A B C D E F G H I J
T U V W X Y
A macromolecule is a large molecule, and may be a protein, a
lipid, a nucleic acid, or a polysaccharide (i.e., a starch). Polysaccharides,
proteins, and nucleic acids are all polymers; lipids are not strictly polymers.
Meiosis is a biological cell division
process in eukaryotes by which a diploid parent cell produces four haploid
daughter cells. It consists of two cycles of nuclear division, usually
accompanied by cell division (especially in multicellular forms, where it is
generally used to produce gametes (gametogenesis), preceded by DNA replication.
A mitochondrion (plural mitochondria;
Fig. 1) is a membrane-enclosed cellular organelle. Mitochondria are distributed
through the cytosol of most eukaryotic cells. Their main function is to convert
the potential energy (via electron transport) of food molecules into ATP (the
universal energy currency of the cell). They are composed of folds called
cristae which give a much increased surface area on which chemical reactions can
The outer membrane encloses the entire organelle and
contains channels made of protein complexes through which molecules and ions
can move in and out of the mitochondrion. Large molecules are excluded from
traversing this membrane.
The inner membrane, folded into cristae, encloses the
matrix (the internal fluid of the mitochondrion). It contains several protein
complexes. Stalked particles are found on the cristae: these are the ATP
synthetase enzyme molecules, which produce ATP.
The intermembrane space between the two
membranes contains enzymes that use ATP to phosphorylate other nucleotides and
that catalyze other reactions.
In biology, Mitosis is the process
of chromosome segregation and nuclear division that follows replication of the
genetic material in eukaryotic cells. This process assures that each daughter
nucleus receives a complete copy of the organism's genome. In most eukaryotes
mitosis is accompanied with cell division or cytokinesis, but there are many
exceptions, for instance among the fungi. There is another process called
meiosis, in which the daughter nuclei receive half the chromosomes of the
parent, which is involved in gamete formation and other similar processes.
Mitosis is divided into several stages, with the
remainder of the cell's growth cycle considered interphase. Properly speaking, a
typical cell cycle involves a series of stages: G1, the first growth phase; S,
where the genetic material is duplicated; G2, the second growth phase; and M,
where the nucleus divides through mitosis. Mitosis is divided into prophase,
prometaphase, metaphase, anaphase, and telophase.
The whole procedure is very similar among most
eukaryotes, with only minor variations. As prokaryotes lack a nucleus and only
have a single chromosome with no centromere, they cannot be properly said to
In chemistry, a monomer (from Greek mono "one" and
meros "part") is any of several small molecular structures that may be
chemically bonded together to form long multi-part polymer molecules.
Amino acids are natural monomers, and polymerize to form proteins. Glucose
monomers can also polymerize to form starches and glycogen polymers. The
polymerization reaction is known as a condensation reaction because of the loss
of a hydrogen atom and a hydroxyl (-OH) group from the two monomer units. The
bond between the monomers is an oxygen molecule with a bond with each monomer
unit. The result of this reaction is H20.
Monosaccharides are carbohydrates in
the form of simple sugars.
Monosaccharides are sweet, water soluble and crystalline. Examples include
the hexoses (glucose, fructose, and galactose) and pentoses (ribose, deoxyribose).
Morphogenesis (from the Greek
morphę shape and genesis creation) describes the process of cellular
differentiation that takes place during the embryonic development of an
organism. The change from a cluster of unitary cells to structured tissues,
specialized cells and organs is controlled by the genetic "program" and can be
modified by environmental factors. The morphogenes (proteins that control
morphogenesis) that determine the fate of cells are proteins that interact with
DNA. They can either activate or deactivate genes that, in turn, can activate
other genes (Fig. 1). The localized expression (production) of a protein results
in a protein gradient. Above a threshold of concentration, the protein is active
and works as a transcription factor. (A transcription factor regulates the
amount of protein that is produced from a gene.)
mRNA runs through several steps during
its usually brief existence: During transcription, an enzyme called RNA
polymerase makes a copy of a gene from the DNA to mRNA as needed. In
prokaryotes, no further processing of mRNA occurs (except in rare cases), and
often translation of the mRNA into protein occurs even while transcription is
going on. In eukaryotes, transcription and translation occur in different parts
of the cell (transcription in the nucleus, where DNA is kept, and translation in
the cytoplasm, where ribosomes reside). Also in eukaryotes, mRNA undergoes
several processing steps before it is ready to be translated:
- addition of a 5' cap - A modified guanine
nucleotide is added to the "front" of the message. This is critical for
recognition and proper attachment of the ribosome.
- splicing - The pre-mRNA is modified to remove
certain stretches of non-coding sequences called introns; the stretches that
remain include protein-coding sequences and are called exons. Sometimes one
pre-mRNA message may be spliced in several different ways, allowing 1 gene to
encode multiple functions. Most RNA splicing is performed by enzymes, but some
RNA molecules are also capable of catalyzing their own splicing (see
- polyadenylation - A sequence (often several
hundred) of adenine nucleotides is added to the 3' end of the pre-mRNA. This
helps increase the half-life of the message, so that the transcript lasts
longer in the cell and consequently is translated more and produces more
After the mRNA has been processed, it is exported
from the nucleus into the cytoplasm, where it is bound to ribosomes and
translated into protein. After a certain amount of time the message degrades
into its component nucleotides, usually with the assistance of RNAses.
Mutations are permanent,
transmissible changes to the genetic material (usually DNA or RNA) of an
organism. Mutations can be caused by copying errors in the genetic material
during cell division and by exposure to radiation, chemicals, or viruses.
Mutations often lead to the malfunction or death of a cell and can cause cancer
in higher organisms. Mutations are considered the driving force of evolution,
where less favorable mutations are removed by natural selection, but favorable
ones tend to accumulate. Neutral mutations do not affect the organism and can
accumulate over time, which might result in what is known as Punctuated
Equilibrium; a modern variation on classic evolutionary theory.