Protein synthesis DNA is the genetic code for all life. DNA literally holds the instructions that make all life possible. Even so, DNA does not directly “do” anything. Therefore, the processes of 1) transcription and 2) translation allow a cell to carry out the process of taking the code of DNA to mRNA and eventually from mRNA to protein. In other words, the flow of information in a cell goes from: DNA mRNA Protein

Transcription – DNA is “transcribed” into mrna What? DNA coding mRNA Where? Nucleus Why? DNA is double stranded and too large to get out of the nucleus through the nuclear pores. (mRNA is single stranded and can escape the nucleus).

Transcription: Key Terms mRNA(messenger RNA)- takes DNA message to ribosomes Promoter DNA – a nucleotide sequence on DNA that signals for transcription to begin at this area This is the site for RNA Polymerase binding and determines which of the two strands of DNA is to be transcribed Terminator DNA – sequence of DNA that signals the end of transcription and the end of the gene

Transcription: Key Terms

Transcription: Key Terms (continued) Helicase – transcription enzyme that breaks the Hydrogen bonds between DNA bases so that transcription can begin RNA (or DNA) Polymerase – transcription enzyme that adds RNA nucleotides to the DNA template by helping to form Hydrogen bonds between the bases of DNA and mRNA

Steps of Transcription 1) Initiation – RNA polymerase binds to promoter DNA 2) RNA elongation – RNA polymerase “slides” down DNA template creating mRNA as it goes by adding RNA nucleotides by correct base pairing rules (A to U and C to G) As RNA synthesis continues, the RNA strand peels away from its DNA template and the two DNA strands come back together 3) Termination - RNA polymerase reaches terminator DNA and the polymerase detaches from the RNA and the gene (DNA)

Steps of Transcription

Mrna processing - mRNA is changed before it leaves the nucleus Changes G-Cap and Poly-A tail – A single Guanine base is added to one end of the mRNA and long tail of 50 to 250 Adenine nucleotides to the other end These help to export mRNA from nucleus, protect mRNA, and help ribosome bind to mRNA Neither of these are translated into the protein RNA Splicing DNA sequences that code for polypeptides are not continuous Intron – internal noncoding regions Exon – coding regions of DNA that are the parts of a gene that are to be expressed as amino acids Introns are “cut” out of the mRNA and the exons are “pasted” together

RNA splicing animation

Other types of Rna produced by transcription tRNA (transfer RNA) – transfers amino acids from cytoplasm to ribosomes Has a site on top for amino acid attachment The bottom of the tRNA is known as an anticodon Acts as the “interpreter” when translating “nucleic acid language” to protein “language” rRNA (ribosomal RNA) – a type of RNA that, along with proteins, make up the 2 subunits of ribosomes

Other types of Rna produced by transcription Continued

Translation – mrna is “translated” into proteins What? mRNA is read by ribosomes and proteins are built from these instructions Where? Ribosomes in the cytoplasm Why? To create proteins to carry out basically every function in the body

Translation: key terms Codon – mRNA is read by the ribosome in groups of 3 bases. Each codon (3 mRNA bases) codes for 1 amino acid Amino acid – monomer (building block) of protein Anticodon – 3 bases on the bottom of tRNA that are complementary (opposite) to the codons on mRNA. Anticodons on the bottom of tRNA ensure that each codon codes for only 1 amino acid Ribosome – Reads mRNA codons and sends out signal to tRNA to bring in appropriate amino acid (by matching codon of mRNA to anticodon of tRNA)

Steps of translation 1) Initiation – binding of mRNA to ribosome mRNA binds to small ribosomal subunit tRNA then binds to the start codon (which is AUG) to bring in first amino acid – MET Large ribosomal subunit binds to the small one, creating a functional ribosome Ribosome now has 2 binding sites P site = holds tRNA with growing polypeptide A site = vacant site where next amino-acid bearing tRNA will bind

Steps of translation continued 2) Elongation – Amino acids are added one by one to first amino acid. Occurs in 3 step process. Codon recognition – Anticodon of incoming tRNA molecule, carrying its amino acid, pairs with mRNA codon in A site Peptide bond formation - Polypeptide separates from tRNA in P site and attaches by a peptide bond to amino acid carried by tRNA in A site Translocation - P site tRNA now leaves the ribosome, and ribosome translocates (moves) the tRNA in the A site, with its attached polypeptide, to the P site. The codon and anticodon remain bonded so tRNA and mRNA move as a unit. This opens the A site for the next amino acid to be brought in by a tRNA

Steps of translation continued

Steps of translation continued 3) Termination – Elongation continues until a stop codon reaches the A site Ribosome then breaks apart and finished polypeptide is released from tRNA where it was growing