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

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• 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

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• 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

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• 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