protein synthesis

Mader: Biology 8th Ed.

Gene Activity: How Genes Work


Outline

• Function of Genes– One Gene-One Enzyme Hypothesis

• Genetic Code• Transcription

– Processing Messenger RNA• Translation

– Transfer RNA– Ribosomal RNA


Function of Genes

• Genes Specify a Polypeptide– The one gene-one enzyme hypothesis

suggests a genetic mutation causes a change in the structure of a protein.

A gene is a segment of DNA that specifies the sequence of amino acids in a polypeptide of a protein.


From DNA to RNA to Protein

• Central Dogma of Molecular Biology:– Genes code for the sequence of

nucleotides in RNA molecules. RNA brings about formation of a protein

coded for by DNA of a particular gene.

TranscriptionDNA

Replication

mRNATranslation PROTEIN

Nucleus Cytoplasm

DNA Replication



RNA

• Uracil (U) replaces thymine (T) of DNA.– Types of RNA

Messenger (mRNA) - Takes message from DNA in nucleus to ribosomes in cytoplasm.

Transfer (tRNA) - Transfers amino acids to ribosomes.

Ribosomal (rRNA) - Help make up ribosomes.


Structure of RNA


Steps of Gene Expression

• During transcription, DNA serves as a template for RNA formation.

• During translation, an mRNA transcript directs the sequence of amino acids in a polypeptide.


Gene Expression


The Genetic Code

• There is a genetic code for each of the 20 amino acids found in proteins.– Genetic code is a triplet code, with each

codon consisting of three nucleotide bases.

Code Properties Universal Degenerate Unambiguous Contains start and stop signals


Transcription

• During transcription, an mRNA molecule is formed with a sequence of bases complementary to a portion of one DNA strand.– RNA polymerase joins the nucleotides

together in 5’ 3’ direction.Promoter defines start of a gene, the

direction of transcription, and the strand to be transcribed.

Elongation continues until polymerase comes to a DNA terminator sequence.


Transcription


Processing Messenger RNA

• Newly formed mRNA molecule, primary mRNA transcript, is modified before it leaves the eukaryotic nucleus.– Cap on 5’ end, and poly-A tail on 3’ end.– Introns are removed during RNA

processing, leaving only exons.Mature mRNA

In most cases, mRNA splicing is done by a spliceosome.


Translation

• Transfer RNA molecules transfer amino acids to the ribosomes.– tRNA molecules bind with one particular

amino acid, and bear an anticodon complementary to the codon for that amino acid.

tRNA synthetases attach correct amino acid to the correct tRNA molecule.


Transfer RNA


Role of Ribosomal RNA

• Ribosomal RNA (rRNA) is produced off a DNA template in the nucleolus of a nucleus.– Packaged with proteins into ribosomal

subunits, one larger than the other.Contain binding sites to facilitate

complementary base pairing between tRNA anticodons and mRNA codons.

• Ribosomes consist of two subunits.

– The large subunit has three binding sites for tRNA.– The small subunit binds to mRNA.


Ribosome Structure and Function

A. Initiation


B. Elongation


C. Termination


Online Activities and Tutorials


http://learn.genetics.utah.edu/content/begin/dna/transcribe/

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120077/micro06.swf::Protein%20Synthesis

http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html

http://www.phschool.com/science/biology_place/biocoach/transcription/tcproc.html

http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_2_.html

protein synthesis

Technology

transcription mader

transfer rna mader

structure of rna mader

function mader

elongation mader

signals mader

tutorials mader

termination mader