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The Molecular Basis of Inheritance - Coggle Diagram
The Molecular
Basis of
Inheritance
DNA is the genetic material
Early in the 20th century, the identification of the molecules of inheritance posed a major challenge to biologists
The Search for the Genetic Material: Scientific Inquiry
When T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material
The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them
Evidence That DNA Can Transform Bacteria
The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928
Griffith worked with two strains of a bacterium, one pathogenic and one harmless
When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic
He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA
Evidence That Viral DNA Can Program Cells
More evidence for DNA as the genetic material came from studies of viruses that infect bacteria
Such viruses are called bacteriophages (or phages)
A virus is DNA (sometimes RNA) enclosed by a protective coat, often simply protein
Phages have been widely used as tools by researchers in molecular genetics
Additional Evidence That DNA Is the Genetic Material
DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group
The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C)
This evidence of molecular diversity among organisms made DNA a more credible candidate for the genetic material
Building a Structural Model of DNA
After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in inheritance
The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases
At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width
Watson and Crick reasoned that the pairing was more specific, dictated by the base structures
They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)
The Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C
Instead, pairing a purine (A or G) with a pyrimidine (C or T) resulted in a uniform width consistent with the X-ray data
The pattern in the photo suggested that the DNA molecule was made up of two strands, forming a double helix
Watson built a model in which the backbones were antiparallel (their subunits run in opposite directions)
CONCEPT 16.2: Many proteins work together in DNA replication and repair
Resemblance of offspring to parents relies on accurate replication of DNA prior to meiosis and its transmission to the next generation
Replication prior to mitosis ensures the faithful transmission of genetic information from a parent cell to the two daughter cells
Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material
The copying of DNA is called DNA replication
The Basic Principle: Base Pairing to a Template Strand
Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication
This yields two exact replicas of the “parental” molecule
Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand
Competing models were the conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new)
DNA Replication: A Closer Look
The copying of DNA is remarkable in its speed and accuracy
More than a dozen enzymes and other proteins participate in DNA replication
Replication in bacteria is best understood, but evidence suggests that the replication process in eukaryotes and prokaryotes is fundamentally similar
Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble”
At the end of each replication bubble is a replication fork, a Y-shaped region where parental DNA strands are being unwound
Helicases are enzymes that untwist the double helix at the replication forks
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