DNA & RNAThe Molecular Basis of Inheritance

DNA & RNAThe Molecular Basis of Inheritance

By the 1940’s, scientists knew that chromosomes carried hereditary material and consisted of DNA and proteins. Most thought proteins were the genetic material because it is a complex macromolecule and little was known about nucleic acids.

DNAGriffith and Transformation

In 1928, Frederick Griffith was trying to determine how bacteria infected people.

He isolated two different strains of pneumonia bacteria

1. smooth strain (S) – polysaccharide coat, on the bacterial cell prevents attach by the immune system

2. rough strain (R) – polysaccharide coat is absent and therefore the immune system can kill the bacteria

DNAGriffith and Transformation

Griffith performed four sets of experiments – Fig. 12-2

Experiment – injected live S strain into the mice; Results – mice developed pneumonia & diedConclusion – S strain causes disease

Experiment – injected live R strain into the mice:Results – mice survivedConclusion – R strain does not cause

disease

DNAGriffith and Transformation

Griffith performed four sets of experiments – Fig. 12-2

Experiment – injected heat killed S strainResults – mice survivedConclusion – polysaccharide coat does

not cause pneumonia

DNAGriffith and Transformation

Griffith performed four sets of experiments – Fig. 12-2

Experiment – Heat killed S strain cells mixed with the live R strain cells and then injected into mice

Results – mice died from pneumonia & blood samples from dead mice contained

living S strain cellsConclusion – R cells had acquired “some

factor” to make polysaccharide coat

DNAGriffith and Transformation

DNAGriffith and Transformation

Transformation – the assimilation of external genetic material by a cell

The disease causing ability was inherited by the bacterial offspring, therefore information for disease might be located on a gene.

Avery & DNAhttp://www.dnalc.org/view/16375-Animation-17-A-gene-is-made-of-DNA-.html

The above link is an explanation of Griffith’s & Avery’s findings. Great site – please review. Avery discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next.

Hershey-Chase ExperimentMore evidence that DNA is the genetic

material

Bacteriophage – a virus that infects a bacterium; made up of DNA or RNA and a protein coat. – Fig. 12-3, 12-4

Hershey-Chase ExperimentMore evidence that DNA is the genetic

material

DNA – contains no sulfur but does have phosphorus

Proteins – contain almost no phosphorus but do have sulfur

Hershey-Chase ExperimentMore evidence that DNA is the genetic

material

Hershey & Chase performed two sets of experiments

1. T2 with radioactive phosphorus infects bacterium – 32P shows up in bacterial

DNA2. T2 with radioactive sulfur infects

bacterium – 35S does not show up in bacterial DNA

3. Conclusion – genetic material of T2 was DNA not protein

Hershey-Chase ExperimentMore evidence that DNA is the genetic

materialhttp://highered.mcgraw-hill.com/olc/dl/120076/bio21.swf (Hershey/Chase experiment animation)

Structure of DNA- Fig. 12-5Nucleotide – functional unit; composed of a phosphate group, sugar (deoxyribose), and a nitrogenous base

T- thymine A – AdenineG – Guanine C – cytosine

Chargaff’s Rules – Fig. 12-6[A] = [T] [C] = [G]

Structure of DNA- Fig. 12-5

X-ray evidence – x shaped pattern shows DNA strands are twisted and nitrogenous bases are in the center (Rosalind Franklin created this image which was used by Watson & Crick to explain the structure of DNA) She probably would have shared in the Nobel Peace Prize with them for this discovery if she had not died.

Structure of DNA- Fig. 12-5

Structure of DNA- Fig. 12-5

Structure of DNA- Fig. 12-5

Chromosomes & DNA ReplicationProkaryotic Cells lack a membrane bound nucleus; only one circular chromosome holds most of the genetic material. Fig. 12-8

Chromosomes & DNA ReplicationEukaryotic cells have a membrane bound nucleus; chromosomes are found in pairs and the number is species specific

DNA is a very long molecule and must be a tightly folded

Chromatin – DNA & histone proteins make up a unit called a nucleosome Fig. 12-10

DNA Replication – Fig. 12-11The two DNA strand separateEach strand is a template for assembling a complementary strand.Nucleotides line up singly along the template strand in accordance with the base-pairing rules ( A-T and G-C)DNA polymerase links the nucleotides together at their sugar-phosphate groups.http://www.youtube.com/watch?v=hfZ8o9D1tus

DNA Replication – Fig. 12-11

RNA and Protein Synthesis

DNA RNA Protein TraitStucture of RNA

Single strandedSugar is ribose instead of deoxyriboseUracil (U) replaces

Thymine (T)

RNA vs DNA

Types of RNA – Fig. 12-12Messenger RNA – mRNA, contains “code” or instructions for making a particular proteinRibosomal RNA – rRNA (part of the ribosome), facilitates the orderly linking of amino acids into polypeptide chainsTransfer RNA – tRNA, brings amino acids from the cytoplasm to the ribosome

mRNA

tRNA

rRNA

Transcription Transcription is the synthesis of RNA using DNA as a template: Fig. 12-14RNA polymerase binds to DNA strand and separates itRNA polymerase will bind to a promoter, a specific “start’ region of the DNA moleculeNucleotides are assembled into a strand of RNATranscription stops when RNA polymerase reaches a specific “stop” region of the DNA molecule

Transcription

Transcription

https://www.youtube.com/watch?v=rKxZrChP0P4

This video also shows translation

RNA Editing Only a small portion of the original RNA sequence leaves the nucleus as mRNA because portions are edited out. Fig. 12-15

Introns are the noncoding sequences in the DNA that are edited out of the pre mRNA moleculeExons are the coding sequences of a gene that are transcribed and expressed (translated into a protein)

RNA Editing

Transcription

The Genetic Code Fig. 12-16, 12-17A codon is a three-nucleotide sequence in mRNA that:• signals the starting place for translation• specifies which amino acid will be added to a growing polypeptide chain• signals termination of translation

Some amino acids are coded for by more than one codon

The Genetic Code

TranslationFig. 12-18Translation is the synthesis of apolypeptide chain, which occurs under the idrection of mRNA• Three major steps of translation include: Initiation, Elongation, and Termination• Initiation - must bring together the mRNA, two ribosomal subunits, and a tRNA

Translation (cont.)Fig. 12-18•Elongation – polypeptide assembly line

1) Codon on mRNA bonds with anticodon site on tRNA2)The amino acid that is brought

in by tRNA is added to the growing polypeptide chain

3) tRNA leaves ribosome

• Termination – stop codon is reached and the entire complex separates

Translation

Translation (cont.)

http://www.youtube.com/watch?v=5bLEDd-PSTQ(translation)

You can also go back to transcription slide to see another video on translation

Translation (cont.)From Polypeptide to Functional Protein – depends upon a precise folding of the amino acid chain into a three-dimentional conformation

MutationsAny change in the genetic material is a mutation.Gene mutations – changes in a single gene – Fig. 12-20

1. point mutations – changes involving only one or a few nucleotides (substitution, insertion, deletion) that affects only one amino acid

Mutations (cont.)2. frameshift mutation (a type of

point mutation) – “reading frame” of the genetic message is changed because of insertion or deletion of a nucleotide, therefore the entire sequence of amino acids can change

Mutations (cont.)Substitution

Mutations (cont.)Insertion and Deletion

Mutations (cont.)Chromosomal mutations – changes in the number of structure of chromosomes; includes – deletion, duplication, inversion, and translocation – Fig. 12-21

Gene RegulationGenes can be switched “on” or “off” depending on the cell’s metabolic needs, (i.e. muscle cell vs. neuron, embryonic cell vs. adult cell) Fig. 12-22

Gene Regulation inProkaryotes – Fig. 12-23

Structural gene – gene that codes for a proteinOperon – a group of genes that operate togetherOperator – a DNA segment between an operon’s promoter and structural genes, which controls access of RNA polymerase to structural genes

Gene Regulation inProkaryotes – (cont.)

Repressor – a specific protein that binds to an operator and blocks transcription of the operonThe lac operon is turned off by repressors and turned on by the presence of lactose.

Gene Regulationhttp://www.sumanasinc.com/webcontent/animations/content/lacoperon.html (lac operon animation)

Gene Expression in EukaryotesEukaryotic genes coding for enzymes of ametabolic pathway are often scatttered over different chromosomes and havew their own promoters. Fig. 12-24

1. TATA box – a repeating sequence of nucleotides that helps position RNA polymerase to the promoter site

http://www.youtube.com/watch?v=7EkSBBDQmpE

TATA box animation

Gene Expression in Eukaryotes

Gene Expression in Eukaryotes2. Enhancer – noncoding DNA

control sequence that enhances a gene’s transcription and that is located thousands of bases away from the gene’s promoter

Development and Differentiation

Differentiation – to become more specialized

Hox – genes – a series of genes that control the differentiation of cells and tissues in the embryo – Fig. 12-25