RNA Synthesis or Transcription Used in the process of protein synthesis!
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Transcript of RNA Synthesis or Transcription Used in the process of protein synthesis!
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RNA Synthesis or “Transcription”
Used in the process of protein synthesis!
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RNA (Ribonucleic acid)
• RNA is the nucleic acid that acts as a messenger between DNA and the ribosomes and carries out the process by which proteins are made from amino acids
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How RNA is Different from DNA
•Single strand•Linear not helical•Contains uracil instead of thymine•Copy of 1 gene, not entire DNA strand (100’s of genes)•Formed in nucleus prior to protein synthesis•Initially in the nucleus BUT then migrates to cytoplasm via a nuclear pore and eventually attaches to a ribosome
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Transcription• The process by which a
gene segment of DNA is copied into a complementary strand of RNA
• The type of RNA formed in this way is called messenger RNA or mRNA
• Occurs so that the information to make a protein can be taken from the nucleus to the ribosome where protein synthesis occurs
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There are two other types of RNA:
• Transfer RNA (tRNA): carries amino acids (the building blocks of proteins) to the ribosome where they can be bonded together
• Ribosomal RNA (rRNA) : small organelle (the ribosome) is made of this kind of rRNA and proteins. The ribosome supplies the site where the reaction to link amino acids together occurs.
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Lets take a closer look at the steps to make mRNA or transcription:
1) The enzyme RNA polymerase attaches to the DNA strand and unzips it at the gene to be copied
- “unzipping” means that the H-bonds between nitrogenous bases are broken
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2) RNA nucleotides base pair with the exposed bases of the gene being copied
- since 2 sides of the DNA strand are exposed, the side being copied is called the “sense” side, while the other uncopied side is called the “missense” side.
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3) Sugar-phosphate bonds form between RNA nucleotides
4) Hydrogen bonds break and mRNA is free to leave the nucleus (through a pore) and attach to ribosome in the cytoplasm (protein used in cell) or on RER (proteins used outside cell)
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• Finally, the H-bonds reform between the DNA strands and the strands rezip.
• http://www.youtube.com/watch?v=ztPkv7wc3yU&feature=related
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A sequence…
• Can refer to either the order the nitrogenous bases come in on a DNA strand or mRNA strand OR the order of the amino acids in a polypeptide chain.
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The sequence of nitrogenous bases on an mRNA strand is read in a specific
way…• Each group of 3
nucleotides codes for one amino acid
• This group of 3 nucleotides is called a codon
• There are 20 different amino acids that the bases code for
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• Since there are 64 possible combinations of 3 nucleotides, each amino acid is coded for by more than one codon!
• AUG is the “start” codon as it shows the ribosome where to start protein synthesis
• There are 3 “stop” codons that signal the end of protein synthesis.
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Closer look at tRNA
• This molecule “reads” the genetic code on the mRNA molecule and supplies the correct amino acids
• One end holds the amino acid, the other contains the “anticodon” that matches up with the codon on the mRNA strand
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Anticodon…
• Each tRNA molecule contains 3 nucleotides on one end
• These nucleotides base pair with a codon on mRNA, ensuring that the appropriate amino acid is added to the polypeptide chain
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Practice!
• Go to this site and play a couple of the games to practice the steps we’ve learned about DNA replication and protein synthesis so far…
• http://mrhartog.blogspot.com/2009/01/transcription-game-httpwww.html
• Homework: Complete part 3 of the notes!