Molecular Biology Flashcards
Viruses infect organisms by
binding to receptors on a host’s target cell,
injecting viral genetic material into the cell, and
hijacking the cell’s own molecules and organelles to produce new copies of the virus.
The host cell is destroyed, and newly replicated viruses are released to continue the infection.
Viruses are not generally considered alive because they
are not cellular and cannot reproduce on their own.
Because viruses have much less complex structures than cells, they are relatively easy to study at the molecular level.
For this reason, viruses are used to study the functions of DNA.
molecular biology
Studies of bacteria and viruses
ushered in the field of molecular biology, the study of heredity at the molecular level, and
revealed the role of DNA in heredity.
In 1928, Frederick Griffith discovered that a “transforming factor” could be transferred into a bacterial cell.
He found that
when he exposed heat-killed pathogenic bacteria to harmless bacteria, some harmless bacteria were converted to disease-causing bacteria and
the disease-causing characteristic was inherited by descendants of the transformed cells.
In 1952, Alfred Hershey and Martha Chase
used bacteriophages to show that DNA is the genetic material of T2, a virus that infects the bacterium Escherichia coli (E. coli).
Bacteriophages
are viruses that infect bacterial cells.
Phages were labeled with radioactive sulfur to detect proteins or radioactive phosphorus to detect DNA.
Bacteria were infected with either type of labeled phage to determine which substance was injected into cells and which
The sulfur-labeled protein stayed with the phages outside the bacterial cell, while the phosphorus-labeled DNA was detected inside cells.
Cells with phosphorus-labeled DNA produced new bacteriophages with radioactivity in DNA but not in protein.
polynucleotide
DNA and RNA are nucleic acids.
One of the two strands of DNA is a DNA polynucleotide, a nucleotide polymer (chain).
nucleotide
A nucleotide is composed of a nitrogenous base, five-carbon sugar, and phosphate group. The nucleotides are joined to one another by a sugar-phosphate backbone. Each type of DNA nucleotide has a different nitrogen-containing base: adenine (A), cytosine (C), thymine (T), and guanine (G).
RNA (ribonucleic acid) is unlike DNA in that it…….
RNA (ribonucleic acid) is unlike DNA in that it…….
uses the sugar ribose (instead of deoxyribose in DNA) and
RNA has the nitrogenous base uracil (U) instead of thymine.
In 1952, after the Hershey-Chase experiment demonstrated that the genetic material was most likely DNA
a race was on to
describe the structure of DNA and
explain how the structure and properties of DNA can account for its role in heredity.
In 1953, James D. Watson and Francis Crick deduced the secondary structure of DNA
using
X-ray crystallography data of DNA from the work of Rosalind Franklin and Maurice Wilkins and
Chargaff’s observation that in DNA,
the amount of adenine was equal to the amount of thymine and
the amount of guanine was equal to that of cytosine.
double helix
Watson and Crick reported that DNA consisted of two polynucleotide strands wrapped into a double helix.
The sugar-phosphate backbone is on the outside.
The nitrogenous bases are perpendicular to the backbone in the interior.
Specific pairs of bases give the helix a uniform shape.
A pairs with T, forming two hydrogen bonds, and
G pairs with C, forming three hydrogen bonds.
In 1962, the Nobel Prize was awarded to
James D. Watson, Francis Crick, and Maurice Wilkins.
Rosalind Franklin probably would have received the prize as well but for her death from cancer in 1958. Nobel Prizes are never awarded posthumously.
The Watson-Crick model gave new meaning to the words genes and chromosomes. The genetic information in a chromosome is encoded in the nucleotide sequence of DNA.
semiconservative model
DNA replication follows a semiconservative model.
The two DNA strands separate.
Each strand is used as a pattern to produce a complementary strand, using specific base pairing.
Each new DNA helix has one old strand with one new strand.
DNA replication begins at the origins of replication
replication where
DNA unwinds
replication proceeds in both directions
Two key proteins are involved in DNA replication
DNA helicase unwinds the strands.
DNA polymerase
adds nucleotides to a growing chain and
proofreads and corrects improper base pairings.
DNA polymerases and DNA ligase also repair DNA damaged by harmful radiation and toxic chemicals.
DNA replication ensures that all the somatic cells in a multicellular organism carry the same genetic information.
Transcription
is the synthesis of RNA under the direction of DNA.
Translation
is the synthesis of proteins under the direction of RNA.
The connections between genes and proteins
The initial one gene–one enzyme hypothesis was based on studies of inherited metabolic diseases.
The one gene–one enzyme hypothesis was expanded to include all proteins.
triplet code
The flow of information from gene to protein is based on a triplet code: the genetic instructions for the amino acid sequence of a polypeptide chain are written in RNA as a series of nonoverlapping three-base “words” called codons.
Translation involves switching from the nucleotide “language” to the amino acid “language.”
Each amino acid is specified by a codon.
64 codons are possible.
Some amino acids have more than one possible codon.
Characteristics of the genetic code
Three nucleotides specify one amino acid.
61 codons correspond to amino acids.
AUG codes for methionine and signals the start of transcription in all living things.
3 “stop” codons signal the end of translation.
The genetic code is
redundant, with more than one codon for some amino acids,
unambiguous in that any codon for one amino acid does not code for any other amino acid,
universal—the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals
Overview of transcription
An RNA molecule is transcribed from a DNA template by a process that resembles the synthesis of a DNA strand during DNA replication.
The DNA molecule is “unwound” by the enzyme helicase
RNA nucleotides are then linked by the transcription enzyme RNA polymerase.
Specific sequences of nucleotides along the DNA are called promoters and they mark where transcription begins and ends
Messenger RNA (mRNA)
encodes amino acid sequences and
conveys genetic messages from DNA to the translation machinery of the cell, which in
prokaryotes, occurs in the same place that mRNA is made, but in
eukaryotes, mRNA must exit the nucleus via nuclear pores to enter the cytoplasm.
Eukaryotic mRNA has
introns, interrupting sequences that separate
exons, the coding regions.
Transfer RNA (tRNA) molecules
function as a language interpreter, converting the genetic message of mRNA into the language of proteins. Transfer RNA molecules perform this interpreter task by picking up the appropriate amino acid and using a special triplet of bases, called an anticodon, to recognize the appropriate codons in the mRNA.
ribosome
Translation occurs on the surface of the ribosome.
Ribosomes coordinate the functioning of mRNA and tRNA and, ultimately, the synthesis of polypeptides.
Ribosomes have two subunits: small and large.
Each subunit is composed of ribosomal RNAs and proteins.
Ribosomes have binding sites for mRNA and tRNAs.
Translation can be divided into three phases just as in transcription:
initiation, elongation, and termination. Initiation brings together mRNA, a tRNA bearing the first amino acid, and the two subunits of a ribosome.
Codon recognition
The anticodon of an incoming tRNA molecule, carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome.
Peptide bond formation
The new amino acid is joined to the chain.
Elongation continues until the termination stage of translation, when
the ribosome reaches a stop codon,
the completed polypeptide is freed from the last tRNA, and
the ribosome splits back into its separate subunits.
A mutation
is any change in the nucleotide sequence of DNA.
Mutations can involve
large chromosomal regions or
just a single nucleotide pair.
Mutagenesis
is the production of mutations.
Mutations can be caused by
spontaneous errors that occur during DNA replication or recombination or
mutagens, which include
high-energy radiation such as X-rays and ultraviolet light and
chemicals.
A virus is essentially “genes in a box,”
an infectious particle consisting of
a bit of nucleic acid,
wrapped in a protein coat called a capsid, and
in some cases, a membrane envelope.
Viral DNA is inserted into the host chromosome
Viral DNA is duplicated along with the host chromosome during each cell division.
emerging viruses
Viruses that appear suddenly or are new to medical scientists are called emerging viruses. These include the AIDS virus, Ebola virus, West Nile virus, and SARS virus.
Three processes contribute to the emergence of viral diseases:
mutation—RNA viruses mutate rapidly.
contact between species—viruses from other animals spread to humans.
spread from isolated human populations to larger human populations, often over great distances.
Viral reproduction
allows researchers to learn more about the mechanisms that regulate DNA replication and gene expression in living cells.
Bacteria are also valuable but for different reasons.
Bacterial DNA is found in a single, closed loop, chromosome.
Bacterial cells divide by replication of the bacterial chromosome and then by binary fission.
Because binary fission is an asexual process, bacteria in a colony are genetically identical to the parent cell.
Bacteria use three mechanisms to move genes from cell to cell.
Transformation is the uptake of DNA from the surrounding environment.
Transduction is gene transfer by phages.
Conjugation is the transfer of DNA from a donor to a recipient bacterial cell through a cytoplasmic (mating) bridge.
Once new DNA gets into a bacterial cell, part of it may then integrate into the recipient’s chromosome.
