Ribonucleic Acid RNA The structure of ribonucleic acid
Ribonucleic Acid RNA
The structure of ribonucleic acid l Learning Objectives: l l l Describe the structure of ribonucleic acid (RNA) Describe the structure and role of messenger RNA (m. RNA) Describe the structure and role of transfer RNA (t. RNA)
RNA l l l RNA = ribonucleic acid RNA is a single -stranded polymer Each RNA nucleotide consists of: l l A pentose sugar (ribose) A phosphate group A base – adenine, cytosine, guanine, uracil (instead of thymine) RNA used in protein synthesis is messenger RNA (m. RNA) or transfer RNA (t. RNA).
RNA contains the pentose sugar ribose. DNA contains the pentose sugar deoxyribose.
The Genetic Code (recap of unit 2) l l l 3 nucleotide bases on m. RNA (a codon) codes for each amino acid in a protein (DNA = triplet code). A few amino acids have a single codon. The code is degenerate – most amino acids have more than one codon. 3 codons code for no amino acids – they are stop codons and mark the end of the polypeptide chain. Non-overlapping – each nucleotide is read only once. Universal – same codon codes for the same amino acid in all organisms (few minor exceptions).
m. RNA (m = messenger)
t. RNA (t = transfer) Amino acid joins here Anticodon
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Messenger RNA (m. RNA) l Used to transfer the DNA code from the nucleus to the cytoplasm, 1 gene at a time. l Complementary to the DNA code. l Small enough to leave the nuclear pores. l Once in the cytoplasm, m. RNA associates with the ribosomes. l Easily broken down and only exists whilst it is needed to manufacture a protein.
Transfer RNA (t. RNA) l l l l Small molecule (~80 nucleotides). Single stranded and folded into a clover leaf shape with one end of the chain slightly longer. This longer section is attached to an amino acid. Each t. RNA carries a different amino acid. 3 bases at the opposite end of the t. RNA are called an anticodon. Each amino acid has a different anticodon. The anticodon pairs with the complementary codon on the m. RNA.
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Polypeptide synthesis – transcription and splicing l Learning Objectives: l How is pre-m. RNA produced from DNA in the process called transcription? l How is pre-m. RNA modified to form m. RNA?
Transcription l l l l Helicase (enzyme) acts on a region of DNA to break the hydrogen bonds between the bases. RNA polymerase moves along one of the two DNA strands – template strand RNA polymerase matches up complementary RNA nucleotides. C matches G and G matches C U matches A and A matches T As the RNA nucleotides join together the pre-m. RNA is formed. The DNA behind the RNA polymerase rejoins into a double helix. When the RNA polymerase reaches a “stop” codon, the chain is terminated and the pre-m. RNA detaches.
Splicing of m. RNA to form m. RNA l Once the introns (non-functional DNA) have been removed, the remaining exon sections join together.
RNA Splicing Spliceosome sn. RNPs
Transcription Animation http: //www. johnkyrk. com/DNAtranscription. html http: //highered. mcgrawhill. com/olcweb/cgi/pluginpop. cgi? it=swf: : 525: : 530: : /sites/dl/free/0 072464631/291136/m. RNA_synthesis. swf: : m. RNA_synthesis. swf m. RNA splicing http: //highered. mcgrawhill. com/olcweb/cgi/pluginpop. cgi? it=swf: : 525: : 530: : /sites/dl/free/0 072464631/291136/spliceosomes. swf: : spliceosomes. swf
Polypeptide synthesis – translation l Learning Objectives: l How is a polypeptide synthesised during the process of translation? l What are the roles of m. RNA and t. RNA in translation?
Translation (Protein Synthesis) t. RNA m. RNA Amino acids
Synthesising the polypeptide (translation) l l l l m. RNA attaches to the ribosome at the “start” codon – AUG. t. RNA with a complementary anticodon attaches to the m. RNA – UAC. This t. RNA is attached to an amino acid – methionine The ribosome moves along the m. RNA bringing in 2 t. RNA molecules at any one time. An enzyme and ATP are used to join the amino acids with a peptide bond. The first t. RNA is released and can collect another amino acid. The process is repeated until a “stop” codon is reached. Many ribosomes can travel along the m. RNA at the same time – polysome.
Translation Animation http: //www. johnkyrk. com/DNAtranslation. html http: //highered. mcgrawhill. com/olcweb/cgi/pluginpop. cgi? it=swf: : 525: : 530: : /sites/dl/free/0072464631/291136/protei n_synthesis. swf: : protein_synthesis. swf
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