CHS

H Biology

Chapter 12DNA

A: The Genetic Code

Genetic Code– the way in which cells store the program that they seem to pass from one generation of an organism to the next generation

Evidence that DNA is the Genetic Material

1928 – Fred Griffith studied pneumonia caused by bacteria. He worked with 2 strains of bacteria, each containing different genetic information.

  • S - strain – virulent produced capsule
  • R- strain – nonvirulant – did not have capsule


Conclusion: Some of the genetic material from the dead, virulent bacteria (S), had entered the living, nonvirulent bacteria (R) changing them to the virulent form. This is called BACTERIAL TRANSFORMATION(one strain of bacteria had been transformed into another)

Oswald Avery, Maclyn McCarty, and Colin MacLeod wanted to know – What factor had transformed the bacteria?

1944 - Made “juice” from heat killed bacteria and treated “juice” with enzymes to destroy lipids, proteins, carbs, and RNA  transformation still occurred BUT when the treated the “juice” with enzymes to destroy DNA  transformation did not occur therefore, DNA was the TRANSFORMING FACTOR

Scientists were still skeptical about the genetic material of higher organisms.

Hershey-Chase 1952 – hypothesized that bacteriophages (made of DNA and a protein coat) don’t enter bacteria intact, but the phages protein coat attaches to the bacterial cell wall and the phage then injects its DNA into the bacterial cell.


Conclusion: DNA and not protein entered the bacteria – strong evidence that the genetic material of bacteriophages is DNA. DNA was the molecule that carried the genetic code

The Role of DNA

  • Storing Information
  • Copying Information
  • Transmitting Information

B: The Structure of DNA

  1. Rosiland Frankiln (&Maurice Wilkins)(early 1950’s)– produced photographs (using X-ray diffraction) showing DNA is twisted into a spiral or HELIX with the bases perpendicular to the length of the molecule. Picture also showed that DNA must be composed of more than one strand and that sugar-phosphate backbone is on the outside of the helix
  2. Erwin Chargaff –Chargaff’s Rule – number of nucleotides containing A (adenine) equals the number of nucleotides containing T (thymine) and that the number of G (guanine) equals the number of C (cytosine)purine with a pyrimidine
  3. James Watson and Francis Crick 1953 – used Franklin and Wilkins x-ray crystallography picture of DNA and information from Chargaff to make a model of DNA (still used today)

C:The Double Helix

DNA is made of nucleotide subunits

  • 5 C-sugar – DEOXYRIBOSESugar
  • One or more phosphate groupsPhosphate
  • One of 4 possible nitrogen-containing basesBase

Model suggests that there are 2 strands of DNA and that the 2 strands are arranged like a ladder.

Sides of the ladder = sugar and phosphate backbone (phosphodiester bonds)

Rungs of the ladder = nitrogen bases – each rungs consists of 2 nitrogen bases COMPLEMENTARY Base Paired. (A=T) (C=G) (held together by H bonds)

  • Purine – Adenine & Guanine- Double Ring (It’s 2x’s as good to be pure)
  • Pyrmidine – Thymine & Cytosine- single ring


D: DNA Replication


Semi-conservative model was suggested by Watson and Crick but proven by Matthew Meselson and Franklin Stahl (1958) –each parent strand is a template for a new complimentary strand – end result is two identical DNA molecules each consisting of one old side “conserved” from parent and one new side

Takes place in the nucleus in eukaryotes

Four Easy steps to remember:

  1. Unwind
  2. Unzip
  3. Add new parts
  4. 2 new molecules of DNA rewind

DNA Replication

UNWIND/UNZIP

  1. DNA helicase separates parental DNA and exposes bases(unwinds/unzips)
  2. Single Stranded Binding Proteins (SSBP) hold strands apart, preventing them from recoiling

Adding New Parts/Elongation

  1. RNA primase lays down short segments of RNA (RNA primer) to which new strands of DNA can be made
  2. DNA polymerase- attaches to the RNA primer and begins to elongate (attach free nucleotides to exposed bases) the strands. Done continuouslyon the leading strand, in shortpieces (Okazaki fragments) on the lagging strand.

Why is there a Leading and a Lagging Strand?????

  • The 2 strands of DNA are antiparallel
  • DNA polymerase can only add nucleotides to the 3’ end (5’  3’) therefore both strands can not be made continuously

5. DNA Polymerasereplaces RNA primer with DNA nucleotides,it also proofreads new strand for base-pair errors (lagging strand requires many RNA primers)

6. DNA ligase joins sugar-phosphate backbone (“glues”) of Okazaki fragments (phosphodiester bonds link fragments)

2 New IDENTICAL Molecules of DNA REWIND

*Telomeres – tips of chromosomes are difficult to copy, the enzyme telomerase adds short repeated DNA sequences to telomeres, lengthening the chromosomes – this makes it less likely that important gene sequences will be lost during replication

The Big Picture

  • Two strands unwind and unzip (HELICASE) splits H bonds between bases
  • Add new parts – new nucleotides are added to the exposed strands by DNA POLYMERASE (RNA PRIMASE – first adds RNA nucleotides)
  • DNA LIGASE “glues”
  • 2 new identical molecules of DNA rewind

Prokaryotic Cellsvs. Eukaryotic Cells: DNA Replication

PROKARYOTICEUKARYOTIC

Single Chromosome (circular) / Many Chromosomes (up to 1000 times more DNA)
1 Origin of Replication – Regulatory protein binds to a single starting point and triggers beginning of S phase - proceeds in two directions until entire chromosome is copied / Many (dozens or hundreds) Origin of Replications proceed in both directions(shorten time for replication)
Okazaki Fragments in lagging strand

Chapter 13 RNA and Protein Synthesis

A. Comparison of DNA and RNA

Nucleic Acid / Monomer / Sugar / Nitrogen Bases / Function / Structure
DNA / Nucleotide
P,S,B / Deoxyribose / A, C, T, G / Contains hereditary information / Double Helix
RNA / Nucleotide
P,S,B / Ribose / A, C, U, G / DNA’s helper / Varies due to type(single strand)
Type of RNA / Function / Structure
mRNA
messenger / Provides the instructions for assembling amino acids into a polypeptide chain / Linear
tRNA
transfer / Delivers amino acid to a ribosome for their addition into a growing polypeptide chain / Clover leaf shaped
rRNA
ribosomal / Combines with proteins to form ribosomes / Globular

B. Gene Expression

  1. Transcription – synthesis of RNA strand from a DNA template
  2. Genes nucleotide sequence is written from DNA to complementary sequences in RNA
  3. mRNA carries the transcript of protein building instructions to ribosome
  4. Translation – synthesis of a polypeptide
  5. Sequences of bases in mRNA translated into sequence of amino acids – polypeptide
  6. tRNA - translator

Central Dogma: DNA  RNA  Protein

A gene is a linear sequence of many nucleotides. 3 Types:

  1. Structural genes: have info to make proteins
  2. Regulatory genes: are on/off switches for genes
  3. Genes that code for tRNA, rRNA, histones

C.Transcription (DNARNA)

Steps:(nucleus of eukaryotes)

  1. Initiation – RNA polymerase splits H bonds in DNA (unzips) and attaches to promoter (sequence on DNA that signals the beginning of transcription)
  2. Elongation – RNA polymerase assembles RNA nucleotides using one strand of DNA (non-coding) as the template;complementary base pair(A=U, C=G)
  3. Termination– ends transcription – special sequence of nucleotides is recognized (terminator sequence) mRNA (or any RNA) is released and DNA winds/rezips back together

mRNA Processing (only in eukaryotes)

RNA Processing – Before mRNA leaves the nucleus it undergoes 2 alterations

  • A “cap” is added to the 5’ end of mRNA and a “tail” is added to the 3’end of the mRNA – both give mRNA stability(help bind), protect & regulate gene expression
  • RNA splicing – cutting andpasting

Exons are Expressed – express a code for a polypeptide

Intronsare INTervening sequences that are non-coding

mRNA can now leave the nucleus (through nuclear pore) and enter the cytoplasm

cut out introns and splice exons together

B. Genetic Code

1. Genetic Code -the language in which instructions for proteins are written in the base sequences

Codon – combination of three nucleotides on the mRNAthat signifies a particular amino acid

  • must be 3 nucleotides (1 or 2 not enough to represent all 20 aa)
  • genetic code has redundancy (more than one codon for each amino acid) but no ambiguity (codons only represent one amino acid each)
  • Universal for all organisms
  • all polypeptide chains start with the codon AUG (methonine)
  1. Translation – process by which mRNA is translated into a polypeptide chain (tRNA is the translator) occurs in cytoplasm at ribosome

Steps:

  1. Initiation- mRNA binds to ribosomal subunit
  2. 1st CODON (AUG) base-pairs with ANTICODON (UAC) on tRNAcarrying amino acid methionine. (*AUG is ALWAYS the start codon and the codon is found on the mRNA)
  3. Elongation – begins when next tRNA brings amino acid from cytoplasm to ribosome (codon and anticodon complementary base pair and place amino acids in the correct order)

Ribosome: 2 protein subunits and ribosomal RNA

• allows aa’s to attach by making peptide bonds

• travels along mRNA strip, tRNA’s join and bring correct amino acids

• 3 sites on ribosome:

• A site – where new tRNA’s and amino acids join

• P site – where protein is growing

• E site – where empty tRNA’s exit ribosome

Translocation: as ribosome moves, tRNA’s move from A site  P site  E site. “A” site is now open for new tRNA with attached amino acid to join

  1. Termination - Polypeptide chain continues growing until a STOP codon is reached (UAA, UAG, UGA- termination codons)