Download File

Unit 9 Notes
Microbial Genetics
History of DNA




In the late 1800’s, scientists discovered that DNA resided in the nucleus of a cell.
In 1902, Walter Sutton proposed that hereditary material resided in chromosomes of cell’s nucleus.
In 1928, Frederick Griffith found out that hereditary material was transmitted from one organism to another.
In 1952, Martha Chase and Alfred Hershey found that DNA was the hereditary substance as opposed to a
protein.
Eukaryotic DNA Structure
 In the 1950’s, Erwin Chargaff found that the amount of adenine in a cell always equaled the amount of
thymine, and that the amount of cytosine is always equal to the amount of guanine. This is known as
Chargaff’s Rules.
 In 1952, Rosalind Franklin and Maurice Wilkins produced an x-ray crystallography of the shape of DNA.
 Scientists now know that DNA consists of long strands of nucleotides. Each nucleotide contains the
following: a sugar (deoxyribose), a phosphate group, and a nitrogen base (either adenine, cytosine, guanine,
or thymine).
 In 1953, James Watson and Francis Crick said that DNA resembles a twisted ladder, and they called it a
double helix.
1
How can so much DNA be packed inside a cell that cannot be seen with the naked eye? It is coiled and folded
as shown below:
DNA Replication
DNA makes copies of itself by a process called replication. Here are the steps of replication:
1. The DNA helix unwinds.
2. Enzymes break the hydrogen bonds that hold the base pairs together. Each single strand of parent DNA
serves as a template for a new complementary strand (semiconservative replication).
3. Another enzyme moves along the separated DNA strands, and matches bases from the parent strand to
the new complementary strand.
4. Bonds re-form between the bases, and you have a new DNA molecule!
The Genetic Code
 We know that a gene is a region of DNA on a chromosome that controls the production of a protein, which is
used for various body functions. There are many different proteins in our bodies.
 The manufacture of these proteins is called protein synthesis. This is a two-part process that involves not
only DNA, but also RNA, or ribonucleic acid.
 RNA differs from DNA as the chart below shows:
# Strands
Type of Sugar
Bases
DNA
2
Deoxyribose
A, T, C, G
RNA
1
Ribose
A, U, C, G
(uracil replaces thymine)
2
 In addition, there are 3 different types of RNA, and each serves a different purpose:



Messenger RNA (mRNA) – carries the coded instructions for protein synthesis from the DNA
in the nucleus to the ribosome in the cytoplasm.
Transfer RNA (tRNA) – brings the amino acids to the ribosome in the correct order so that they
can be built into the new protein
Ribosomal RNA (rRNA) – works with several proteins to make up the structure of a ribosome.
 Each amino acid is coded for by a sequence of 3 bases, and this is called a codon. Each codon produces an
amino acid. Look at codon chart below, and be sure you know how to read it:

The amino acids can be joined together in any number of combinations, and depending on the combination,
a different protein is produced!
Protein Synthesis
Bacteria, like humans, must have the capability of producing proteins. What do bacteria use proteins for? Well,
there are two reasons:
1. To provide structural proteins for the cell membrane, cell wall, etc. of the replicating bacterial
cell.
2. To provide enzymes (all enzymes are proteins) for the physiological processes of the bacterial
cell.
There are two parts to the protein synthesis process: transcription and translation.
1. Transcription – this is the first step. Transcription is the process of transferring information from a
strand of DNA to a strand of RNA in the nucleus. Here are the steps:
a. The DNA strand in the nucleus unwinds and separates, like in replication.
b. The ½ of the DNA strand that contains the gene for a protein acts as the template.
c. An enzyme matches RNA base pairs with their complementary DNA base pair.
3
d. The nucleotides of the RNA are bonded together to form a strand of mRNA. This mRNA
contains the complete code for the original strand of DNA.
e. mRNA leaves the nucleus and moves into the cytoplasm on a ribosome, where the actual protein
synthesis will occur.
2. Translation – this is the second step. Translation is the process where ribosomes synthesize proteins
with the help of mRNA molecules in the cytoplasm. Here are the steps:
a. The first codon (set of 3 nitrogen bases) of the mRNA attaches to a ribosome.
b. Then, tRNA molecules, each carrying a specific amino acid from the cytoplasm, approaches the
ribosome.
c. The tRNA anticodon (a set of 3 complementary bases floating around in the cytoplasm) pairs
with the mRNA codon, and the two molecules join with the tRNA molecule still attached.
d. Often, the first codon on mRNA is AUG, which is called the “start” codon and codes for
methionine. This tells the protein synthesis process to begin. When this signal is given, the
nRNA slides along the ribosome to the next codon.
e. Another tRNA molecule carrying an amino acid will pair with the mRNA codon, and so on, and
so on, down the chain.
f. When the different amino acids are synthesized, an enzyme joins them all together with peptide
bonds, thus forming a polypeptide.
g. As the process continues, a chain of amino acids is formed until the ribosome reaches a “stop”
codon on the mRNA strand. The process ends, and a protein is produced.
4
Prokaryotic DNA
Bacterial DNA is made up of the same things that eukaryotic DNA is made of, but the overall structure is
different. In bacteria, DNA exists in two forms:
1. Chromosome– bacteria have one chromosome, and it consists of DNA in a double helix in a closed
loop. This chromosome occupies about ½ of the total volume of the bacterial cell, and if extended its
full length, is about 1.5 mm long. In order for all of this DNA to fit inside a microscopic bacterial cell, it
is looped into a “flower” shape. The chromosome of a typical bacteria contains about 4,000 genes
compared to the 50,000 genes on a human chromosome.
2. Plasmid – in addition to the central chromosome, bacteria also have a number of closed loops of DNA
called plasmids. They exist independent from the bacterial chromosome. They contain about 2% of the
genetic information of the bacteria. They do not appear to be essential to a bacterium’s life; however,
they offer a selective advantage to the bacteria. Certain plasmids carry genes that confer resistance to
antibiotics, others allow the bacteria to transfer their genetic material to another bacteria. Still others
contain genes that encode for the production of toxic proteins.
Bacterial Chromosome Replication
The bacterial chromosome replicates itself in a process called binary fission. See unit 2 notes for an explanation
of this.
Gene Mutations
Humans usually go through their lives with the same set of genes; but bacteria do not necessarily do the same.
There are two main methods by which bacteria can change their genes:
1. Mutations – mutations are permanent changes in an organism’s DNA. They can occur in the following
ways:
a. Spontaneous Changes in DNA – since bacteria multiply rapidly, and there are so many of them, if
stands to reason that one or more of them will have defective DNA (kind of like a bacterial birth
defect). Suppose one bacteria spontaneously becomes resistant to an antibiotic. The antibiotic will
kill all of the others, but the resistant one will survive and multiply. Pretty soon there are a lot of
these!
b. Mutagens – these are outside factors that cause mutations, such as UV radiation, chemicals in the
environment, antibiotics, etc.
c. Transposons – these are small segments of DNA that have the ability to move from one position to
another in the bacterial chromosome. They do NOT contain genes, but insert themselves into areas
in the DNA strand where they interrupt the coding sequence of another gene, thus causing an
incorrect protein or no protein to be formed. These are commonly known as “jumping genes”, and
can even jump across species, such as from bacteria to plants!
2. Genetic Recombination – bacteria can transfer genes to other bacteria, in one of the three following ways.
It is estimated that as much as 20% of the DNA of an E.coli bacterium comes from other microbes!
5
a. Conjugation – when two live bacterial cells come together and transfer genetic material by
transferring plasmids.
6
b. Transduction – this is when gene transfer occurs with the assistance of a bacterial virus called a
bacteriophage. The virus actually transfers DNA from the donor cell to the recipient cell, kind of like
a mosquito.
7
c. Transformation – this is when a bacteria acquires fragments of DNA from a dead bacteria and
incorporates them into their own genome. An example of when this happens is this: suppose you are
taking antibiotics for a Strep throat, and an E.coli from your intestines picks up the DNA from the
dead Strep bacteria.
Comparison of Transformation, Conjugation, and Transduction
Characteristic
Transformation
Conjugation
Transduction
Method of DNA transfer
Movement across wall and
membrane of recipient
Few genes
Through channel after cell-tocell contact
Few genes to entire
chromosome
Yes
Sometimes
No
Yes
No
Yes
By a virus
Amount of DNA transferred
Plasmid Transferred?
Entire chromo. trans?
Virus required?
Live bacteria required?
Dead bacteria required?
Used to acquire antibiotic
resistance?
Yes
No
No
Yes
Yes
Yes
Few genes
Not likely
No
Yes
Yes
No
Not likely
8