Chapter 3
The Biological Basis of Life
Key Terms

Nucleus
A structure (organelle) found in all eukaryotic
cells. The nucleus contains chromosomes
(nuclear DNA).

Molecules
Structures made up of two or more atoms.
Molecules can combine with other molecules to
form more complex structures.

Deoxyribonucleic acid (DNA)
The double-stranded molecule that contains
the genetic code. DNA is a main component of
chromosomes.

Ribonucleic acid (RNA)
A single stranded molecule, similar in structure
to DNA. The three forms of RNA are essential
to protein synthesis.

Cytoplasm
The portion of the cell contained within the cell
membrane, excluding the nucleus. The
cytoplasm consists of a semifluid material and
contains numerous structures involved with cell
function.

Proteins
Three-dimensional molecules that serve a wide
variety of functions through their ability to bind
to other molecules.

Protein synthesis
The assembly of chains of amino acids into
functional protein molecules. The process is
directed by DNA.

Mitochondria
Structures contained within the cytoplasm of
eukaryotic cells that convert energy, derived
from nutrients, into a form that is used by the
cell.

Ribosomes
Structures composed of a specialized form of
RNA and protein. Ribosomes are found in the
cell’s cytoplasm and are essential to the
manufacture of proteins.

Somatic cells
Basically, all the cells in the body except those
involved with reproduction.

Gametes
Reproductive cells (eggs and sperm in
animals) developed from precursor cells in
ovaries and testes.

Zygote
A cell formed by the union of an egg and a
sperm cell. It contains the full complement of
chromosomes (in humans, 46) and has the
potential of developing into an entire organism.

Nucleotides
Basic units of the DNA molecule, composed of
a sugar, a phosphate, and one of four DNA
bases.

Complementary
Referring to the fact that DNA bases form base
pairs in a precise manner. For example,
adenine can bond only to thymine. Thus, these
two bases are said to be complementary
because one requires the other to form a
complete DNA base pair.

Enzymes
Specialized proteins that initiate and direct
chemical reactions in the body.

Replicate
To duplicate. The DNA molecule is able to
make copies of itself.

Hemoglobin
A protein molecule that occurs in red blood
cells and binds to oxygen molecules.

Hormones
Substances (usually proteins) that are
produced by specialized cells and that travel to
other parts of the body, where they influence
chemical reactions and regulate various
cellular functions.

Amino acids
Small molecules that are the components of
proteins.

Messenger RNA (mRNA)
A form of RNA that is assembled on a
sequence of DNA bases. It carries the DNA
code to the ribosome during protein synthesis.

Codons
The triplets of messenger RNA bases that
correspond to specific amino acids during
protein synthesis.

Transfer RNA (tRNA)
The type of RNA that binds to specific amino
acids and transports them to the ribosome
during protein synthesis.

Gene
Sequence of DNA bases that specifies the
order of amino acids in an entire protein or, in
some cases, a portion of a protein. A gene may
be made up of hundreds or thousands of DNA
bases.

Mutation
A change in DNA. Technically, mutation refers
to changes in DNA bases (specifically called
point mutations); also refers to changes in
chromosome number and/or structure.

Chromosomes
Discrete structures composed of DNA and
protein found only in the nuclei of cells. Visible
only under magnification during certain stages
of cell division.

Centromere
The constricted portion of a chromosome. After
replication, the two strands of a doublestranded chromosome are joined at the
centromere.

Homologous
Referring to members of chromosome pairs.
Homologous chromosomes carry genes that
govern the same traits. During meiosis,
homologous chromosomes pair and exchange
segments of DNA.

Autosomes
All chromosomes except the sex
chromosomes.

Sex chromosomes
In mammals, the X and Y chromosomes.

Mitosis
Simple cell division; the process by which
somatic cells divide to produce two identical
daughter cells.

Meiosis
Cell division in specialized cells in ovaries and
testes. Meiosis involves two divisions and
results in four daughter cells, each containing
only half the original number of chromosomes.
These cells can develop into gametes.

Recombination
The exchange of DNA between homologous
chromosomes during meiosis; also called
“crossing over.”

Nondisjunction
The failure of homologous chromosomes or
chromosome strands to separate during cell
division.

Polymerase chain reaction (PCR)
A method of producing thousands of copies of
a DNA segment using the enzyme DNA
polymerase.

Recombinant DNA technology
A process in which genes from the cell of one
species are transferred to somatic cells or
gametes of another species.

Clone
An organism that is genetically identical to
another organism. The term may also be used
to refer to genetically identical DNA segments
and molecules.

Human Genome Project
An international effort aimed at sequencing and
mapping the entire human genome.

Genome
The entire genetic makeup of an individual or
species. In humans, it is estimated that each
individual possesses approximately 3 billion
DNA nucleotides.