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

cannot reproduce on their own

Until the 1940s, the case for proteins serving as the genetic material was stronger than the case for DNA.

Why?

Proteins were thought to be structurally more complex because they were made from 20 different amino acids.

DNA was known to be made from just four kinds of nucleotides.

Studies of bacteria and the viruses that infected them

the study of heredity at the molecular level, and

revealed the role of DNA in heredity

He was studying two strains (varieties) of a bacterium that causes pneumonia in mice.  

He was surprised when he exposed heat-killed virulent (disease-causing) bacteria to living non-virulent (harmless) bacteria, 

because some non-virulent bacteria were converted to disease-causing bacteria.

In addition, the disease-causing characteristic was inherited by descendants of the transformed bacteria.

protein  and DNA 

A phage functions like a needle and syringe, injecting a drug. The “needle and syringe” are the protein components of the phage. The “drug” to be injected is the phage DNA.

Bacteria were infected with either type of labeled phage to determine which substance was injected into cells and which remained outside the infected cell.

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!

A nucleotide is composed of a 

nitrogenous base,

five-carbon sugar, and

phosphate group.

adenine (A), cytosine (C), thymine (T), and guanine (G).

Each type of DNA nucleotide has a different nitrogen-containing base:

RNA uses the sugar five-carbon sugar, ribose (instead of deoxyribose in DNA) 

RNA has the nitrogenous base uracil (U) instead of thymine.

RNA is single-stranded

X-ray crystallography data of DNA from the work of Rosalind Franklin and Maurice Wilkins 

In 1953, American James D. Watson and Englishman Francis Crick deduced the secondary structure of DNA, using

the amount of adenine was equal to the amount of thymine and 

the amount of guanine was equal to that of cytosine.

between bases (A, T, C, G) hold the strands together.

  Each base pairs with a complementary partner:  A with T, and G with C.

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 (parental) strand with one new (daughter) strand (semiconservative

short stretches of DNA having a specific sequence of nucleotides where proteins attach to the DNA and separate at the strands.

DNA unwinds at the origin to produce a “bubble,”

replication proceeds in both directions from the origin, and

replication ends when products from the bubbles merge with each other.

such that hundreds or thousands of “bubbles” can be present at once shortening the time for replication.

It takes an E. Coli bacterium less than an hour to replicate its DNA, for humans a few hours.

DNA polymerase

adds nucleotides to a growing chain and

proofreads and corrects improper base pairings.

rewrites (transcribes) the DNA code into RNA, using the same nucleotide “language.” 

The transcription of DNA into RNA is like a reporter’s transcription of a political speech. In both situations, the language remains the same, although in the case of the reporter, it changes its form from spoken to written language.

codon.

64 codons are possible.

Some amino acids have more than one possible codon

Transcription and Translation of Codons

in that codons are adjacent to each other with no gaps in between.

In other words, the nucleotides making up the codons occur in a linear order along the DNA and RNA, with no gaps separating the codons.

molecules function as a language interpreter,

converting the genetic message of mRNA

into the language of proteins.

initiation,

elongation, and

termination.

 Initiation brings together

mRNA,

a tRNA bearing the first amino acid, and

the two subunits of a ribosome

start codon.

The start codon reads AUG and codes for methionine.

The first tRNA has the anticodon UAC

Initiation occurs in two steps:

A mRNA molecule binds to a small ribosomal subunit and the first tRNA binds to mRNA at the 

Translocation

Peptide bond formation

Codon recognition

stop codon (UAA, UAG, UGA),

the completed polypeptide is freed from the last tRNA, and

the ribosome splits back into its separate subunits.

Elongation continues until the termination stage of translation, when

the ribosome reaches a 

is the synthesis of RNA from a DNA template. In eukaryotic cells,

transcription occurs in the nucleus and

the mRNA must travel from the nucleus to the cytoplasm.

Amino acid attachment,

Initiation of polypeptide synthesis,

Elongation, and

Termination.

Translation can be divided into four steps, all of which occur in the cytoplasm:

Nucleotide substitutions may

have no effect at all, producing a

change the amino acid coding, producing a 

which produces a different amino acid,

reading frame (triplet grouping) of the mRNA, so that nucleotides are grouped into different codons,

lead to significant changes in amino acid sequence downstream of the mutation, and

produce a nonfunctional polypeptide.

Mutations can result in deletions or insertions that may

alter the

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.

capsid, and 

in some cases, a membrane env

A virus is essentially “genes in a box,” an infectious particle consisting of

a bit of nucleic acid,

wrapped in a protein coat called 

are parasites that can replicate (reproduce) only inside host cells.

The host cell provides most of the goodies to replicate, transcribe, and translate the viral nucleic acid

viral particles are produced using host cell components,

the host cell lyses (breaks open), and

viruses are released

Viral DNA is inserted into the host chromosome by recombination.

Viral DNA is duplicated along with the host chromosome during each cell division.

The inserted phage DNA is called a prophage. 

Most prophage genes are inactive.

Environmental signals (radiation, drought, low nutrients) can cause a switch to the lytic cycle, causing the viral DNA to be excised from the bacterial chromosome and leading to the death of the host bacterial cell.

stunt their growth and decrease crop yield.

To infect a plant, they must get past the outer protective layer of the plant.

Viruses spread from cell to cell through their plasmodesmata.

Infection can spread to other plants by insects, herbivores (cows, deer), humans, or farming tools.

Mutation—RNA viruses mutate rapidly such as influenza virus.

Contact between species—viruses from other animals spread to humans such as hantavirus in rodents and bird “avian” flu.

Spread from isolated human populations to larger human populations, often over great distances due to international travel such as AIDS virus.

AIDS (acquired immunodeficiency syndrome) is caused by HIV (human immunodeficiency virus).

HIV

is an RNA virus,

carries molecules of reverse transcriptase, an enzyme, which causes reverse transcription, producing DNA from an RNA template, RNA→DNA

is the transfer of DNA from a donor to a recipient bacterial cell through a cytoplasmic (mating) bridge.  The donor cell uses its sex pili to attach itself to the recipient cell.

Plasmids

Some plasmids can bring about conjugation and move to another cell in linear form.

The transferred plasmid re-forms a circle in the recipient cell.