DNA & RNA The Molecular Basis of Inheritance

DNA & RNA The 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.

DNA Griffith 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

DNA Griffith and Transformation Griffith performed four sets of experiments – Fig. 12-2 Experiment – injected live S strain into the mice; Results – mice developed pneumonia & died Conclusion – S strain causes disease Experiment – injected live R strain into the mice: Results – mice survived Conclusion – R strain does not cause disease

DNA Griffith and Transformation Griffith performed four sets of experiments – Fig. 12-2 Experiment – injected heat killed S strain Results – mice survived Conclusion – polysaccharide coat does not cause pneumonia

DNA Griffith 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 cells Conclusion – R cells had acquired “some factor” to make polysaccharide coat

DNA Griffith and Transformation

DNA Griffith 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 & DNA http://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 Experiment More 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 Experiment More 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 Experiment More 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 DNA 2. 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 Experiment More evidence that DNA is the genetic material http://highered.mcgraw-hill.com/olc/dl/120076/bio21.swf (Hershey/Chase experiment animation)

Structure of DNA- Fig. 12-5 Nucleotide – functional unit; composed of a phosphate group, sugar (deoxyribose), and a nitrogenous base T- thymine A – Adenine G – 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 Replication Prokaryotic Cells lack a membrane bound nucleus; only one circular chromosome holds most of the genetic material. Fig. 12-8

Chromosomes & DNA Replication Eukaryotic 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. 12-11 The two DNA strand separate Each 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  Trait Stucture of RNA Single stranded Sugar is ribose instead of deoxyribose Uracil (U) replaces Thymine (T)

RNA vs DNA

Types of RNA – Fig. 12-12 Messenger RNA – mRNA, contains “code” or instructions for making a particular protein Ribosomal RNA – rRNA (part of the ribosome), facilitates the orderly linking of amino acids into polypeptide chains Transfer 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. 12-14 RNA polymerase binds to DNA strand and separates it RNA polymerase will bind to a promoter, a specific “start’ region of the DNA molecule Nucleotides are assembled into a strand of RNA Transcription 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 molecule Exons 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, 12-17 A 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

Translation Fig. 12-18 Translation is the synthesis of a polypeptide 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 tRNA 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 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

Mutations Any 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 Regulation Genes 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 in Prokaryotes – Fig. 12-23 Structural gene – gene that codes for a protein Operon – a group of genes that operate together Operator – a DNA segment between an operon’s promoter and structural genes, which controls access of RNA polymerase to structural genes

Gene Regulation in Prokaryotes – (cont.) Repressor – a specific protein that binds to an operator and blocks transcription of the operon The lac operon is turned off by repressors and turned on by the presence of lactose.

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

Gene Expression in Eukaryotes Eukaryotic genes coding for enzymes of ametabolic pathway are often scatttered over different chromosomes and havew their own promoters. Fig. 12-24 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 Eukaryotes 2. 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