

What is the fourth step of translation?

At any given time 2 tRNA can bind to a ribosome. Peptide bonds form in between 2 amino acids on adjacent tRNA

What is the fifth step of translation?

tRNA detach from mRNA, ribosome moves along mRNA

What is the sixth step of translation?

An amino acid chain is formed, producing a protein/polypeptide

What is the start codon?

AUG

What are the stop codons?

UAA, UAG, UGA

What are codons?

Sequence of 3 nucleotides that code for an amino acid

Why is the genetic code degenerate?

More than 1 codon may code for the same amino acid

Why is the genetic code universal?

Same code is used in all life forms

Why is the genetic code non-overlapping?

Codons are read sequentially

What is the triplet code?

3 bases = 1 codon = 1 amino acid

How does the ribosome move along mRNA?

5’ → 3’, A → P → E

What is sickle cell anaemia?

Single base substitution mutation - Glu replaced by Val in each beta chain → changes haemoglobin interactions and RBC shape

What are point substitution mutations?

Silent (no amino acid change), missense (different amino acid), nonsense (premature stop)

What are insertion/deletion mutations?

Cause frameshifts, altering all downstream codons