Protein Synthesis

How does the information coded in DNA affect a cell?

DNA directs protein synthesis

The sequence of nitrogen bases contains the coded information in DNA and is translated into the amino acid sequence of a protein according to a set of rules called the genetic code. The DNA nucleotides are read in sequence, in groups of three called a triplet. Each triplet specifies 1 amino acid. The sequence of nucleotides that codes for a specific polypeptide is called a gene. Each gene also contains regulator sequences such as promoter and repressor areas or those that indicate "start" and "stop" ends.

Transcription Process:

In eukaryotes

1. DNA is transcribed into mRNA, process occurs in the nucleus
2. Need precursor nucleotides (A, G, C, U) & RNA polymerase
3. A portion of DNA (gene) separates & 1 DNA strand acts as a template to form mRNA
4. RNA polymerase attaches to DNA promoter site and binds free nucleotides to complementary nitrogen bases and links nucleotides together to form mRNA.  Multiple copies can be made.  3-base mRNA is called a codon
5. Single strand mRNA is released & double-stranded DNA reforms (conservative process)
6. RNA processing occurs in the nucleus.  mRNA contains introns that are noncoding and exons that code for protein.  Introns are cut out and exons are spliced together

There are three types of RNA involved in protein synthesis:

mRNA

1. single strand
2. produced in nucleus & travels into cytoplasm
3. half-life of minutes to hours

rRNA

1. combined with protein forms two subunits of the ribosome
2. ribosomes act as nonspecific mechanisms for assembling amino acids into proper sequence
3. ribosomes are found attached to RER or in groups in the cytoplasm

tRNA

1. short molecule with clover shape that transports amino acids
2. stalk attaches to a specific amino acid
3. on upper loop is an anticodon complementary to a mRNA codon & able to bind to it

Translation Process

1. Codons of mRNA are translated into amino acid sequences (polypeptide) in the cytoplasm
2. mRNA attaches to the light subunit of ribosome & is read sequentially from "start" codon (AUG) to "stop" (3') codon, generally nonoverlapping, occurs in about 20 sec
3. A complementary tRNA binds to 1^st codon and heavy ribosomal subunit attaches
4. A complementary tRNA binds with 2^nd codon, enzymes of the heavy subunit detach amino acid from 1^st tRNA and attach it to 2^nd amino acid
5. Ribosome moves along one codon on mRNA, process repeats until reaching the “stop” codon
6. Ribosomal subunits detach from the mRNA, polypeptide is released into ER or cytoplasm
7. mRNA can be read multiple times or by multiple ribosomes

Effects of mutations on protein synthesis:

Point mutation

1 base change codes for wrong amino acid {ex. sickle cell anemia}

Frame-shift mutation

insert or delete 1 base, whole sequence is read out of phase

Despite thousands of changes per cell per day, only a few mutations accumulate in a cell each year because DNA repair mechanisms (based on double-stranded characteristic of DNA) fix most changes