Chapter 16:The Molecular Basis of Inheritance (1)

April 1953: __ and __ discovered the double helical model for the structure of DNA.
__ and __ on chromosomes are composed of __.

Of all nature's molecules, __ are unique in their ability to direct their own replication from monomers.
It is the __ program that directs the development of your biochemical, anatomical, physiological, and to some extent, behavioral traits.

The chemical components of chromosomes are __ and __.
- __ were believed to be the genetic material, since biochemists had identified them as a class of macromolecules with great __ and __ of function, essential requirements for the hereditary material.

We trace the discovery of the genetic role of DNA back to 1928.
-While attempting to develop a vaccine against __, __studied Streptococcus pneumonia, a bacterium that causes pneumonia in mammals.
oHad two strains of the bacterium, one __(disease-causing) and one __(harmless).
oSurprised to find that then he killed the __bacteria with heat and then mixed the cell remains with living bacteria of the __strain, some of the living cells became __.
oFurthermore, this newly acquired trait of __was inherited by all the descendents of the transformed bacteria.
§Some chemical component of the dead pathogenic cells caused this heritable change, although the identity of the substance was not known.

___: a change in genotype and phenotype due to the assimilation of external DNA by a cell

Griffith’s work set the stage for a 14-year search by ___ for the identity of the transforming substance: __, __, or __·
-Broke open the heat-killed pathogenic bacteria and extracted the cellular contents.
-Used specific treatments that inactivated each of the three types of molecules.
-Then tested each treated sample for its ability to __live __bacteria.
-----Only when __was allowed to remain active did __occur.
Transforming agent: __

Many biologists were not convinced that the genes of bacteria would be similar in composition and function to those of more complex organisms. But the major reason for the continued doubt was that so little was known about __.
-
Additional evidence for DNA as the __ came from studies of viruses that infect bacteria.
- These viruses are called __, a virus that infects bacteria (meaning “bacteria-eaters”), or just __.

__ are much simpler than cells. It is a little more than __(or sometimes __) enclosed by a protective coat, which is often simply protein.
To reproduce, a __must infect a cell and take over the cell’s metabolic machinery.

__ and __ in 1952 performed experiments showing that __ is the genetic material of a __ known as T2.
· This is one of many __ that infect E. coli, a bacterium that normally lives in the intestines of mammals.
· At that time, biologists already knew that T2, like many other __, was composed almost entirely of __ and __·
They also knew that the T2 phage could quickly turn an E. coli cell into a T2-producing factory that released many copies when the cell ruptured.
· Somehow, T2 could reprogram its host cell to produce viruses.

Which viral component- proteins or DNA- was responsible? (don't answer)
o __ and __answered this by devising an experiment showing that only one of the two components of T2 actually enters the E. coli cell during infection.
§ Used a radioactive isotope of __to tag __in one batch of T2
§ Used a radioactive isotope of __to tag __in a second batch.

§ In the experiment, separate samples of nonradioactive E. coli cells were allowed to be infected by the protein-labeled and DNA- labeled batches of T2.

Why did Hershey and Chase use sulfur to tag protein?

Why did Hershey and Chase use phosphorus to tag DNA?

__found that the phage __entered the host cells, but the phage __did not. Moreover, when these bacteria were returned to a culture medium, the infection ran its course, and the E. coli released phages that contained some radioactive __, further showing that the __inside the cell played an ongoing role during the infection process.

Hershey and Chase concluded that the __injected by the phage must be the molecule carrying the genetic information that makes the cells produce new viral __.
__experiment provided powerful evidence that __, rahter than __ are the hereditary material, at least for viruses.
--

DNA is a polymer of nucleotides, with three components: a __, __, and __.
· Bases: __, __, __, __

__ analyzed the base composition of DNA from a number of different organisms. In 1950, he reported that the base composition of DNA varies from one species to another.
Healso noticed a peculiar regularity in the ratios of nucleotide bases within a single species.
· The number of __equaled the number of __; the number of __equaled the number of __.
· Equivalences for any given species between the number of A and T bases and the number of G and C bases became known as __

Biologists were determined to find out how the structure of DNA could account for its role in inheritance.
· In 1950, arrangement of __ in a nucleic acid polymer was well established, and researchers focused on discovering the three-dimensional structure of DNA.
· __ and __were the first two come up with the right answer.

__ and __were the first two come up with the right answer.
__ was studying protein structure with __.
__ saw an __ image of DNA produced by Wilkins’s colleague __·
---Images produced by __ are not actually pictures of molecules. The spots and smudges were produced by X-rays that were diffracted (deflected) as they passed through aligned fibers of purified DNA.
· Crystallographers used mathematical equations to translate such patterns into information about the three-dimensional shapes of molecules, and __was familiar with the types of patterns that __models produce.

A careful study of Franklin’s X-ray diffraction photo of DNA not only told him that DNA was __ in shape, but also enabled him to approximate the width of the helix and the spacing of the nitrogenous bases along it.
· Width of the helix suggested that it was made up of __strands, contrary to a three-stranded model that Linus Pauling had proposed a short time earlier.
·The presence of two strands accounts for the now-familiar term __.

__ and __began building models of a __ that would conform to the X-ray measurements and what was then known about the chemistry of DNA.
They knew that the __ were on the outside of the double helix.
· This arrangement was appealing because it put the relatively __ in the molecule’s interior and thus away from the surrounding aqueous solution

__ constructed a model with the __ facing the interior of the __.
· In this model, the two __ are __- that is, their subunits run in opposite directions to each other.

Franklin’s X-ray data indicated that the helix makes one full turn every __along its length. With the bases stacked just __apart, there are __ layers of base pairs in each full turn of the helix.
The nitrogenous pairs:
· __; __

True or False:
They first assumed bases paired like with like. This didn’t fit X-ray data.
True False

Adenine and guanine are __, nitrogenous bases with two organic rings. They are twice as wide as __.

Cytosine, thymine, and uracil are __, which have a single ring.

Each base has __ that can form __ with its appropriate partner: adenine can form __with only thymine; guanine forms __ with only cytosine.

Watson - Crick Model explained the basis for __.
· Wherever one strand of a DNA molecule has an A, the partner strand is T. And a G in one strand is always paired with a C in the complementary strand.
o Amount of adenine and thymine are equal; amount of guanine and cytosine are equal
The beauty of the __ was that the structure of DNA suggested the basic mechanism of its replication.