Glossary ❮ 327lymphocyteWhite blood cell. There are two main
types of lymphocyte: B cells and T cells. These
cells are formed in the bone marrow of the body
and arise from stem cells.
lysogenic cycle The virus falls dormant and incor-
porates its DNA into the host DNA as an entity
called a provirus.The viral DNA is quietly repro-
duced by the cell every time the cell reproduces
itself, and this allows the virus to stay alive from
generation to generation without killing the host
cell.
lysosomeMembrane-bound organelle that spe-
cializes in digestion and contains enzymes that
break down proteins, lipids, nucleic acids, and
carbohydrates.
lysozyme An enzyme, present in saliva and tears,
that can kill germs before they have a chance to
take hold.
lytic cycleThe cell actually produces many viral off-
spring, which are released from the cell, killing the
host cell in the process.
macroevolutionThe big picture of evolution, which
includes the study of evolution of groups of species
over very long periods of time.
macronucleus A nucleus present in some protists
(Ciliophora) and which controls the everyday activ-
ities of organisms.
macrospores Female gametophytes produced by
heterosporous plants.
map unit Also termed centigram.Unit used to geo-
graphically relate the genes on the basis of
crossover frequencies. One map unit is equal to a
1 percent recombination frequency.
matter Anything that has mass and takes up space.
mechanical digestion The physical breakdown of
food that comes from chewing.
medullaInner region of the kidney.
medulla oblongataThe control center for involun-
tary activities such as breathing.
medusa A cnidarian that is flat and roams the waters
looking for food (e.g., jellyfish).
melatonin Hormone that is known to be involved in
our biological rhythms (circadian).
memory cells Stored instructions on how to handle
a particular invader. When an invader returns to
the body, the memory cells recognize it, produce
antibodies in rapid fashion, and eliminate the
invader very quickly.
meristemic cells Cells that allow plants to grow
indeterminately.
mesoderm Intermediate germ layer that gives rise to
muscle, the circulatory system, the reproductive
system, excretory organs, bones, and connective
tissues of the gut and exterior of the body.
mesophyll Interior tissue of a leaf.
mesophyll cells Cells that contain many chloroplasts
and host the majority of photosynthesis.
methanogens Archaebacteria that produce methane
as a by-product.
microevolution Evolution at the level of species and
populations.
microfilamentsSubstances built from actin that play
a major role in muscle contraction.
micronucleus A nucleus present in some protists
(Ciliophora) and which functions in conjugation.
microspores Male gametophytes produced by het-
erosporous plants.
microtubules Substances constructed from tubulin;
play a lead role in the separation of cells during cell
division; are also important components of cilia
and flagella.
migrationThis is a cyclic movement of animals over
long distances according to the time of year.
mismatch repair Process during DNA replication by
which DNA polymerase replaces an incorrectly
placed nucleotide with proper nucleotide.
missense mutation Substitution of the wrong
nucleotides into the DNA sequence. These substi-
tutions still result in the addition of amino acids
to the growing protein chain during translation,
but they can sometimes lead to the addition of
incorrectamino acids to the chain.
mitochondrionDouble-membraned organelle that
specializes in the production of ATP; host
organelle for the Krebs cycle (matrix) and oxida-
tive phosphorylation (cristae).
mitotic spindleApparatus constructed from micro-
tubules that assists in the physical separation of the
chromosomes during mitosis.
monocot (monocotyledon) Angiosperm with a single
cotyledon.
monohybrid cross A cross that involves a single
character in which both parents are heterozygous
(Bb×Bb). A monohybrid cross between heterozy-
gous gametes gives a 3:1 phenotype ratio in the
offspring.
monosaccharide The simplest form of a carbohy-
drate. The most important monosaccharide is glu-
cose, which is used in cellular respiration to
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