From Gene to Protein

Chapter 17

Hon. Adv. Biology Notes

11/29/10

• Connection between Genes and Proteins– Evidence that genes specify proteins

– Overview: “Central Dogma of Molecular Genetics”

– “The Code” – nucleotide triplets specify a.a.’s

– Evolution of the code

• Synthesis and Processing of RNA– A closer look: Transcription

– RNA processing occurs in eukaryotic cells

• Synthesis of Protein– A closer look: Translation

– Signal peptides – target specific cell destinations

– Review: Roles of RNA in the cell

– Review: Comparison of protein synthesis in prokaryotes and eukaryotes

– Point mutations can affect protein structure and function

– Wrap-up: “So, What is a gene?”

Lecture Objectives

• Compare the structure of RNA with that of

DNA.

• Summarize the process of transcription.

• Relate the role of codons to the sequence of

amino acids that results after translation.

• Outline the major steps of translation.

• Discuss the evolutionary significance of the

genetic code.

Figure 17.1 Beadle and Tatum’s evidence for the one gene-one enzyme hypothesis

RNA vs. DNA

• there are 3 major differences between the 2 forms of nucleic acids

– Composition of sugar backbone (reflects name)

• RNA - ribonucleic acid (ribose)

• DNA - deoxyribonucleic acid (deoxyribose)

– number of strands: RNA = 1 and DNA = 2

– RNA does not have thymine as a nitrogen base, instead uracil (U) is substituted in its place (back)

Central Dogma of

Molecular Genetics

• Explains the flow of information from gene to protein.

– Replication: flow of gene from one generation to the next

– Transcription: flow of info from DNA to RNA

– Translation: conversion of info from nucleic acid language into protein

•Transfer of Information from

DNA to RNA

• Transcription: mRNA production, or the

synthesis of other types of RNA

• Short segment of DNA (gene) is transcribed

into mRNA for movement out of the

nucleus to the ribosome

• DNA is used as a template to make a

complementary piece of “messenger

RNA”. This requires the aid of the RNA

polymerase enzyme.

Translation: Protein Synthesis

• mRNA transcript arrives at the ribosome to

act as the genetic code for protein assembly

• proteins are composed of a unique sequence

of amino acids.

• mRNA carries the code for the order and

type of amino acids to be included in the

protein. The “genetic code” is the unique

order of triplet codons located on the

mRNA strand. (back)

The Genetic Code: Three-

Nucleotide “Words”

• Genetic Code The genetic code

(codons) used by most organisms to

translate mRNA is nearly universal.

Sometimes called a triplet code.

Figure 17.3 The triplet code

Figure 17.4 The dictionary of the genetic code

What does a “Gene” really do?

• It is simply a blueprint in

the DNA that tells how to

build a protein.

• “WHO” we are

genetically is really just a

collection of “WHAT”

proteins are expressed in

our phenotype.

• Figure 17.5 A tobacco plant expressing a

firefly gene

– (back)

Evolution of the Genetic Code

• must have evolved very early in the history

of life…. Why?

• Answer: nearly universal, shared by

organisms from the simplest bacteria to

complex plants and animals

• Ex. CCG codes for the amino acid proline

in every organism ever studied (back)

The Synthesis/Processing of RNA

• The Stages of Transcription

– initiation

– elongation

– termination

• RNA processing occurs in eukaryotes

– alteration of mRNA ends

– RNA splicing

– ribozymes

– introns (back)

Protein Synthesis (Translation)

• structure and function of tRNA

• aminoacyl-tRNA synthetases

• ribosomes

• building a polypeptide

– initiation

– elongation

– termination

• signal peptides target protein destinations

(back)

Point Mutations

• types of point mutations

– base-pair substitutions

– insertions

– deletions

• Results of point mutations

– silent mutation

– missense mutations

– nonsense mutations

– frameshift mutations (back)

Chapter 18: Genetics of

Viruses and Bacteria

Part 1: Viruses

Part 2: Bacteria

What is a Virus?

• Infectious particle consisting of a genome

– either single- or double-stranded

– either RNA or DNA

– inclosed in a capsid - protein that surrounds it.

• Replicate using metabolic machinery of

host cells: bacteria, plant, animal

• Possibly evolved from plasmids or

transposons

TMV

• The search for the cause of tobacco mosaic

disease led to discovery of viruses.

• No microbe could be found and no agent

could be filtered out

• Crystallization of infectious particle.

Structural components of viruses

• Genome: genes are contained on a single

linear or circular nucleic acid molecule

• Capsid: protein shell, often large numbers

of identical subunits (capsomeres)

– rod-shaped (helical)

– polyhedral

– complex

• Envelope: derived from membranes of hosts

What is a “phage”?

• Bacteriophage: complex viruses that attack

bacteria

• Host Range: proteins on the outside of virus

only recognize a few specific receptors on

host cell surfaces.

Three patterns of Genome

Replication

• DNA viruses

• RNA viruses

• retroviruses

– reverse transcriptase

Viral Life Cycles

• Lytic cycle

• Lysogenic cycle

Defenses against Infection

• restriction enzymes

• barriers

• antibodies

• vaccines

Viroids and Prions and Viral

Evolution

• viroid

• prion

• evolution

Chapter 18 (part 2): The

Genetics of Bacteria

• short generation span aids their evolutionary

adaptation to changing environments

– circular bacterial chromosome found tightly

packed in a region called the nucleoid.

• 100x the DNA of a virus

• 1/1000 of the DNA of a eukaryote

– plasmids are also found in many strains

• genetic recombination produces new strains

Binary Fission

• replication of chromosome proceeds in two

directions from the single point of origin

prior to binary fission.

• See diagram on board.

• Due to semiconservative replication, most

bacteria in a colony are identical.

Genetic Recombination: Bacteria

• Transformation: (recall Griffith’s exp.)

– bacteria take up naked DNA

• Transduction: (recall Hershey and Chase)

– phages transfer genes from one bacterium to

another

– generalized and specialized transduction

• Conjugation and Plasmids

– 2 cells temporarily join by appendages called

sex pili

Closer Look at Conjugation

• bacteria join and transfer DNA thru pili

• ability to form pili and donate DNA

depends on the presence of an F factor

• if the F factor is on the F plasmid they are

called F+; after conjugation the F- (female)

is converted into an F+ (male)

• Hfr cells; F factor inserted into chromosome

• R plasmids: inheritance of resistance

Key Terms for Review!!

• See the key terms on the Campbell Place

website for important terms for Ch. 16-18!!!