Chapter 12: Molecular Genetics

What I KNOW already about DNA / What I WANT to know about DNA / What I have LEARNED about DNA

Learning Targets for Section 12-1 (DNA: The Genetic Material)

•Summarize the relationship between genes and DNA?

•Describe the overall structure of DNA?

Research Behind DNA : Griffith and Transformation

  • In 1928, British scientist Frederick Griffith tried to determine which bacteria producedpneumonia.
  • Griffith isolated two different strains.

1. disease-causing = smooth colonies

2. harmless strain = rough colonies.

  • Griffith injected mice
  1. Disease-causing bacteria- mice developed pneumonia and died.
  2. Harmless strain - didn’t get sick at all
  3. Griffith’s then mixed heat-killed, disease-causing bacteria with live, harmless ones and injected the mixture into mice
  4. Mice developed pneumonia and many died.
  5. Found their lungs filled with the disease-causing bacteria

Griffith’s Conclusion:

•Griffith hypothesized some factor transformed harmless cells into the heat-killed harmful cells

Avery and DNA

•Avery and his colleagues repeated Griffith’s experiment then:

•Used enzymes that destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA

•When they destroyed the nucleic acid (DNA), transformation did not occur

Avery’s Conclusion:

•Avery and other scientists discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation of an organism to the next

The Hershey Chase Experiment: Martha Chase and Alfred Hershey

•Studied viruses, nonliving particles smaller than a cell that can infect living organisms

•Hershey and Chase reasoned that if they could determine which part of the virus—the protein coat or the DNA core—entered the infected cell, they would learn whether genes were made of protein or DNA

•They grew viruses in cultures of radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S).

•Proteins contain almost no phosphorus and DNA contains no sulfur

•If 35S was found in the bacteria, it would mean that the viruses’ protein had been injected, If 32P was found in the bacteria, then it was the DNA that had been injected

Hershey and Chase’s Conclusion:

Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.

The Structure of DNA

•DNA is a long molecule made up of units called nucleotides

•Each nucleotide is made up of three parts:

1. a 5-carbon sugar called deoxyribose,

2. a phosphate group,

3. and a nitrogenous (nitrogen-containing) base

There are four kinds of nitrogenous bases in DNA

•Purines:

–Adenine: Expressed A

–Guanine: Expressed G

•Pyrimidines:

–Thymine: Expressed T

–Cytocine: Expressed C

Chargaff’s Rules

•Erwin Chargaff, an American biochemist, discovered that the percentages of guanine [G] and cytosine [C] are almost equal in any sample of DNA

•The same thing is true for adenine [A] and thymine [T]

•Despite the fact that DNA samples from organisms obeyed this rule, neither Chargaff nor anyone else had the faintest idea why

X-Ray Evidence

•In the early 1950s, a British scientist named Rosalind Franklin began to study DNA using a technique called X-ray diffraction

•The X-shaped pattern in the image shows that the strands in DNA are twisted around each other like the coils of a spring, a shape known as a helix

The Double Helix

•At the same time Francis Crick, and James Watson, were trying to understand the structure of DNA by building three-dimensional models of the molecule

•In 1953, Watson was shown a copy of Franklin’s X-ray pattern. In his book The Double Helix, Watson wrote: “The instant I saw the picture my mouth fell open and my pulse began to race.”

Watson and Crick’s Conclusion:

  1. DNA is a double helix in which two strands are wound around each other.
  2. Each strand is made up of a chain of nucleotides.
  3. The two strands are held together by hydrogen bonds between Adenine and Thymine and between Guanine and Cytosine.

Learning Targets for Section 12.2

•Summarize the events that happen in DNA replication

•Relate the DNA molecule to chromosome structure.

Chromosomes and DNA Replication: DNA and Chromosomes

Most prokaryotes have a single circular DNA molecule that contains nearly all of the cell’s genetic information

•Differences between Prokaryotes and Eukaryotes

•Eukaryotic DNA is a bit more complicated. Many eukaryotes have as much as 1000 times the amount of DNA as prokaryotes

•DNA LengthDNA molecules are surprisingly long

•The chromosome of the prokaryote E. coli, contains 4,639,221 base pairs

Length of DNA

•This means that the nucleus of a human cell contains more than 1 meter of DNA

•How can so much DNA be packed into each and every cell in our body?

Chromosome Structure

•Eukaryotic chromosomes contain both DNA and protein, tightly packed together to form a substance called chromatin

•Chromatin consists of DNA that is tightly coiled around proteins called histones

•Together, the DNA and histone molecules form a beadlike structure called a nucleosome

•This allows the chromosomes to be very tightly coiled up in the nucleus

DNA Replication

•When Watson and Crick discovered the double helix structure of DNA, there was one more remarkable aspect that they recognized immediately.

•The structure explained how DNA could be copied, or replicated

•Each strand of the DNA double helix has all the information needed to reconstruct the other half by the mechanism of base pairing

•During DNA replication, the DNA molecule separates into two strands

•Then produces two new complementary strands following the rules of base pairing.

•Each strand of the double helix of DNA serves as a template, or model, for the new strand

How DNA Replicates

•Start with a double strand of DNA

•DNA replication is carried out by a series of enzymes. which “unzip” a molecule of DNA

Replicate the following strand of DNA

•ATG GGA CCG TAT ACG GAG

•TAC CCT GGC ATA TGC CTC

DNA and Enzymes

•DNA replication involves a host of enzymes and regulatory molecules

•The principal enzyme involved in DNA replication is called DNA polymerase

•In addition to replicationDNA polymerase also “proofreads” each new DNA strand, helping to maximize the odds that each molecule is a perfect copy of the original DNA

Learning Targets for Section 12.3

•What are the three main types of RNA?

•What is transcription?

•What is translation?

12–3RNA and Protein Synthesis

•The double helix structure explains how DNA can be replicated but it does not explain how a gene works

•Genes are coded DNA instructions that control the production of proteins

RNA

•The first step is to copy part of the nucleotide sequence from DNA into RNA a process called transcription

•RNA molecules then carry out the process of making proteins.

•RNA molecule is a working copy of a single gene.

•Using RNA makes it possible for a single gene to produce hundreds or even thousands of RNA molecules

•T

There are three main differences between RNA and DNA:

  1. The sugar in RNA is ribose instead of deoxyribose
  2. RNA is generally single-stranded
  3. RNA contains uracil in place of thymine

Types of RNA

There are three main types of RNA: messenger RNA, ribosomal RNA, and transfer RNA.

  1. messenger RNA (mRNA) serves as “messengers” from DNA to the rest of the cell.
  2. ribosomal RNA (rRNA) is the site where Proteins are assembled on ribosomes
  3. transfer RNA (tRNA) transfers each amino acid to the ribosome during the construction of a protein.

Transcription

•During transcription, RNA polymerase binds to DNA and separates the DNA strands.

•RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA

Practice Transcribing

•Remember A goes with T, C goes with G and Uracil takes the place of Thymine

•TAC GCA CCA TAT CCG ATT

•AUG CGU GGU AUA GGC UAA

Where to Begin?

•The enzyme will bind only to regions of DNA known as promoters, which have specific base sequences.

•Promoters are signals in DNA that indicate to the enzyme where to bind to make RNA

•Similar signals in DNA cause transcription to stop when the new RNA molecule is completed

Editing RNA

  • Many RNA molecules have sections, called introns, edited out of them before they become functional.
  • The remaining pieces, called exons, are spliced together.
  • Then, a cap and tail are added to form the final RNA molecule.

The Genetic Code

•The “language” of mRNA instructions is called the genetic code.

•RNA contains four different bases: A, U, C, and G.

•In effect, the code is written in a language that has only four “letters.”

•

STOP THINK!

•What is the process of transcription making?

•Where does transcription take place?

•What enzymes are used to complete transcription?

•Compare and Contrast DNA and RNA?

Genetic Code

  • The genetic code is read three letters at a time
  • Each three-letter “word” in mRNA is known as a codon
  • A codon consists of three consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide
  • Because there are four different bases, there are 64 possible three-base codons (4 × 4 × 4 = 64).
  • The codon, AUG, that can either specify methionine or serve as the initiation, or “start,” codon for protein synthesis
  • There are three “stop” codons that do not code for any amino acid

Translation

•During translation, the cell uses information from messenger RNA to produce proteins

•This process is known as translation

•Before translation can occur, messenger RNA must first be transcribed from DNA in the nucleus and released into the cytoplasm

•Translation begins when an mRNA molecule in the cytoplasm attaches to a ribosome

Translation

•As each codon of the mRNA molecule moves through the ribosome, the proper amino acid is brought into the ribosome and attached to the growing polypeptide chain

•That job is done by transfer RNA

•The codon matches up with complementary bases on the tRNA to tell it which amino acid to bring in

•The three bases on the tRNA molecule, called the anticodon, are complementary to one of the mRNA codons

•The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule

•At that point the protein is released to be modified in the Golgi apparatus or to be shipped out to perform its function