from gene to protein chapter 17. from gene to protein the “central dogma of molecular biology”...
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From Gene To Protein
Chapter 17
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From Gene to Protein
• The “Central Dogma of Molecular Biology” is DNA RNA protein
• Meaning that our DNA codes our RNA which provides instructions for making protein
• Proteins (you may remember) do many things: structure, support, communication, transportation, enzymes etc.
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Transcription and Translation
• Transcription is the synthesis of RNA from DNA
• Translation is the synthesis of a polypeptide (protein) from RNA.
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Codons
• Proteins are made of amino acids.
• Each amino acid is coded for by a triplet of nucleotides called a codon.
• For example, AGT = serine
• There are only 20 amino acids, but 64 codons.
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Codons
• You must memorize these codons: • AUG = “start” the
process of transcription
• UAA, UAG, and UGA = “stop”
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Transcription: DNA to RNA
• First, RNA Polymerase unzips a strand of DNA.
• Transcription can only go from 5’ to 3’
• RNA Polymerase II attaches to DNA at a promoter
• The portion of DNA being transcribed is called a transcription unit
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Transcription: DNA to RNA
• The promoter has a region called the TATA box, showing the enzyme where to bind.
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Transcription: DNA to RNA
• RNA is now synthesized, as base pairs are added to the unzipped DNA strand.
• RNA is ribonucleic acid. It is a single helix. Instead of T (thymine) RNA has U (uracil).
• So every A in DNA now pairs with U (instead of T).
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Transcription: DNA to RNA
• The RNA that is made is called mRNA which stands for messenger RNA.
• Before it leaves the nucleus, it is modified in several ways.• A 5’ cap is added, made of a special
guanine molecule • The 3’ end gets a poly-A tail, a long
chain of 50 to 250 adenines• These help the mRNA bind to ribosomes
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RNA splicing
• Some of the RNA isn’t needed to code for proteins, so it is cut out through RNA splicing.
• The non-coding regions that are cut out are called introns, the coding portions the cell needs are called exons.
• Little molecules called small nuclear ribonucleoproteins, snRNA, join with a molecule called a spliceosome to slice the RNA.
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Transcription
• http://www.hhmi.org/biointeractive/media/DNAi_transcription_vo1-lg.wmv
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Translation: RNA to protein
• The mRNA now leaves the nucleus and binds to a ribosome, where protein synthesis occurs.
• As it passes through the ribosome, tRNA (transfer RNA) molecules, each carrying an amino acid, begin to form a long chain of amino acids.
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Translation: RNA to protein
• At one end of tRNA is a triplet code called an anticodon which matches the mRNA.
• At the other end of the tRNA is an amino acid.
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Translation: RNA to protein
• The ribosome where this all happens has two pieces, and is made of proteins and RNA called ribosomal RNA (rRNA)
• The subunits are called “large” and “small”
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Translation: RNA to protein
• Each ribosome has 3 binding sites for tRNA.
• The “P” site holds the tRNA with the polypeptide going into the big chain
• The “A” site holds the next tRNA in line
• The “E” site is the exit site, once the tRNA drops off its amino acid, it detaches from this site.
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• Mutations are changes in the DNA base sequence• These are caused by errors in DNA replication or
by mutagens (a physical or chemical agent such as UV light, x-rays, chemicals)
• Mutagenesis is the creation of mutations
10.16 Mutations can change the meaning
of genes
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Translation: RNA to protein
• As the polypeptide chain grows, it begins to coil and fold into its secondary structures (helixes and sheets)
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Translation
• http://www.hhmi.org/biointeractive/media/DNAi_translation_vo1-lg.wmv