Molecular Biology Lecture 4 Translation By Dr Ghaidaa

Molecular Biology Lecture 4 Translation By Dr. Ghaidaa Jihadi

Protein Synthesis or Gene Expression • 1 step: Transcription occurs in the nucleus • 2 step: Translation occurs in the cytoplasm

nd 2 step Translation: RNA Protein • Translation: synthesizing a code from amino acids, according to the sequences of the nucleotides in m. RNA. • Occurs at the ribosome, in cytoplasm of cell • Ribosomal RNA, r. RNA, is needed for protein synthesis – helps m. RNA bind to the ribosome • t. RNA, brings specific amino acids to the ribosome to be assembled as proteins.

– nucleotide sequence of m. RNA transcript is translated into amino acid sequence in the polypeptide – moves three nucleotides at a time » disengages at stop signal • Gene expression - Collective of transcription and translation

Translation overview a. m. RNA attaches to the ribosome b. t. RNA's attach to free amino acids in the cytoplasmic "pool" of amino acids c. t. RNA carries its specific amino acid to the ribosome

d. t. RNA "delivers" its amino acid based on complementary pairing of a triplet code (anticodon) with the triplet code (codon) of the m. RNA. e. Enzyme "hooks" the amino acid to the last one in the chain forming a peptide bond. f. Protein chain continues to grow as each t. RNA brings in its amino acid and adds it to the chain. - This is translation!!` `

Translation • Begins when initial portion of m. RNA molecule binds to r. RNA in a ribosome – t. RNA molecule with complimentary anticodon binds to exposed codon on m. RNA • some t. RNA molecules recognize more than one codon

The Genetic Code • Each 3 consecutive bases on the m. RNA is a code word, CODON, that specifies an amino acid. • The genetic code consists of 64 codons, • but only 61 code amino acids. • Three codons act as signal to stop the process • One codon, AUG, codes for methionine, and is also the Start signal for translation.

Genetic Code • Genetic code consists of a series of information blocks called codons. – reading frame (triplet) • each codes for one amino acid – genetic code is nearly universal » mitochondria » chloroplasts

1. A triplet code comprised of three nucleotide bases in a sequence. 2. How many triplet codes? 20 common amino acids in a protein 4 diff. bases on DNA A, T, C, & G | 4 diff. bases on RNA | | | U, A, G, & C 4 things put together in combinations of 3 = 43= 64 Therefore - 64 different DNA triplet codes or RNA codons

The 64 triplet codes • 60 code for amino acids • 4 act as "stop" and "start codes • Degenerate Code- more than one triplet code for some amino acids e. g. , GGG GGU GGC GGA All code for the amino acid glycine

Translation • Start and stop signals – start signal coded by AUG codon – stop signal coded by one of three nonsense codons: UAA - UAG - UGA • Initiation – Polypeptide synthesis begins with the formation of an initiation complex. • initiation factors

Translation • Translocation – ribosome moves nucleotides along m. RNA molecule

• Termination Translation – Nonsense codons are recognized by release factors that release the newly made polypeptide from the ribosome.

Steps in Translation 1. m. RNA leaves the nucleus and migrates to ribosome 2. m. RNA binds to small ribosomal subunit 3. t. RNA brings an amino acid to the ribosome, where anticodon on the t. RNA binds to the codon of the m. RNA 4. The amino acid bonds to its adjoining amino acid to form a growing polypeptide molecule 5. The t. RNA without the amino acid is released from the ribosome 6. Other t. RNA’s bring amino acids to the ribosome to complete the protein molecule

Transfer RNA (t. RNA) • Transport molecule that carries specific amino acid to a ribosome • Each t. RNA recognizes the correct codon on the m. RNA molecule

Translation

Protein translation

Differences Between Prokaryotic and Eukaryotic Gene Expression • Most eukaryotic genes possess introns. • Individual bacterial m. RNA molecules often contain transcripts of several genes. • Eukaryotic m. RNA molecules must be completely formed and must pass across the nuclear membrane before translation. • In prokaryotes, translation begins at the AUG codon preceded by a special nucleotide sequence.

Differences Between Prokaryotic and Eukaryotic Gene Expression • Eukaryotic m. RNA molecules have introns cut out and exons joined together before translation. • Eukaryotic ribosomes are larger than prokaryotic ribosomes.