chapter 14 from dna to protein. byssus marine mussel manufactures the ultimate underwater adhesive,...
TRANSCRIPT
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 TranslationDNA RNA proteins
TranscriptionTranslation
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