Name ______________________________________________________________________________ Test Date _______________

UNIT VI – DNAI. DNA – THE DISCOVERY OF THE MOLECULE OF INHERITANCE DNA contains the _genetic code_______ and the _working instructions______________ for a cell. Scientists in the early 1900s realized that inheritance was dependent on _genes_____ found on _chromosomes_____ and that chromosomes were composed of _DNA__ and _protein_____. It wasn’t until the 1940s that scientists established _DNA______ as the molecule of inheritance, and still another 15 years passed before the structure of DNA was discovered.

A. Griffith (1928) Discovered that bacteria can take up genetic material from environment in a process known as

_transformation______. Performed experiments with _mice_____ and bacteria known as _Pneumococcus______.

o Mixed harmless living bacteria with dead _pathogenic_______ bacteria and injected them into mice.

o Mice died.

B. Avery, et.al. (1944)

Set up experiments to identify “transforming substance” discovered by Griffith Used enzymes that destroyed carbohydrates, lipids, proteins, and RNA . . . transformation still

occurred. Used an enzyme that destroyed DNA, _transformation did not occur; mice survived_________________.

Concluded that _DNA_____ stores and transmits the genetic information from one generation of an organism to the next.

Scientific community did not completely accept their conclusion

C. Hershey & Chase (1952) Used a virus known as a _bacteriophage_____ which infects _bacteria_____. Viruses are composed of _DNA___and _protein____; require a _host___ cell in order to replicate. Hershey & Chase recognized that virus must inject its genetic material into the bacterial cell for

replication to take place. Used labeled _isotopes_______ to determine whether DNA or protein was injected into bacterium. Hershey & Chase concluded that the genetic material of the bacteriophage was _DNA____.

D. Watson & Crick (1953) Used molecular models to solve the 3-dimensional structure of DNA “Borrowed” an x-ray of DNA taken by _Rosalind Franklin_________ to solve the puzzle

Proposed a _double helix_______ structure with a backbone of alternating _deoxyribose______ and _phosphate groups______ with _nitrogen bases____ paired in the middle

II. STRUCTURE OF DNA DNA is a _nucleic acid____; a biomolecule composed of monomers called _nucleotides___. A. Each nucleotide of DNA consists of:

1. _5 __ carbon sugar - Deoxyribose 2. _Phosphate group(PO4)3. _Nitrogen base

Strong, stable _covalent_____ bonds hold the

components of each nucleotide together. In addition, nucleotides form strong

_covalent_____ bonds with other nucleotides, resulting in a long strand of nucleotides.

Double helix created by weak _hydrogen_____ bonding between the _nitrogen bases_____

The diameter of DNA is uniform due to specific pairing of nitrogen baseso Purines – double-ringed nitrogen bases

_guanine______________

_adenine______________

o Pyrimidines - single-ringed nitrogen bases _thymine______________ _cytosine______________

o _Purine____ always base pairs with a _pyrimidine_____, specifically … _Adenine____ base pairs with _thymine_____ _Guanine____ base pairs with _cytosine_____

o _Watson____ and _Crick_____ determined this structurally because the _diameter____ of DNA is uniform.

o _Chargaff____ determined this chemically. His chemical analysis of DNA from many different organisms always showed that the % of adenine = % _thymine___ and the % of guanine = the % _cytosine___. These findings are known as _Chargaff’s rules_____.

The two strands of nucleotides are _anti-parallel_________; that is, they run opposite to each other.

It is the _pattern_____ and _number____ of nucleotides that makes each gene unique. **5’ and 3’ – refers to the location on the 5C sugar deoxyribose to which phosphate groups attach.

DEOXYRIBONUCLEIC ACID

III. DNA REPLICATION Prior to cell division, the DNA must _replicate_____. This occurs in _S____ of _interphase____.

During this part of the cell cycle, DNA is in the form of _chromatin____. DNA can replicate itself exactly due to _Chargaff’s (base pairing) rules____. Replication occurs in the _nucleus_____ of the cell and requires the participation of several _enzymes______.

Replication may be summarized in the following steps: DNA is “unzipped” by the enzyme, _helicase__. Helicase breaks the _hydrogen______ bonds

between the _nitrogen bases______. The point where the unzipping begins is known as the _origin of replication_____. The continuation of the unzipping is called the _replication fork_____. This occurs in several places in each _chromosome_, much like a _broken zipper_____.

_Single-strand binding proteins___________ hold the separated DNA strands apart. Another enzyme, _topoisomerase_____ moves ahead of the _helicase______ to relieve

tension in the double helix as it is being unzipped. The enzyme, _DNA polymerase_______ adds nucleotides to both sides of the DNA molecule

according to _base pairing______ rules; however, DNA polymerase has two restrictions:1) It can only add nucleotides in a _5’ – 3’____ direction.2) It can only add nucleotides to an existing strand.

Before DNA polymerase begins moving in nucleotides, an _RNA primer_____ must be put in place. Eventually this primer is removed and replaced with DNA nucleotides.

The two sides of the DNA molecule are replicated differently because nucleotides can only be added in a _5’ – 3’____ direction:

o One side of the DNA molecule is being replicated in the same direction as the _replication fork____. This is known as the _leading____ strand. Nucleotides are moved in by _DNA polymerase____ in a _5’ – 3’_____ direction as the DNA is unzipped.

o On the other side of the DNA molecule, nucleotides are added _away_______ from the replication fork. This is known as the _lagging_________ strand. Short segments of nucleotides are synthesized, each with a new _RNA primer_______, as the DNA is unzipped. These short segments are known as _Okazaki fragments_______. Eventually, another enzyme known as _ligase____ “glues” together the _Okazaki_____ fragments to create a continuous strand of nucleotides.

Another type of _DNA polymerase______ proofreads the replicated DNA to minimize the chance of _mutation_____.

The result is two _identical________ DNA molecules, each new DNA consists of one _original___ strand and one _new___ strand. This is known as the _semi-conservative______ model of replication.

These two identical DNA molecules are the _sister chromatids_____ that proceed through _mitosis________.

Once replication has been completed, the cell moves into _G2___ and then, _prophase___ of _mitosis___.

IV. A DETAILED VIEW OF DNA REPLICATION