Encyclopedia of Biology

fertilization The combining of two gametes from
different sexes to form a zygote, e.g., the penetration
ofsperm into the egg and the resulting combining of
genetic material from both that develops into an
embryo. The process involves karyogamy, the fusion
of nuclei of both gametes, and plasmogamy, the
fusion of cytoplasm. Each gamete contains a haploid
set of chromosomes, with the resulting nucleus con-
taining a diploid set of chromosomes. Fertilization
can also be self-induced by the fusion of male and
female gametes from the same euploid (nucleus of a
cell contains exact multiples of the haploid number of
chromosomes) organism; cross fertilized by the fusion
of male and female gametes from different euploid

individuals; or double fertilized, in which two sepa-

rate sperm cells unite with two cells in the embryo

sacto form the zygote and endosperm, such as in

angiosperms.

Inagriculture, fertilization means the application

of nutrients, a fertilizer, to soil in order to promote

growth and development of domestic or crop plants.

[2Fe-2S] Designation of a two-iron, two-labile-sulfur

CLUSTERin a protein, comprising two sulfido-bridged

iron atoms. The oxidation levels of the clusters are

indicated by adding the charges on the iron and sulfide

atoms, i.e., [2Fe-2S]2+; [2Fe-2S]+. The alternative desig-

nation, which conforms to inorganic chemical conven-

tion, is to include the charges on the LIGANDs; this is

more appropriate where the ligands are other than the

usual cysteine sulfurs, such as in the RIESKE IRON–SUL-

FUR PROTEINs.

See alsoFERREDOXIN.

[4Fe-4S] Designation of a four-iron, four-labile-sul-

fur CLUSTERin a protein. (See[2FE-2S].) Possible oxida-

tion levels of the clusters are [4Fe-4S]3+; [4Fe-4S]2+;

[4Fe-4S]+.

See alsoFERREDOXIN;HIPIP.

fetus An unborn offspring in the postembryonic

stage where the major features of the organism can be

seen.

F factor A bacterial plasmid, which is a piece of

DNA that is able to replicate independently of the

chromosome, that allows a prokaryote (cell with no

nucleus) to join together with and pass DNA. An epi-

some that can replicate by itself or in integrated form

and move from one bacterium to another while conju-

gating. A circular piece of DNA that can replicate inde-

pendently of the bacterial chromosome or integrate and

replicate as part of the chromosome.

fiber Along-walled plant cell that is often dead at

maturity, is lignified, and reinforces the xylem of

angiosperms, giving elasticity, flexibility, tensile

fiber 127

Colored scanning electron micrograph (SEM) of sperm clustered
around a human egg (ovum) during fertilization. The rounded egg
(at center) is seen on human tissue. Sperm attached to its surface
appear as fine hairlike structures; each sperm has a rounded
head and a long tail. They are penetrating the thick spongy sur-
face of the zona pellucida of the egg, a surface layer that attracts
sperm to the egg and enables the sperm to attach. The human
female usually produces a single large egg, and only one of the
millions of male sperm may penetrate the egg’s wall to fuse with
the egg nucleus. Once fertilized, the egg begins its process of
growth by cell division.(Courtesy © K.H. Kjeldsen/Photo
Researchers, Inc.)