DNA and Replication

DNA and Replication

Essential Content

A. Experiments and History

• 1. Bacterial Transformation (16.12)

  2. Experiments: (Griffith, Hershey & Chase)

B. Universal code for all organisms (16.9)

• 1. Role of DNA

  2. Similarities evidence of ancestry

C. Review of Structure of DNA and location in the cell

• 1. Components of DNA

  2. Double helix

  3. Location in cell and importance

  4. Genes

D. Role of Nucleic acids in organisms (18.1)

• 1. Molecular structure

  2. Primary function

E. DNA Replication in Cell Cycle (16.3, 16.17)

• 1. DNA template

  2. Cell cycle (S Phase); Mitosis, Meiosis, and Mutations

  3. Location of process (nucleus)

F. Types of Mutations and effects

• 1. Effects on offspring (16.4)

  2. Effects on individual: Cancer (16.8)

Objectives

• Summarize the experiments that lead to the discovery of DNA as the genetic material, its structure, location in the cell, and function.

• Explain that the basic components of DNA are universal in organisms.

• Explain how similarities in the genetic codes of organisms are due to common ancestry and the process of inheritance.

• Identify the nitrogen base pairs found in DNA and summarize its structure.

• Summarize the basic molecular structure and describe the primary function of nucleic acids in organisms.

• Explain the basic processes of DNA replication and/or its role in the transmission and conservation of genetic information.

• Distinguish among the cellular processes of DNA replication, transcription, and/or translation.

• Explain how gene and chromosomal mutations may or may not result in a phenotypic change.

• Assess how uncontrolled growth may result from mutations that affect the proteins that regulate the cell cycle.

The Components of DNA DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds. Nucleotides may be joined in any order.

A DNA nucleotide is a unit made of a nitrogenous base, a 5-carbon sugar called deoxyribose, and a phosphate group.

DNA has four kinds of nitrogenous bases: adenine, guanine, cytosine, and thymine.

Solving the Structure of DNA

Erwin Chargaff showed that the percentages of adenine and thymine are almost always equal in DNA. The percentages of guanine and cytosine are also almost equal.

Rosalind Franklin’s X-ray diffraction studies revealed the double-helix structure of DNA.

James Watson and Francis Crick built a model that explained the structure of DNA.

The Double-Helix Model The double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held together. The model showed the following:

The two strands in the double helix run in opposite directions, with the nitrogenous bases in the center.

Each strand carries a sequence of nucleotides, arranged almost like the letters in a fourletter alphabet for recording genetic information.

Hydrogen bonds hold the strands together. The bonds are easily broken allowing DNA strands to separate.

Hydrogen bonds form only between certain base pairs–adenine with thymine, and cytosine with guanine. This is called base pairing.

Bacterial Transformation

In 1928, Frederick Griffith found that some chemical factor from heat-killed bacteria of one strain could change the inherited characteristics of another strain.

He called the process transformation because one type of bacteria (a harmless form) had been changed permanently into another (a disease-carrying form).

Because the ability to cause disease was inherited by the offspring of the transformed bacteria, he concluded that the transforming factor had to be a gene.

In 1944, Oswald Avery tested the transforming ability of many substances. Only DNA caused transformation. By observing bacterial transformation, Avery and other scientists discovered that the nucleic acid DNA stores and transmits genetic information from one generation of bacteria to the next.

Bacterial Viruses

A bacteriophage is a kind of virus that infects bacteria. When a bacteriophage enters a bacterium, it attaches to the surface of the bacterial cell and injects its genetic material into it. (See image just below)

In 1952, Alfred Hershey and Martha Chase used radioactive tracers to label proteins and DNA in bacteriophages.

Only the DNA from the bacteriophage showed up in the infected bacterial cell.

Hershey and Chase concluded that the genetic material of the bacteriophage was DNA.

Their work confirmed Avery’s results, convincing many scientists that DNA was the genetic material found in genes—not just in viruses and bacteria, but in all living cells.

The Role of DNA The DNA that makes up genes must be capable of storing, copying, and transmitting the genetic information in a cell.

Copying the Code Each strand of the double helix has all the information needed to reconstruct the other half by the mechanism of base pairing. Because each strand can be used to make the other strand, the strands are said to be complementary. DNA copies itself through the process of replication:

The two strands of the double helix unzip, forming replication forks.

New bases are added, following the rules of base pairing (A with T and G with C).

Each new DNA molecule has one original strand and one new strand.

DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA.

During replication, DNA may be lost from the tips of chromosomes, which are called telomeres.

Replication in Living Cells The cells of most prokaryotes have a single, circular DNA molecule in the cytoplasm. Eukaryotic cells have much more DNA. Nearly all of it is contained in chromosomes, which are in the nucleus.

Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied.

In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.

Tutorial DNA Replication