TRANSCRIPTION AND TRANSLATION
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Genes are instructions for proteins
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In transcription, genes are converted into mRNA, which is then read at ribosomes
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Ribosomes translate mRNA into polypeptides (sequences of amino acids determined by genes)
In RNA uracil replaces thymine, thus A2UC3G
Transcription (in nucleus)
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RNA Polymerase makes a mRNA molecule that is complementary to the DNA strand
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The antisense strand (template strand) is complementary to the sense strand (coding strand)
PROMOTER REGION
TERMINATOR REGION
CODING REGION
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Enzymes work in 3’ to 5’ direction
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Transcription and translation are both carried out in a 5’ to 3’ direction
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RNA polymerase attaches at the promoter region and uses free nucleoside triphosphates (found in nucleus) to make a strand of mRNA
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mRNA is elongated until it reaches the terminator region
- DNA is transcribed in triplets (sequences of three bases)
- Start (AUG) and stop codons (UAG, UAA, UGA)
- 64 codons, only 20 amino acids (+ stop)
- The same amino acid can be coded for by more than one codon
- Introns are removed from mRNA to produce mature DNA through splicing
Translation (at ribosomes)
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tRNA delivers amino acids to growing polypeptide chain in translation
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Ribosomes are made up of protein and rRNA (2 subunits – small and large 80s)
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Polysomes are multiple ribosomes that work together on the same mRNA at the same time to complete translation more efficiently
1) INITIATION: Ribosome binds with mRNA at the AUG codon. First tRNA molecule brought in at P site.
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2) ELONGATION: tRNA beings in another amino acid at A site. Both tRNA molecules move forward, and peptide bond is formed between two amino acids, making a polypeptide.
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3) TRANSLOCATION: Ribosomes release used tRNA and moves along the mRNA to the next codon. Next tRNA and amino acid are brought in, elongating the polypeptide further.
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4) TERMINATION: When stop codon is reached, polypeptide is released. No amino acid with tRNA that has anticodon of stop codon.
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Figure 1. Translation at ribosome
- Translation in free floating ribosomes gives proteins for use within cell.
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- Translation in ribosomes of RER give proteins for use outside cell.