3.4.2 DNA and protein synthesis Flashcards
protein synthesis stages
Transcription: production of mRNA from DNA
Translation: production of polypeptides from the sequence of codons carried by mRNA
MRNA
Made by transcription in the nucleus
Acts as a template for translation in the cytoplasm
Sequence of bases on RNA determined by sequence of amino acids in polypeptide chain
Straight chain molecule
Sequence of bases on RNA determined by sequence of bases on DNA
Chemically unstable so breaks down after a few days
TRNA
Carries an amino acid
Each tRNA specific to one amino acid, in relation to its anticodon
Single polynucleotide strand - three-leafed clover shape held by hydrogen bonds
Anticodon
3 bases
Bases complementary to mRNA codon
Similarities & differences between the structure of mRNA and tRNA molecules
Both single polynucleotide strand
MRNA single helix/straight, tRNA is folded into clover shape
MRNA is a longer, variable length whereas tRNA is shorter
MRNA contains no paired bases or hydrogen bonds whereas tRNA has some paired bases and hydrogen bonds 2
Transcription
In nucleus
DNA double helix unzipped by helicase, hydrogen bonds broken
RNA nucleotides align next to their complementary bases on the template strand, forming temporary hydrogen bonds, uracil replaces thymine in RNA
RNA polymerase joins adjacent nucleotides - condensation reaction
Forming phosphodiester bonds
When RNA polymerase reachers stop codon, mRNA or pre-mRNA detaches from DNA
MRNA leaves nucleus via nuclear pore
Post transcriptional modification
Eukaryotic genes contain exons (coding regions) and introns (non-coding regions)
Whole gene transcribed to pre-mRNA which contains introns and exons
Splicing - introns removes, exons spliced together in different combos for different proteins
Prokaryotic DNA doesn’t contain introns, mRNA is produced directly from DNA, no splicing
Translation
Sequence of mRNA codons determines sequence of amino acids
TRNAs carry specific amino acids, in relation to their anticodon
At ribosome, tRNA codon binds to mRNA codon
- tRNA anticodon complementary to mRNA codon
- hydrogen bonds formed
- first codon = start codon
Two amino acids joined via a condensation reaction, forming a peptide bond
- using energy from ATP
TRNA detaches, ribosome moves along mRNA to next codon
Continues until stop codon (polypeptide released)
ATP role in translation
Hydrolysis of ATP to ADP + Pi, releases energy
For the bond between the amino acid and its corresponding tRNA molecule
- amino acid attaches at amino acid binding site
For peptide bond formation between amino acids
TRNA role in translation
TRNA attaches to and transports a specific amino acid, in relation to its anticodon
TRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds
Two tRNAs bring amino acids together for the formation of a peptide bond
About 60 types of tRNAs to carry 20 different amino acids
-genetic code is degenerate
Ribosomes role in translation
Attaches to mRNA and houses tRNA, allowing codon-anticodon complementary base pairing
Allows peptide bonds to form between amino acids
Base sequence of nucleic acids & amino acid sequence of polypeptides
TRNA anticodons are complementary to mRNA codon
MRNA sequence of bases/codons are complementary to sequence of bases/triplets on DNA template strand
In RNA, uracil replaces thymine
