Chapter 14 From DNA to Protein

Byssus Marine mussel manufactures the ultimate underwater adhesive, a protein called byssus

DNA is like a book of instructions in each cell The instructions are written in the alphabet of A,T,G,C. But merely knowing the letter does not tell us how the genes work DNA consist of two strands of nucleotides twisted together in a double helix. – In replication, the two strands unwind to serve as templates for assembly of new complenetary strands

Continue… Each gene is a linear stretch of DNA nucleotides that codes for the assembly of amino acids into a polypeptide

Three things to move Chromosomes are made of DNA Segments of DNA code for a protein Protein in turn, relates to a trait (eye color, enzymes, hormones..)

The path from genes to proteins has two steps: Transcription: molecules of RNA are produced on the DNA templates in the nucleus Translation: RNA molecules shipped from the nucleus to the cytoplasm are used as templates for polypeptide assembly Transcription Translation DNA  RNA  proteins

Transcription Translation

How is RNA Transcribed from DNA? Three classes of RNA 1- Messenger RNA (mRNA) carries the blueprint for protein assembly to the ribosome (Goes in the nucleus and translates the material from DNA to RNA) 2- Transfer RNA (tRNA) brings the correct amino acid to the ribosome and pairs up with an mRNA code for that amino acid

Continue… 3- Ribosomal RNA (rRNA) combines with proteins to form ribosome upon which polypeptide are assembled

DNA VS RNA Double Stranded A, T, G, C- Nitrogen Bases Sugar: Deoxyribose Purpose: hereditary Single Stranded A, U, G, C- Nitrogen Base (notice that thymine changes to uracil Sugar: Ribose Purpose: Protein Synthesis

Transcription Difference from Replication Only one region of one DNA strand is used as a template RNA polymerase is used instead of DNA polymerase

Continue… Transcription begins when RNA polymerase binds to a promoter region (a base sequence at the start of a gene) and then moves along to the end of a gene

Finishing Touches on mRNA Transcripts New formed mRNA is an unfinished molecule, not yet ready for use mRNA transcripts are modified before leaving the nucleus – The 5’ end is capped with a special nucleotide that may serve as a “start” signal for translation – Noncoding portions (introns) are snipped out, and actual coding regions (exons) are spliced together to produce the mature transcript

What is a gene code? Both DNA and its RNA transcript are linear sequences of nucleotides carrying the hereditary code

Continue… Every three bases (a triplet) specifies an amino acid to be included into a growing polypeptide chain; the complete set of triplets or is called the genetic code – Each base triplet in RNA is called codon – The genetic code consists of sixty-one triplets that specify amino acids and three that serve as the stop protein synthesis – AUG: Starts protein sythesis – UAA, UAG, UGA: Stop protein synthesis

Codon Chart

DNA Strand: T G C A T C A G A RNA Strand: A C G U A G U C U Hyperlink\animationstranscription.htm

Structure and Function of tRNA and rRNA Each kind of tRNA has an anticodon that is complementary to an mRNA codon; each tRNA also carries one specific amino acid After the mRNA arrives in the cytoplasm, anticodon on a tRNA bonds to the codon on the mRNA, and thus a correct amino is brought into place

Continue… The first bases of the anticodon must pair up with the codon by the usual rules base pairing (A with U and G with C), but there is some latitude in the pairing of the third base (called the wobble effect) A ribosome has two subunits (each composed of rRNA and proteins) that perform together only during translation

Stages of Translation Initation – a complex forms in this sequence: initiator tRNA + small ribosomal subunit + mRNA + large ribosomal subunit Elongation: Start codon on mRNA defines the reading frame; a series of tRNA deliver amino acids in sequence by codon-anticodon matching; peptide bond joins each amino acids to the next in sequence

Continue… Termination: a stop codon is reached and the polypeptide chain is released into the cytoplasm or enters the cytomembrane system for further processing

What happens to the new polypeptides? Three steps just outlined can be repeated many times on the same mRNA at the same time Some polypeptide joins the cytoplasm’s pool of free proteins; other enter the rough ER of the cytomembrane system LOOK DRAWINGS (REPLICATION, TRANSCRIPTION, AND TRANSLATION)

Do mutations affect protein synthesis? Gene mutation is a change in one to several bases in the nucleotide sequence of DNA, which can result in a change in the protein synthesized

Mutations Mutations (“GENE MISTAKES”) can results from base-pair substitutions, insertions (frameshift mutations), deletion Results when DNA regions (called transposable elements) move form one location to another in the same DNA molecule of different one

Causes of Gene Mutation Mutations are rare, chance events but each gene has a characteristics mutations rate Mutations can be caused by mutagens such as ultraviolet radiation, ionizing radiation (gamma rays and X-rays) and chemical such as alkylating agents, which act as carcinogens

The proof is in the protein If a mutation arises in a somatic cells, it will affect only the owner of the at cell and will not be passed on to offspring If mutations arises in a gamete, it may be passed on and thus enter the evolutionary arena Mutations may prove to be harmful, benefical, or neutral in its effects