Section 10: Genetics and DNA
I. Essential
terms in genetics
II.
Inheritance
III. The
discovery and structure of DNA
IV. DNA
replication
I. Essential
terms in genetics:
v Genetics: the study of genes; the
field of study began in 1866, the year that Gregor Mendel
published his landmark paper on inheritance in pea plants. Prior to this work, no one could explain how
inherited characteristics are passed from parent to offspring. We have learned a great deal about the
physical and chemical properties of genes in the years since Mendel’s original
work.
v Chromosome: an elongated structure
found in the nucleus of a cell; composed of DNA packaged with proteins
v Gene: a segment of DNA within the
long DNA molecule of the chromosome; most genes contain instructions for the
synthesis of a single protein or protein subunit.
o
Each
of the trillions of cells in our bodies contains the same set of genes (two
copies-one inherited from each parent).
The exceptions to this are the sperm and egg cells which are haploid and
only contain one copy of each gene and sex-linked genes, in which only one copy
is inherited in males.
·
Sex-linked
genes:
o
Allele:
alternative versions of a gene
v Trait: feature of an organism such as
height, flower color, chemical structure of a protein, etc. Traits are specified by genes.
v Genotype: genetic makeup of an
organism
v Phenotype: observable physical
characteristics
v Mutation: a change in the DNA that
makes up a gene; can be harmful because they can lead to the production of a
protein that functions poorly; common for mutations to have little effect; occasionally can produce genes that
improve on the original protein.
II. Basic patterns of Inheritance
v Alternative versions of genes cause
variation in inherited traits.
v Offspring inherit one copy of a gene
from each parent.
v An allele is dominant if it
determines the phenotype of an organism even when paired with a different
allele.
v The two copies of a gene separate
during meiosis and end up in different gametes.
v Gametes fuse without regard to which
alleles they carry.
v Genes and inheritance: some traits
are controlled by a single gene and are little affected by environmental
conditions. Many other traits are
influenced by sets of genes that interact with one another and with the
environment.
o
Ex:
Many human diseases are influenced by multiple genes and by many different
environmental factors.
v DNA tweak turns vole mates into soul
mates
III. The discovery and structure of DNA
v Each chromosome consists of a single
large DNA molecule that codes for hundreds or even
thousands of different genes.
o
This
molecule is very long (6 feet unwound per cell!). Thus, it is not surprising that it is highly
organized.
·
DNA
is wound about special proteins called histones that
keep the DNA organized.
·
The
histone plus the DNA is called a nucleosome.
1. The nucleosomes
are what make up part of the chromatin, which condenses to form visible
chromosomes.
v Although DNA is a very large
molecule, only recently have we been able to “see” it in detail.
o
Much
of the progress in analyzing the three dimensional arrangement of atoms in
nucleic acids has come from the application of x-ray diffraction.
·
If
a stream of x-rays is passed through a crystallized substance (such as DNA),
they are scattered (i.e. differentiated) as they encounter the atoms of the
crystal.
·
The
differentiated x-rays then interfere with each other and produce spots of
different intensities.
·
These
“spots” are like a molecular fingerprint that can then be recorded on
photographic film.
v X-ray Diffraction
o
Rosalind
Franklin and Maurice Wilkins were the key scientists that studied the structure
of DNA using x-ray diffraction.
o
At
that time, DNA could not be crystallized, but two different types of fibers
could be taken from DNA. These fibers were enough like a crystal to produce an
x-ray diffraction pattern.
o
One
of the fibers produced better images with more dark spots. This is what was
used to identify the 3-dimensional structure of DNA.
o
The
x-ray diffraction photographs of DNA show us that it has a helical shape (X
pattern in center of photograph).
o
The
x-ray photos also suggested that some portion of the helix is repeated. This
repetitious element was later determined to be the hydrogen-bonded bases.
v Watson and Crick Model
o
Understanding
the helical structure of DNA was a major breakthrough, but it had to be shown
that this helical shape was consistent with DNA’s chemical composition.
o
Other
scientists had demonstrated that the backbone of DNA consists of alternating
sugar and phosphate units with four kinds of bases attached (adenine, thymine,
cytosine, and guanine.)
·
Erwin
Chargaff was the first to demonstrate how these bases were arranged by
discovering complementary base pairing.
1. This means that the amount of thymine
in a sample of DNA was equivalent to the amount of adenine and the amount of
cytosine was equal to the amount of guanine.
o
Two
scientists, James Watson and Francis Crick, worked together to utilize the
discoveries of other scientists in order to build a model of DNA.
·
Chargaff’s
discoveries led them to propose that DNA is not a single, but a DOUBLE helix,
composed of two strands twisted about one another much like a spiral staircase.
These two strands are held together by bases that connect with one another,
pairing up in a complementary fashion.
o
Watson
and Crick built a model of the DNA molecule as they understood it.
·
Model
was built of accurately scaled models of atoms and groups of atoms, with both
sizes and bond angles properly proportioned.
·
When
finished, the model was huge, and all the pieces fit properly. It worked!!!
·
This
was one of the greatest discoveries in the history of science.
v So, what does the model show us?
o
DNA
is composed of nucleotides. Nucleotides are the building blocks (monomers) of
DNA.
·
Nucleotides
are made of a five-carbon sugar (deoxyribose), a
phosphate group, and a base (nitrogen-containing organic compound).
1. DNA has four bases—cytosine and
guanine are smaller, single-ring bases (pyrimidines);
adenine and thymine are double-ring bases (purines).
2. The bases project like the rungs of a
ladder at right angles from the sugar-phosphate backbone of the DNA molecule.
o
DNA
consists of two strands attached to each other by complementary base pairing.
So, if you know the base sequence of one of the two strands, you can predict
the base sequence of the other strand.
·
No
other pairing relationship is possible because of the nature of the hydrogen
bonds that form between the bases.
o
The
length of the “rungs” of the ladder are always the
same, regardless of the paired bases. So, a double strand of DNA is the same
width from one end to the other.
o
The
two strands run in opposite directions. One runs downward from what is known as
the 5’ (FIVE PRIME) to the 3’ (THREE PRIME) end. The other runs downward from
the 3’ to the 5’.
IV. DNA
replication
v DNA replication
o
Remember
that when cells divide, DNA has to be replicated and then redistributed into
the daughter cells.
o
This
is a complex process involving over a dozen enzymes and proteins.
o
DNA
replication is very precise, meaning that DNA makes an exact duplicate of
itself.
v In sum, the process of DNA
replication is as follows:
1. Hydrogen bonds connecting two strands
of DNA are broken (thanks to the help of helicase!).
2. DNA strand unwinds and separates.
3. Each strand is used as a template for
the construction of a new strand of DNA. (DNA polymerase is the enzyme
here.)
4. When this process is completed, there
are two identical copies of the original DNA molecule, each with the same
sequence of bases. Each copy is composed
of one “old” strand of DNA (from the original molecule) and one newly
synthesized strand of DNA. One old
strand + one new strand = semi-conservative
replication.
v DNA replication in Eukaryotes
o
DNA
replication in eukaryotes is surprisingly accurate and relatively quick
overall.
·
The
fast speed of replication is due to the ability of the DNA to unzip at
different points along the strand.
v DNA replication is never 100%
accurate. Some errors in replication do occur.
o
Examples
include noncomplementary base pairing or insertions
and deletions of base pairs in the DNA molecule.
o
These
accidental changes are called mutations. Mutations occur infrequently and are
usually harmless. Beneficial mutations are quite rare. Think in terms of
computers—randomly removing a part of the computer would much more likely
damage than benefit it. The same goes for living organisms.
v Why not more mutations?
o
All
organisms are subject to mutations, but they do not happen as frequently as we
might expect. Mutations occur when a
mistake in the copying process is not corrected. (They can also occur when cells are exposed
to mutagens, which are substances or energy sources that alter DNA.)
o
Many
mutations are “neutral” and do not affect organisms. (Primary reason: silent codon
shuffling)
o
Cells
possess backup enzymes that repair or remove damaged sections of DNA. Thus,
most mutations are not passed on to other cells.
v Examples of serious genetic disorders
involving a single gene: