protein synthesis. dna rna protein 2 major steps 1.transcription – dna is transcribed (copied!)...
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![Page 1: Protein Synthesis. DNA RNA Protein 2 Major Steps 1.Transcription – DNA is transcribed (copied!) into single stranded mRNA (DNA code transcribed into](https://reader035.vdocuments.us/reader035/viewer/2022062320/56649d0b5503460f949de43a/html5/thumbnails/1.jpg)
Protein Synthesis
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DNA RNA Protein
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2 Major Steps
1. Transcription – DNA is transcribed (copied!) into single stranded mRNA (DNA code transcribed into RNA code)
2. Translation – mRNA is translated into protein (amino acids)
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Transcription• Occurs in nucleus (where the DNA is located!)• Generally divided into three steps – initiation,
elongation, termination
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A. Initiation• RNA polymerase (enzyme) attaches to promoter
regions of DNA at TATA box• RNAP requires TATA box and other transcription
factors to be present
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B. Elongation
• RNAP can only add nucleotides to the 3’ of the growing mRNA strand; thus mRNA is synthesized in the 5’ 3’ direction
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C. Termination• Transcription ends when
mRNA polymerase reaches a specific STOP sequence:– ATT– ATC– ACT
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mRNA processing• After transcription, mRNA must be modified so
that it can exit the nucleus1. GTP cap added to 5’ end of mRNA (stability & attachment point for ribosome down the road)2. Poly-A tail added to 3’ end of mRNA (stability & guidance so mRNA goes from nucleus to ribosome)3. Removal of introns
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Introns vs. Exons• Exons – sequences that contain the code for a
polypeptide (protein); exons are expressed• Introns – non-coding sequences of mRNA
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• Original, unprocessed mRNA contains both introns and exons
• Before mRNA exits nucleus for translation at ribosome, snRNPs (small nuclear ribonucleoproteins) remove introns and splice exons together
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Translation
• Occurs in cytoplasm where processed mRNA meets ribosome and is translated into protein
• Steps:– Initiation– Elongation– Termination
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For translation to occur, you need…
1. Activated tRNA -tRNA in cytoplasm finds the correct amino acid-Attachment (activation) requires 1 ATP and enzyme
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2. Ribosomal subunits• Ribosomes consist of 2 subunits (small and large)– Each subunit is made of rRNA and protein– Each is built separately in nucleolus
• The two subunits exit nucleolus and form the ribosome in the cytoplasm when translation starts
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The Genetic Code• Codon – triplets of bases• mRNA makes a template strand (from DNA) that
is translated into protein via a triplet code– Every three base pairs codes for a particular aa
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1. Initiation
• Brings together mRNA (from nucleus), tRNA (in cytoplasm), and ribosomal subunits (from nucleolus)
1. Small ribosomal subunit binds to mRNA and a specific tRNA (methionine)
2. Small subunit scans mRNA until it finds AUG (start codon)
3. Large subunit attaches the complex
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2. Elongation
• Begins with the next tRNA arriving at the P site of the ribosome
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Polyribosome
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3. Termination• Occurs when the ribosome encounters one of
three STOP codons located on mRNA• At this point, polypeptide synthesis is
complete and the ribosome detaches from the polypeptide
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Point Mutations
• Chemical changes in just one base pair of a gene
• 2 general types:– Base pair substitutions– Base pair insertions or deletions
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1. Base pair substitutions• Substitution of one base pair with another• Often silent mutations because they don’t have
an effect on encoded protein due to redundancy of genetic code: – DNA: CCG CCA– mRNA: GGC GGU– AA: gly gly
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Example: Sickle Cell Anemia• Results from a single point
mutation in a gene that codes for one of the polypeptide chains that form hemoglobin
• Fatigue, paleness, rapid heart rate, shortness of breath, jaundice
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2. Insertions & Deletions• Additions or deletions of nucleotide base pairs• BAD NEWS… these are often called frameshift
mutations because they literally shift the reading frame of the mRNA protein
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Example: Tay-Sachs Disease• Inherited frameshift
mutation on chromosome 15
• Mutation results in malformation of hexoaminidase A, a protein that breaks down a particular chemical in gangliosides (nerve tissue)
• Deafness, blindness, dementia, paralysis, slow growth, mental retardation etc
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Mutagens• Spontaneous mutations – errors that result from
cellular machinery malfunction• Environmental mutations – damage to DNA
caused by environment– Mutagens – chemical agents that interact with DNA
and cause problems• X-rays, UV light, various drugs (seriously)