Protein Synthesis

DNA RNA Protein

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)

Transcription• Occurs in nucleus (where the DNA is located!)• Generally divided into three steps – initiation,

elongation, termination

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

B. Elongation

• RNAP can only add nucleotides to the 3’ of the growing mRNA strand; thus mRNA is synthesized in the 5’ 3’ direction

C. Termination• Transcription ends when

mRNA polymerase reaches a specific STOP sequence:– ATT– ATC– ACT

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

Introns vs. Exons• Exons – sequences that contain the code for a

polypeptide (protein); exons are expressed• Introns – non-coding sequences of mRNA

• 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

Translation

• Occurs in cytoplasm where processed mRNA meets ribosome and is translated into protein

• Steps:– Initiation– Elongation– Termination

For translation to occur, you need…

1. Activated tRNA -tRNA in cytoplasm finds the correct amino acid-Attachment (activation) requires 1 ATP and enzyme

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

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

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

2. Elongation

• Begins with the next tRNA arriving at the P site of the ribosome

Polyribosome

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

Point Mutations

• Chemical changes in just one base pair of a gene

• 2 general types:– Base pair substitutions– Base pair insertions or deletions

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

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

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

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

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)