CH 11.4 & 11.5“DNA to Polypeptide”

Genes are coded DNA instructionsthat control theproduction of proteinswithin the cell…

DNA → RNA → Protein

…just how are those instructions decoded???

…by copying the nucleotidesequence of DNA intoRNA: ribonucleic acid!

Step 1:

Like DNA, RNA is a longchain made up of nucleotides.

There are 3 main differences:

- the sugar in RNA is ribose (not deoxyribose);

- RNA is mostly single-stranded;

- RNA contains uracil instead of thymine.

Types of RNA

Messenger RNA

Messenger RNA

Carries copies of gene instructions (to synthesize proteins out of amino acids) from DNA to the rest of the cell.

Ribosomal RNA

Proteins are assembledon ribosomes, which are made up of many proteins as well as ribosomal RNA.

Transfer RNA

Transfer RNA delivers each amino acid coded for by messenger RNA to the ribosome for proteinassembly.

Transcription

…part of the nucleotide sequence of DNA is copied into a complementary sequence of RNA.

• RNA polymerase opens up DNA and separates the strands.

• RNA polymerase uses one DNA strand as a template to produce a strand of RNA.

• RNA polymerase binds only to promoters, special DNA regions with specific base sequences…tell where to start and stop transcription.

Introns & Exons: RNA Editing

• Introns are intervening sequences of nucleotides (non-coding segments of DNA) that are cut out from the mRNA before it travels out to the ribosomes.

• Exons are expressed sequences of nucleotides (they will be synthesized into proteins) that get spliced together as mRNA.

Can YOU transcribe?

RNA strand: A U U G A C U G A A C U G A

DNA strand: T A A C T G A C T T G A C T

REMEMBER: in transcription, uracil replaces thymine in RNA!

Translation: Breaking the genetic code

The process of decoding mRNA nucleotides into

proteins• Proteins are made by

joining amino acids into long chains called polypeptides.

• The order of these amino acids determines which protein is made.

• The sequence of nitrogenous bases in DNA determines the order of amino acids, thus dictating the protein.

The genetic code is written in a language that only has four letters:

A,U,G &C!These letters (nucleotides) combine in different ways to form the code for 20

different amino acids.

The genetic code is read 3 letters (nucleotides) at a time in groups called codons.

64 possible codons(4 bases3)

…some amino acidscan be coded forby more than onecodon…

…can you give an example?

One codon, AUG, canspecify methionine or “start” codon for protein synthesis.

Note the three “stop” codons that do note code for any amino acid – they signalthe end of a polypeptidechain.

Consider the following mRNA sequence:

U C G C A C G G U

This sequence would be read three bases at a time as:

U C G - C A C - G G UThe codons represent different amino acids:

U C G - C A C - G G U (Serine - Histidine – Glycine)

TranslationmRNA leaves thenucleus, attaches to ribosome…

…starting at AUG, mRNA’s codon attracts tRNA ‘s anticodon with its amino acid…

…in the ribosome, the amino acid is transferred to the growing polypeptide chain.

http://www.google.com/url?sa=i&rct=j&q=peptide+synthesisdiagram&source=images&cd=&cad=rja&docid=CTseCm6YEExo5M&tbnid=GLw1_0z1JA9EuM:&ved=0CAUQjRw&url=http://www.crtdb.net/showthread.php/4917-dna-chain-antiparallel-ladder-diagram-protein-synthesis-steps-for-kids&ei=S4kiUeukN8HXygH6mYCgBA&bvm=bv.42553238,d.aWc&psig=AFQjCNEfuipRcDdy5yjrQBylxHMPw36PTA&ust=1361304121822860

Transcription AND Translation!

http://www.sciencephoto.com/image/210289/large/G1100840-Protein_synthesis-SPL.jpg

Videos for Review

• Replicationhttp://www.dnalc.org/resources/3d/DNAReplicationBasic_w_FX.html

• Transcription http://www.dnalc.org/resources/3d/TranscriptionBasic_withFX.html

• Translationhttp://www.dnalc.org/resources/3d/TranslationBasic_withFX0.html