FROM DNA TO PROTEIN Transcription – Translation

I. Overview Although DNA and the genes on it are responsible for inheritance, the day to day operations of the cell are done by proteins. The information from DNA has to be converted into a functioning protein. The molecule that carries, edits and regulates this information to make proteins are various forms of RNA

Central Dogma of Molecular Biology The flow of information in the cell starts at DNA, which replicates to form more DNA. Information is then ‘transcribed” into RNA, and then it is “translated” into protein. The proteins do most of the work in the cell. Information flows in the other direction only in cases of some viruses and prions.

Points to review:  Difference between DNA and RNA  Cell organelles – nucleus, ribosomes RNA molecules are smaller polynucleotide chains that are formed from ribose, nitrogen- containing bases (A, T, G and U) and phosphate. They are usually single stranded Some types of RNA:  Messenger RNA (mRNA) – carries information  Transfer RNA (tRNA) – carries amino acids  Ribosomal RNA (rRNA) – forms ribosomes

II. Transcription Transcription – the copying of a part of a DNA molecule to make an mRNA (messenger RNA) molecule. Active transcription occurs in specific parts of the nucleus The process of transcription: 1. Initiation – Transcription protein factors bind to the promoter region of the DNA molecule. This region is recognized by the presence of a “TATA box”

2. The transcription factors help RNA polymerase enzyme to find the proper region to be copied. 3. After binding, RNA polymerase begins to unwind the DNA double helix – the template chain of the DNA molecule will be copied. 4. Elongation – RNA polymerase assembles the RNA nucleotides that complement the template DNA nucleotides. This enzyme also reads from the 3’ – 5’ direction and builds the new RNA strand in the 5’-3’ direction. 5. As the RNA polymerase continues to copy the new strand, the copied section of the DNA molecule joins back because the RNA chain separates from the template.

6. Termination – The transcription continues until the enzyme encounters a terminator signal – a sequence of nucleotides that signals the end of transcription. 7. After the terminator nucleotides are transcribed the pre-mRNA molecule is released. 8. When the pre-mRNA is released the DNA reforms its original double helix structure. Game:

III. mRNA Processing Process that protects and allows the mRNA to leave the nucleus. Before the mRNA leaves the nucleus, its ends are modified with a 5’cap and a polyA tail to protect it from damage in the cytoplasm. The introns are also removed from the pre-mRNA. Introns are part of the primary transcript but do not provide useful information for protein coding. Exons are segments that will code amino acids for proteins.

After introns are removed the exons join together to form the final mRNA molecule

IV. The Genetic Code The genetic code – triplets of mRNA nucleotides that determine an amino acid These triplets are called codons There are 64 variations of codons The genetic code is  Universal  Continuous  Redundant (wobble)

Start codon – AUG (also determines methionine) – start translation Stop codons – UAA, UGA, UAG – stops translation

V. Transfer RNA (tRNA) Clover leaf” structure  anticodon on “clover leaf” end  amino acid attached on 3 end

VI. Ribosomes Small organelles that are made up of rRNA and proteins. Composed of two subunits (small and large) Located on the rough ER or in the cytoplasm. There are also a few ribosomes in mitochondria and chloroplast Function: makes proteins by reading the code on the mRNA molecule

Binding sites:  A-site binds to the tRNA anticodon that carries the proper amino acid  P-site binds to the last entering tRNA that holds the growing polypeptide chain  E-site allows the exit of the empty tRNA

VII. Translation Review: protein structure and function Translation – The process of converting the information from the “language” of nucleic acids to the “language” of proteins. It takes place in ribosomes in the cytoplasm or on the surface of the endoplasmic reticulum The product of this process is a polypeptide chain (primary protein structure)

The actual process is composed of an initiation, elongation, and termination stage. During initiation all participants of translation come together: mRNA (for genetic information) bind to the small subunit of the ribosome. The tRNA molecule that carries Met (start codon) also attaches to the small subunit (on the A-site). Finally, the large subunit binds to the small subunit. The copying starts at the 5’ end of the mRNA

This process requires energy (GTP provides it)

During elongation: The ribosome moves over and reads the next codon on the mRNA. The matching tRNA enters to the A-site, the previous tRNA moves over to the P-site. The amino acids on the two tRNA molecules bind by peptide bond. This process keeps repeating until a stop signal on the mRNA is reached.

Termination starts when the stop codon on the mRNA is reached. Instead of the a tRNA, a release factor comes in, reads the triplet and makes the entire complex of mRNA, tRNA, polypeptide fall apart.

Related animations:   class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3. html class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3. html 

VIII. Point mutations Point mutations small changes on the nucleotide sequence of the DNA Can be caused by deletion, insertion or a single base change. Single base changes only change one amino acid Deletion and insertion can shift the entire reading frame and can result in a disfunctional or no polypeptide at all.