Download BL414 Genetics Spring 2006 Lecture 1 Outline January 18, 2006

BL414 Genetics Spring 2006
Lecture 1 Outline
January 18, 2006
Introduction to genetics:
What are genes?
Genes are the elements of heredity transmitted from parents to
offspring.
The questions that follow from the understanding of genes existing in
organisms are:
What exactly are they made of?
How do they affect an organism?
How do genes actually work?
What exactly are genes made of?
1.1 DNA the Genetic Material
Historical progress and context of the search for the answers to these
questions:
 1869 Miescher found what was later to be known as DNA in the
nucleus
 1880’s chromosomes were observed in cells undergoing mitosis (not
seen in nucleus, only when nucleus disappears during cell division)
 1900’s became generally thought that chromosomes carry “genes” but
still not known exactly how; Emil Fischer showed that proteins consist
of long strings of amino acids – perhaps thousands of different amino
acids were thought to exist in proteins. (Later it was found that there
are 20 amino acids. Protein folding accounts for some of the incredibly
diverse forms and functions of proteins).
 1920’s Robert Fuelgen observed that DNA is exclusively in the
chromosomes. DNA was known to consist of 4 types of nucleotides,
and therefore was thought to not be complex enough to be the
substance of genes. It was thought that proteins were most likely to be
the substance of genes, with DNA perhaps in a scaffolding role.
Key experiments finally led to the realization that genes were comprised
exclusively of DNA.
 1920’s Griffith
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o injected live non-virulent and killed virulent bacteria into mice,
and the mice died
o showed that genetic material can be transferred from one
bacterial cell to another
1940’s Avery, MacLeod, and McCarty at Rockefeller University
o isolated DNA, transformed bacteria
o showed that transforming substance was not susceptible to
protease, but it is susceptible to DNases  therefore the
transforming substance, i.e. the genetic material, was DNA!
1952 Hershey and Chase
o showed that only the DNA enters bacterial cell from phage
virus; protein coat stays outside
1.2 DNA Structure
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The Double Helix, 1953 Watson and Crick
o DNA consists of two strands of phosphodiester linked
nucleotides with four different base constituents: adenine,
guanine, cytosine and thymine. The adenine nucleotides are
always arranged in a base pair with thymine and the
guanosine is always paired with cytosine. Therefore, the two
strands are complementary to each other.
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DNA Replication
o In order for DNA to replicate, the two strands of the double
helix are unfolded and a new complementary strand is
created for each half. The resulting two daughter double
helices are identical to the original parent double helix.
So, how do genes affect an organism?
1.4 Genes and Proteins:
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1908 Archibald Garrod through his studies of “black urine disease”
(alkaptonuria) found that certain diseases can be inherited
according to the same rules described by Mendel – that is, genes
can cause a defect in some normal biological function, leading to
disease
These hereditary diseases were dubbed “inborn errors of
metabolism.”
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This idea was expanded to the understanding that a given gene is
responsible for a single enzyme or a single protein
At the time of Garrod, it was not understood how genes directed
the activity of a protein – it might have been assumed that genes
were the proteins – some how they replicated and were activated
Then, how do genes actually work?
1.5 Gene Expression: The central dogma.
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The discovery of the double helix was followed by a chase after the
“code” – the understanding of how the nucleotide sequence on a
strand of DNA directed the creation of specific proteins.
A new player was realized to be of great importance, messenger
RNA (mRNA), which carries the genetic sequence of a gene that
has been transcribed from the DNA, and is then used as a template
for translation into protein by ribosomes.
RNA also has other functional roles in the cell, for example there is
ribosomal RNA within every ribosome, which operates in the
catalytic center of the ribosome and is also involved in many
interactions with the many ribosomal proteins that make up this
small organelle. Transfer RNA (tRNA) is very important to the
central dogma and to translation – each tRNA is attached to a
particular amino acid – tRNA is the linker molecule between the
genetic code and the amino acid sequence of proteins.
The central dogma of molecular genetics is that DNA is
transcribed into RNA, which is in turn translated into protein.
The genetic code consists of three nucleotides that encode a
particular amino acid. The three nucleotide group is called a codon.
Transcription is the process by which and RNA strand is created
from a DNA template
– the RNA sequence is complementary to one of the DNA
strands, and therefore it carries the code of the gene encoded
on the DNA. The RNA molecule that is created is called a
transcript. This process is carried out by RNA polymerase.
Translation is the process of creating a protein, or polypeptide
chain, that has the amino acid sequence encoded on the mRNA
transcript. This process is carried out by ribosomes and tRNA’s
which carry amino acids.
1.6 Mutation refers to a heritable change in a gene – physically it is created by a
change in the sequence of a gene, which could be the change of a single
nucleotide, or something more extensive such as an insertion or deletion of
multiple bases. The term mutant refers to the result of a particular mutation, and
could refer to the mutant gene, a mutant protein, or a mutant organism.
 The mutant gene can affect the proper functioning of a gene in
several ways – the protein could be not expressed at all (loss of
function), the protein could be expressed but not function properly
(loss of function), the protein could be expressed but in a form that
actively disables or disrupt other biological functions in a cell (gain
of function), or the protein could be overexpressed so there is too
much of that protein and it does not fulfill its role properly or it
disrupts other biological functions in the cell (gain of function).
 Protein folding has an important role in the proper functioning of
proteins, because function depends upon form. Your book uses the
PAH gene as an example. The gene encodes phenylalanine
hydroxylase (PAH) which converts phenylalanine to tyrosine.
1.7 Genes and environment:
 The expression of genes and how they function is also affected by the
environment and experiences of the particular organism. e.g. patients with
PKU disease can live healthy lives if they restrict phenylalanine (no meat,
poultry, fish, eggs, milk or milk products, etc.) in their diet. If
phenylalanine is not restricted a PKU patient can develop mental
retardation.
 Pleiotropy: the condition in which a single mutant gene affects two or
more distinct and semmingly unrelated traits. Some genes have
pleiotropic effects, whereby they affect multiple yet seemingly unrelated
traits through secondary or downstream effects. For example, some
children with PKU also have reduced body pigment because PAH blocks
the production of tyrosine, which is a precursor form of melanin.
 Many traits are affected by multiple genes. For example, genes involved in
the maintenance of the blood-brain barrier are important to determining
the severity of the PKU disease.
Genome: The total set of genes contained in a cell or virus; in eukaryotes,
commonly used to refer to all genes present in one complete haploid set of
chromosomes.
Proteome: The total set of proteins present in a cell or organism. (differs from
genome in that is depends on which genes are expressed in particular cells,
tissues, at different times of development, or in response to certain
environmental factors.
Chapter 2: DNA Structure and DNA Manipulation
2.2 The molecular structure of DNA
more detail on the biochemical nature of DNA strands and DNA folding
2.3 Separation and Identification of Genomic DNA Fragments
Hybridization: annealing of a DNA fragment of known sequence, to a larger
amount of DNA
Polymerase chain reaction: amplification of a small amount of DNA using the
cycles of annealing and replication of DNA, depends upon small fragments of
DNA called primers which select the sequence boundaries of DNA to be
amplified
Restriction enzymes: recognize and cleave DNA at short characteristic
sequences, also called restriction endonucleases
Restriction site: the particular site on DNA that is recognized by a restriction
enzyme
Gel electrophoresis: process by which DNA fragments can be separated
according to size, depends on the movement of the charged DNA molecules
through a gel through which an electric current is applied. DNA is visualized as
bands in the gel.
Denaturation: the separation of two strands in a DNA double helix
Renaturation: the coming together of two complementary strands of DNA – also
called hybridization