DNA: The Genetic Material
description
Transcript of DNA: The Genetic Material
DNA: The Genetic MaterialChapter 14
2
3
The Genetic Material
Griffith’s conclusion:- information specifying virulence passed
from the dead S strain cells into the live R strain cells
- Griffith called the transfer of this information transformation
4
The Genetic Material
Avery, MacLeod, & McCarty, 1944repeated Griffith’s experiment using purified
cell extracts and discovered:- removal of all protein from the transforming material did not destroy its ability to transform R strain cells- DNA-digesting enzymes destroyed all transforming ability- the transforming material is DNA
5
The Genetic Material
Hershey & Chase, 1952- investigated bacteriophages: viruses that
infect bacteria- the bacteriophage was composed of only
DNA and protein- they wanted to determine which of these
molecules is the genetic material that is injected into the bacteria
6
7
8
9
10
DNA Structure
Determining the 3-dimmensional structure of DNA involved the work of a few scientists:
– Erwin Chargaff determined that • amount of adenine = amount of thymine• amount of cytosine = amount of guanine
This is known as Chargaff’s Rules
11
DNA Structure
Rosalind Franklin and Maurice Wilkins– Franklin performed X-ray diffraction
studies to identify the 3-D structure– discovered that DNA is helical– discovered that the molecule has a
diameter of 2nm and makes a complete turn of the helix every 3.4 nm
12
DNA Structure
James Watson and Francis Crick, 1953– deduced the structure of DNA using
evidence from Chargaff, Franklin, and others
– proposed a double helix structure
13
DNA Structure
The double helix consists of:– 2 sugar-phosphate backbones– nitrogenous bases toward the interior of
the molecule– bases form hydrogen bonds with
complementary bases on the opposite sugar-phosphate backbone
14
15
DNA Structure
The two strands of nucleotides are antiparallel to each other– one is oriented 5’ to 3’, the other 3’ to 5’
The two strands wrap around each other to create the helical shape of the molecule.
16
17
DNA Replication
Matthew Meselson & Franklin Stahl, 1958investigated the process of DNA replicationconsidered 3 possible mechanisms:
– conservative model– semiconservative model– dispersive model
18
19
DNA Replication
Bacterial cells were grown in a heavy isotope of nitrogen, 15N
all the DNA incorporated 15N cells were switched to media containing
lighter 14NDNA was extracted from the cells at various
time intervals
20
DNA Replication
The DNA from different time points was analyzed for ratio of 15N to 14N it contained
After 1 round of DNA replication, the DNA consisted of a 14N-15N hybrid molecule
After 2 rounds of replication, the DNA contained 2 types of molecules:– half the DNA was 14N-15N hybrid– half the DNA was composed of 14N
21
22
DNA Replication
Meselson and Stahl concluded that the mechanism of DNA replication is the semiconservative model.
Each strand of DNA acts as a template for the synthesis of a new strand.
23
Prokaryotic DNA Replication
The chromosome of a prokaryote is a circular molecule of DNA.
Replication begins at one origin of replication and proceeds in both directions around the chromosome.
24
25
Prokaryotic DNA Replication
The double helix is unwound by the enzyme helicase
DNA polymerase III (pol III) is the main polymerase responsible for the majority of DNA synthesis
DNA polymerase III adds nucleotides to the 3’ end of the daughter strand of DNA
26
27
28
Eukaryotic DNA Replication
The larger size and complex packaging of eukaryotic chromosomes means they must be replicated from multiple origins of replication.
The enzymes of eukaryotic DNA replication are more complex than those of prokaryotic cells.
29
Eukaryotic DNA Replication
Synthesizing the ends of the chromosomes is difficult because of the lack of a primer.
With each round of DNA replication, the linear eukaryotic chromosome becomes shorter.
30
31
Eukaryotic DNA Replication
telomeres – repeated DNA sequence on the ends of eukaryotic chromosomes– produced by telomerase
telomerase contains an RNA region that is used as a template so a DNA primer can be produced
32
33
DNA Repair
- DNA-damaging agents- repair mechanisms- specific vs. nonspecific mechanisms
34
DNA Repair
Mistakes during DNA replication can lead to changes in the DNA sequence and DNA damage.
DNA can also be damaged by chemical or physical agents called mutagens.
Repair mechanisms may be used to correct these problems.
35
DNA Repair
DNA repair mechanisms can be:– specific – targeting a particular type of
DNA damage• photorepair of thymine dimers
– non-specific – able to repair many different kinds of DNA damage• excision repair to correct damaged
or mismatched nitrogenous bases
36
37