Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Red tide

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In 1973, Stanley Cohen and Herbert Boyer created the

first recombinant DNA organism, by adding recombinant

plasmidsto E. coli.Since that time, advances in molecular

biologytechniques, in particular the development of the poly-

merase chain reaction, have made the construction of recom-

binant DNA swifter and easier.

Recombinant DNA has been of fundamental importance

in furthering the understanding of genetic regulatory processes

and shows great potential in the genetic design of therapeutic

strategies.

See alsoChromosomes, eukaryotic; Chromosomes, prokary-

otic; DNA (Deoxyribonucleic acid); Genetic regulation of

eukaryotic cells; Genetic regulation of prokaryotic cells;

Laboratory techniques in immunology; Laboratory techniques

in microbiology; PCR; Plasmid and plastid

RRecombinationECOMBINATION

Recombination, is a process during which genetic material is

shuffled during reproduction to form new combinations. This

mixing is important from an evolutionary standpoint because

it allows the expression of different traits between generations.

The process involves a physical exchange of nucleotides

between duplicate strands of deoxyribonucleic acid(DNA).

There are three types of recombination; homologous

recombination, specific recombination and transposition.

Each type occurs under different circumstances. Homologous

recombination occurs in eukaryotes, typically during the first

phase of the meiotic cell division cycle. In most eukaryotic

cells, genetic material is organized as chromosomesin the

nucleus. A nick is made on the chromosomal DNA of corre-

sponding strands and the broken strands cross over, or

exchange, with each other. The recombinant region is

extended until a whole geneis transferred. At this point,

further recombination can occur or be stopped. Both

processes require the creation of another break in

the DNA strand and subsequent sealing of the nicks by

special enzymes.

Site specific recombination typically occurs in prokary-

otes. It is the mechanism by which viral genetic material is

incorporated into bacterial chromosomes. The event is site-

specific, as the incorporation (integration) of viral genetic

material occurs at a specific location on the bacterial genome,

called the attachment site, which is homologous with the

phage genome. Under appropriate conditions alignment and

merging of the viral and bacterial genomes occurs.

Transposition is a third type of recombination. It

involves transposable elements called transposons. These are

short segments of DNA found in both prokaryotes and eukary-

otes, which contain the information enabling their movement

from one genome to another, as well as genes encoding other

functions. The movement of a transposon, a process of trans-

position, is initiated when an enzyme cuts DNA at a target site.

This leaves a section that has unpaired nucleotides. Another

enzyme called transposase facilitates insertion of the transpo-

son at this site. Transposition is important in genetic engineer-

ing, as other genes can be relocated along with the transposon

DNA. As well, transposition is of natural significance. For

example, the rapid reshuffling of genetic information possible

with transposition enables immunocytes to manufacture the

millions of different antibodies required to protect eukaryotes

from infection.

See alsoCell cycle (eukaryotic), genetic regulation of; Cell

cycle (prokaryotic), genetic regulation of; Microbial genetics

RRed tideED TIDE

Red tides are a marine phenomenon in which water is stained

a red, brown, or yellowish color because of the temporary

abundance of a particular species of pigmented dinoflagellates

(these events are known as “blooms”). Also called phyto-

plankton, or planktonic algae, these single-celled organisms of

the class Dinophyceae move using a tail-like structure called a

flagellum. They also photosynthesize, and it is their photosyn-

thetic pigments that can tint the water during blooms.

Dinoflagellates are common and widespread. Under appropri-

ate environmental conditions, various species can grow very

rapidly, causing red tides. Red tides occur in all marine regions

with a temperate or warmer climate.

The environmental conditions that cause red tides to

develop are not yet understood. However, they are likely

related to some combination of nutrient availability, nutrient

ratios, and water temperature. Red tides are ancient phenom-

ena. Scientists suspect that human activities that affect nutri-

ent concentrations in seawater may be having an important

influence on the increasingly more frequent occurrences of red

tides in some areas. In particular, the levels of nitrogen, phos-

phorous, and other nutrients in coastal waters are increasing

due to runoff from fertilizers and animal waste. Complex

global changes in climate also may be affecting red tides.

Water used as ballast in ocean-going ships may be introducing

dinoflagellates to new waters.

Sometimes the dinoflagellates involved with red tides

synthesize toxic chemicals. Genera that are commonly asso-

ciated with poisonous red tides are Alexandrium, Dinophysis,

and Ptychodiscus. The algal poisons can accumulate in

marine organisms that feed by filtering large volumes of

water, for example, shellfish such as clams, oysters, and mus-

sels. If these shellfish are collected while they are signifi-

cantly contaminated by red-tide toxins, they can poison the

human beings who eat them. Marine toxins can also affect

local ecosystems by poisoning animals. Some toxins, such as

that from Ptychodiscus brevis, the organism that causes

Florida red tides, are airborne and can cause throat and nose

irritations.

Red tides can cause ecological damage when the algal

bloom collapses. Under some conditions, so much oxygen is

consumed to support the decomposition of dead algal biomass

that anoxic (lack of oxygen) conditions develop. This can

cause severe stress or mortality in a wide range of organisms

that are intolerant of low-oxygen conditions. Some red-tide

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