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Protein Synthesis - AHL

Protein Synthesis - AHL. Transcription & Translation. Transcription. Transcription is carried out in a 5’  3’ direction. The 5’ end of the free RNA nucleotide is added to the 3’ end of the RNA molecule that is already synthesized.

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Protein Synthesis - AHL

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  1. Protein Synthesis - AHL Transcription & Translation

  2. Transcription • Transcription is carried out in a 5’  3’ direction. • The 5’ end of the free RNA nucleotide is added to the 3’ end of the RNA molecule that is already synthesized.

  3. During transcription, the DNA double helix unwinds and the two strands separate. Only one strand is then transcribed into mRNA. This is the antisense strand. The sense strand has the same base sequence as mRNA, with U instead of T. A T G C C A Sense & Antisense DNA Strands Antisense strand mRNA Sense strand

  4. Transcription in Prokaryotes • Genes are only transcribed if RNA polymerase binds to a region of DNA close to the start of the gene called the promoter. The promoter is on the sense strand. • RNA polymerase splits the DNA strands. • The mRNA strand produced continues to grow by the addition of free nucleoside triphosphates, again due to RNA polymerase.

  5. Transcription continued…. • Two phosphates are removed as they are linked on, converting them into RNA nucleotides. • The process continues until a region called the terminator is reached. This is a base sequence on the sense strand that causes RNA polymerase to stop transcription. • RNA polymerase is released after transcribing the terminator. • The DNA is rewound into a double helix by RNA polymerase.

  6. Transcription in Eukaryotes • In eukaryotic cells, repetitive sequences are called INTRONS and coding sequences (genes) are called EXONS. • Both introns and exons are transcribed. • The introns are then removed (“spliced”) leaving only exons. • This mature mRNA then travels to the cytoplasm for translation.

  7. Translation • Translation occurs in a 5’  3’ direction. • During translation the ribosome moves along the mRNA towards the 3’ end. The start codon is nearer to the 5’ end. • Translation consists of initiation, elongation, translocation & termination.

  8. Ribosomes • Made of proteins and ribosomal RNA (rRNA) • Consist of 2 subunits – one large, one small. • There are 3 tRNA binding sites on the surface of the ribosome. 2 tRNA molecules can bind at the same time. • There is a mRNA binding site on the surface.

  9. Ribosomes continued…. • Free ribosomes synthesize proteins for use primarily within the cell. • Membrane bound ribosomes on rough er synthesize proteins primarily for secretion or for lysosomes.

  10. tRNA • All tRNA molecules have an anticodon. • All have 3 loops. • All have the base sequence CCA at the 3’ terminal. This is the site for attachment of the amino acid. • All have sections that become double stranded by base pairing.

  11. tRNA continued • Each type of tRNA has a distinctive 3D shape and chemical properties. • Each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to it at the 3’ terminal. • There are 20 different tRNA activating enzymes – one for each amino acid. • ATP is used to provide energy for the attachment of amino acids. • A high-energy bond is formed between the amino acid and the tRNA which is later broken and used to make a peptide linkage during translation.

  12. Initiation • A particular codon on mRNA is called the start codon. • tRNA with the anticodon complementary to this binds to the small subunit of the ribosome. • The small subunit carrying the tRNA binds to the 5’ end of the mRNA. • The small subunit slides along until it reaches the start codon. • The large subunit binds to the small subunit.

  13. Elongation & Translocation • One of the binding sites for tRNA is vacant. The small subunit of the ribosome ensures only the tRNA with the complementary anticodon binds to it. • The large subunit slides over the small subunit

  14. Termination • The ribosome moves along the mRNA until it reaches a stop codon. • No tRNA has the complementary anticodon. • The large subunit moves over the small subunit releasing the polypeptide from the tRNA. • The tRNA detaches. The large subunit, the small subunit and mRNA all separate.

  15. Polysomes • Several ribosomes can translate an mRNA at the same time, forming what is called a polysome. • This makes it possible to produce many polypeptides simultaneously from a single mRNA.

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