LESSON 14: BACTERIAL GENETICS Flashcards
is the science of heredity; it includes the study of genes and information it carries.
Genetics
How genes are replicated and transferred to other organisms
or how particular genes are expressed in an organism and how does it influence its
characteristics
is the genetic information that a cell carry that includes chromosomes and plasmid.
genome
are containing DNA that carry
hereditary information; the chromosomes carry the genes.
Chromosomes
are segments of DNA (except in RNA viruses) that code for functional products.
GENES
A DNA or deoxyribonucleic acid is a macromolecule composed of repeating units called
nucleotides
nucleotide is consist of of a
Nitrogenous base (adenine, thymine, cytosine or guanine),
Deoxyribose (a pentose sugar), and a Phosphate group
The cells DNA exist as long strands of nucleotides twisted together in pairs to form a
Double helix
Each strand has a string of sugar and phosphate group and nitrogenous base attached to each sugar.
The two strands are held together by
hydrogen bonds
DNA
The base pairs are
(Thymine and adenine)
(Cytosine and guanine)
These strands have two designated ends called 5’ and 3’ (you can read that as 5 prime end and 3 prime end).
These numbers indicate end-to-end chemical
orientation.
is the end, which joins a phosphate group that attaches to another nucleotide.
5
end is important as during replication the new nucleotide is added to this end.
3
In terms of direction, if one strand is 5’ to 3’ while reading from left
to right, the other strand will be 3’ to 5’. Simply put, the strands run in opposite
directions. This orientation is kept for easy binding between nucleotides of the
opposite strands.
is the genetic makeup of an organism that codes for all its characteristics.
It is the organisms collection of genes.
Genotype
actual expressed properties of an organism
or the manifestation of a genotype
phenotype
A typical bacterial chromosome have a single circular DNA molecule and associated proteins.
The chromosome is folded and looped and attached to the plasma membrane of a bacteria.
The flow of genetic information from one generation to the next is made possible through DNA replication, or mRNA transcription.
If there is adenine in the parent or old
strand, complementary thymine will be added to the new strand.
Similarly, if there is cytosine in the parent strand, complementary guanine will be copied into the new daughter strand.
To summarize DNA replication
-the two strands uncoil and permanently
separate from each other.
- The base sequence of parent or old strand directs the base sequence of new or daughter strand.
initiated at the origin of replication.
DNA replication
Before the DNA synthesis begins, both the parental strands must unwind (due to an enzyme called
and separate permanently into single stranded state made possible by enzyme helicase
topoisomerase or gyrase
Primers are short sequences of RNA, around 10 nucleotides in length
Primase synthesizes the primers.
The next step involves the addition of new complementary strands.
The choice of nucleotides to be added in the new strand is dictated by the sequence of
bases on the template strand.
New nucleotides are added one by one to the end of growing strand by an enzyme called D
DNA polymerase
The strand, which is synthesized in the same direction as the replication fork, is known as the
‘leading’ strand.
The DNA polymerase has to attach only once and it can continue its work as the replication fork moves forward.
for the strand being synthesized in the other direction, which is known as the —-
the polymerase has to synthesize one fragment of DNA thus creating many fragments as it moves towards the replication fork.
‘lagging’ strand
These fragments are known as
Okazaki fragments
These gaps are filled by
ligase
Reiji okazaki
When DNA polymerase is adding nucleotides to the lagging strand and creating Okazaki
fragments, it at times leaves a gap or two between the fragments
The Replication process is finally complete once all the primers are
removed and Ligase has filled in all the remaining gaps.
This process gives us two
sets of genes, which will then be passed on to two daughter DNA molecules which
are identical to the parent molecule.
The information in a DNA are used to make proteins that controls cell activities.
the synthesis of a complimentary strand of RNA from a DNA template.
transcription
The encoded information is then used by cell to synthesize protein in a process called
translation
wherein genetic information in DNA is
copied or transcribed into a complimentary base sequence of RNA.
transcription
carries the coded
information for making specific proteins from DNA to ribosomes, where proteins are
synthesized.
Messenger RNA (mRNA)
There are three types of RNA in a bacterial cell:
messenger RNA,
ribosomal RNA, and transfer RNA
The process of transcription is similar to DNA replication wherein it also uses complementary base pairing as a guide, except that thymine is replaced by a uracil to pair with adenine in an RNA strand
Protein synthesis is called
translation
because it decodes and translates the genetic codes (codons) made during transcription into specific proteins which consists of a series of amino acids.
Sense codons codes into a particular amino acid
while nonsense codons signal the ends of protein synthesis thus it is also called
stop codons
The site of translation is the
ribosomes
while transfer RNA (tRNA) recognize the specific codon and transport the corresponding
amino acid
The functions of the
ribosomes are to direct the orderly binding of tRNAs to codons and to assemble the
amino acids brought there into a chain, ultimately producing protein.
is a regulatory mechanism
that inhibits gene expression and decreases synthesis of enzymes, usually in response to overabundance of an end-product (protein).
repression
RNA synthesis starts at a site in the strand called
promoter
until it reaches a site on the DNA called
terminator
is the process that
turns on the transcription of genes.
induction
is a change in the base sequence of a DNA which sometimes cause a change in the end-product (protein)
mutation
a single base in one point of a DNA is
replaced with another
- Base mutation (point mutation):
Types of mutation:
- Base mutation (point mutation):
- Missense mutation:
- Frame-shift mutation:
happens when as a result of base mutation, an
incorrect amino acid is inserted into the synthesized protein.
Missense mutation
when one or a few nucleotide pairs are inserted or
deleted in the DNA
Frame-shift mutation
are environmental agents that directly or indirectly cause mutations
Mutagens
mutagens examples:
1.Chemical mutagens
2. Radiation
Examples of chemical mutagens:
- nitrous acid
- nucleoside analog
- aflatoxins
exposure of bacteria to nitrous acid can convert the base adenine (A) to a form no longer unpairable with thymine at a random location.
also have altered base-pairing property
nucleoside analog
Radiation examples:
1.Xrays and gamma rays
2. . Ultraviolet (UV) light
are potent mutagens due to their ability to ionize
atoms
Xrays and gamma rays
a frame-shift mutagen and also a potent carcinogen.
aflatoxin
harmful (at 260 nm wavelength) due to its ability to form covalent bonds between bases.
ultraviolet (UV)
Cell damage due to UV light can be repaired by photolyases or light repair enzymes by destroying covalent bonds to return its original sequence.
is the exchange of genes between two DNA
molecules to form new combinations of genes on a chromosome that results to
genetic diversity in a population.
Genetic recombination
occurs when genes are passed from an organism to
its offspring (plants and animals).
Vertical gene transfer
the transfer involves a donor cell that gives a portion of its DNA to a recipient cell.
Horizontal gene transfer
The recipient cell that incorporates donor DNA to its own DNA is called
recombinant
In bacteria, genetic recombination results from these three types of
gene transfer :
- Transformation
- Conjugation
- Transduction
is the transfer of genes from one bacterium to another as “naked” DNA in a solution.
Transformation
in bacteria is a mechanism by which genetic material is
transferred by a plasmid.
conjugation
A is a circular piece of DNA that replicates
independently from the cells chromosome.
plasmid
is a genetic transfer wherein bacterial DNA is transferred
from a donor cell to recipient cell inside a virus that infects bacteria called
bacteriophage,
transduction
requires cell to cell contact, only donor cell must carry the plasmid.
conjugation
is an extrachromosomal genetic element that is capable of autonomous replication in the cytoplasm of a bacterial cell.
plasmid
The plasmids can also be
present as integrated with bacterial chromosomes, and plasmids integrated with host chromosome are known as
Episomes
Plasmids are present in both Gram-positive
and Gram-negative bacteria.
Many plasmids control medically important properties of pathogenic bacteria.
These include
(a) resistance to one or several antibiotics,
(b) production of toxins, and
(c) synthesis of cell surface structures required for adherence or colonization.
transfer antibiotic resistant genes to some
organism.
R plasmid
2 categories of plasmid:
- Transmissibility
- Nature of plasmid
cell to cell genetic transfer through conjugation
Transmissible
Responsible for synthesis of the sex pilus and
for the synthesis of enzymes required for their transfer.
a. transmissible
empty of genes, thus unable to transfer
b. non- transmissible
nature of plasmid
a. F factor
b. R (resistance) factor
c. Col factor
contains the genetic information, essential for controlling mating process of the bacteria during conjugation.
f factor
These genes determine
(a) expression of pili,
(b) synthesis and transfer of DNA during mating,
(c) interference with the ability of F bacteria to serve as recipients, and
(d) other functions.
2 sizes; large plasmid (mol. wt. 60 million); contains R factor and are conjugative, while small plasmids (mol. wt. 10 million)contains
“r” factor and non-conjugative
is responsible for conjugational
transfer
resistance transfer factor
R factor consists of two components:
The resistance transfer factor (RTF)
and resistant determinant (r)
carries resistance for one of the several
antibiotics
R determinants
or colicinogenic factor, resembles the F factor in promoting conjugation, leading to self-transfer and also at times transfer of segments of
chromosomes
col factor
encodes for production of colicins, which are antibiotics-like substances that are specifically and selectively lethal to other enteric
bacteria.
They also encode for production of diphthericin and pyocyanin
produced by
which are substances similar to colicins.
diphtheriae and Pseudomonas pyocyanea,
