Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

Glossary 357

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

Motor pathways or tracts: Long fiber tracts that carry message from the brain

through the spinal cord to the muscles of the body.

MRI: see Magnetic resonance imaging.

mRNA: see Messenger RNA.

Multilevel selection: A theory of natural selection that includes both kinship

and groups of different sizes.

Natural selection: A key mechanism of evolution in which the environment

plays a selective role for the survival of species whose inherited characteris-

tics fit the particular environment. In the Darwinian theory, these heritable

characteristics tend to vary over generations.

Neocortex: see Cortex of the brain.

Neuromodulators: Any molecule that modifies or regulates the function of a

neurotransmitter.

Neuron: Cellular unit of the nervous system that conducts message from one

cell to another, through action potential in the axon and neurotransmitters

across the synapse between two neurons.

Neurotransmitters: Chemicals of small molecules that carry message across a

neuronal synapse.

NMDA receptor: A type of glutamate receptor in the post-synaptic part of

a synapse (located in the dendritic spine) believed to be important in the

formation of memory and learning, through the mechanism of long-term

potentiation (LTP). NMDA stands for N-methyl-D-aspartate.

Nucleic acid: A macromolecule made up of nucleotides linked together with

phosphate bonds. It is involved in information transfer and heredity. Nucleic

acids are of two types: the ribose (RNA) and the deoxyribose (DNA) vari-

eties. DNA tends to form a double helix whereas RNA assumes irregular

shape unique to each molecule.

Nucleobase: Also known simply as the “base.” A component of nucleic acid

consisting of heterocyclic nitrogenous compounds. It is joined to a ribose

sugar molecule to form a nucleoside (in an RNA), or to a deoxyribose to

form a deoxyribo-nucleoside (in a DNA). The four nucleobases in DNA are

adenine (A), guanine (G), thymine (T), and cytosine (C), with pairing of A-T

and G-C. The four nucleobases in RNA are adenine, guanine, uracil, and

cytosine.

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