Chapter 10 DNA, RNA, and Protein Synthesis

Section 10.1 Discovery of DNA

3 experiments lead to the discovery of DNA: – Griffith – Avery – Hershey-Chase

Griffith’s Experiment: – Studying a bacterium  S. pneumoniae – Some strains can cause lung disease pneumonia in mammals. – Trying to develop a vaccine against this virulent strain. – Bacterium has a capsule made of polysaccharides that protect it from the body’s defense system.

2 strains of bacterium: – S strain: Smooth-edge colonies Causes pneumonia – R strain: Does not cause pneumonia Lacks capsule Rough colonies 4 experiments:

Conclusions: – Heat-killed virulent bacterial cells release a hereditary factor that transfers the disease – causing ability to the live harmless cells. – Transformation

Avery’s Experiment: – Testing Griffith’s experiment to see if the transforming agent was RNA or DNA – Used enzymes specific to what they were testing – Conclusions: DNA is responsible for the transformation in bacteria

How did Avery’s Experiment differ from Griffith’s Experiment? Griffith found that 1 strain of bacteria could be transformed to another and concluded that a transforming factor was involved. Avery tested to see which molecule changed the R strain into the S strain of bacteria. He found that DNA was the transforming molecule.

Hershey-Chase Experiment: – Test whether DNA or protein is the hereditary material viruses transfer when they enter a bacterium. Bacteriophages – used radioactive isotopes to label protein and DNA – 35 S  protein – 32 P  DNA Conclusion: – All of the viral DNA entered the E.coli – Little of the protein entered the E.coli – DNA is the hereditary molecule in viruses.

Section 10.2 DNA Structure

DNA Double Helix James Watson Francis Crick Found DNA is made of two chains that wrap around each other  double helix – Explained how DNA replicates Received Noble Prize in medicine

DNA Nucleotides DNA: – Nucleic acid – 2 long chains (strands) of repeating subunits called nucleotides Consist of 3 parts: – 5-carbon sugar: » deoxyribose – Phosphate group: » P atom bonded to 4 oxygen atoms – Nitrogenous base: » Face toward center of molecule – form H bonds » Bonded in pairs – by 2 /3 H bonds Form the steps of the staircase – H bonds hold 2 chains of helix together

Nitrogenous Bases 4 bases: – Thymine  T – Cytosine  C – Adenine  A – Guanine  G Purines: – Double ring of C and N atoms A and G Pyrimidines: – Single ring of C and N atoms C and T

Complementary Bases Structure of DNA paired by base-pairing rules – Amount of adenine = thymine – Amount of cytosine = guanine Complementary base pairs: – A – T – C – G – Each pair contains 1 purine and 1 pyrimidine Base sequence: – ATTC – TAAG

Complementary base pairing: – Important in structure and function: H bonds between base pairs help hold 2 strands of DNA together Complementary nature of DNA helps explain how DNA replicates before cell division. – 1 strand serves as a template

What would be the complementary strand of the follow: – AGTA: TCAT – GACT: CTGA – GGTA: CCAT – CACT: GTGA

Section 10.3 DNA Replication

Steps of DNA Replication 1.An enzyme - helicase separates the DNA strands. – Forms the replication fork 2.Enzymes – DNA polymerase add complementary nucleotides to each original strand 3.DNA polymerase finish replicating the DNA and fall off. – Result = 2 separate & identical DNA molecules that are ready to move to a new cell in cell division

Each new DNA double helix: – Semi-conservative replication: 1 strand from the original 1 new strand DNA replication occurs in different directions – Forms gaps which are joined together by an enzyme  DNA ligase.

DNA Errors in Replication 1 error for every billion paired nucleotides added. DNA polymerase has repair enzymes that “proofread” DNA. When mistakes do occur: – Base sequence of newly formed DNA differs from the base sequence of original  mutation

Section 10.4 Protein Synthesis

Flow of genetic information Gene: – Segment of DNA that is located on a chromosome and that codes for a hereditary character. Nucleic acid  ribonucleic acid (RNA) 2 steps in protein synthesis (gene expression): – Transcription: Nucleus of cells DNA acts as a template for the synthesis of RNA – Translation: cytoplasm RNA directs the assembly of proteins DNA  RNA  Protein

RNA Sugar ribose Uracil instead of thymine – U will pair with A Single stranded Shorter in length than DNA

3 types of RNA: – mRNA: Carries instruction from a gene to make a protein Eukaryotic cells, mRNA carries genetic “ message” from DNA in the nucleus to the ribosomes in the cytosol. – rRNA: Part of the structure of ribosomes Protein synthesis occurs in ribosomes – tRNA: Transfer amino acids to the ribosome to make a protein

Transcription Process by which the genetic instruction in a specific gene are transcribed or “rewritten” into an RNA molecule Nucleus – eukaryotic cells Cytoplasm – prokaryotic cells

3 steps: 1.RNA polymerase bind to a promoter. DNA strands unwind and separate 2.Free RNA nucleotides are added on one of the DNA strands  RNA molecule DNA strand  ATCGAC RNA Strand  UAGCUG 3.RNA polymerase reaches a termination signal: End of the gene  stop signal New gene is transcribed – mRNA, rRNA, tRNA

Genetic Code Next process of gene expression, amino acids are assembled based on the genetic code. – 3 adjacent nucleotides (“letters”) in mRNA =amino acid “word” – 3 letters = codon – 64 possibilities

How many codons code for amino acids? 61 How many codons code for stop? 3 What amino acid is coded by the codon AUG? Methionine What is special about this particular codon (AUG)? Start codon for all mRNAs.

Translation All 3 types of RNA are involved Protein structure: – 20 different amino acids – 3-D structure – Shape is important to its function Translation or decoding of the genetic instructions to form a polypeptide involves 5 steps:

Steps: 1.Initiation: tRNA and mRNA join together Specific a.a is attached to tRNA Other end of tRNA is anticodon 2.Elongation: Polypeptide chain is put together 3.Elongation continued: Chain continues to grow 4.Termination: Ribosomes reach a stop codon. Polypeptide falls off 5.Disassembly: Components of translation come apart

Human Genome Entire gene sequence of the human genome is known. 3.2 billion base pairs in the 23 pairs of human chromosomes Large = 10yrs to read entire sequence Important: – Help diagnose, treat, and prevent genetic disorders, cancers, and infectious diseases in the future.