Evolution__3rd_Edition

22 PART 1 / Introduction
DNA is carried in different ways in
prokaryotic and in eukaryotic cells
Figure 2.1
The cells of a body have a fine
structure (or “ultrastructure”)
made up of a number of
organelles. Not all the
organelles illustrated here are
found in all cells. Animal and
fungal cells, for example,
lack plastids; but all
photosynthesizing organisms
have them. Eukaryotes (i.e.,
all plants and animals) have
complex cells with a separate
nucleus. Within the nucleus the
DNA is here illustrated in the
diffuse form called chromatin;
when the cell divides, the
chromatin coalesces into
structures called chromosomes.
Prokaryotes are simpler
organisms, particularly
bacteria, and they lack a distinct
nucleus; their DNA lies naked
within the cell.
2.1 Inheritance is caused by DNA molecules, which are
physically passed from parent to offspring
The molecule called DNA (deoxyribose nucleic acid) provides the physical mechanism
of heredity in almost all living creatures. The DNA carries the information used to
build a new body, and to differentiate its various body parts. DNA molecules exist
inside almost all the cells of a body, and in all the reproductive cells (or gametes). Its
precise location in the cell depends on cell type.
There are two main types of cell: eukaryotic and prokaryotic (Figure 2.1). Eukaryotic
cells have a complex internal structure, including internal organelles and a distinct
region, surrounded by a membrane, called the nucleus. Eukaryotic DNA exists within
the nucleus. Prokaryotic cells are simpler and have no nucleus. Prokaryotic DNA lies
within the cell, but in no particular region. All complex multicellular organisms,
including all plants and animals, are built of eukaryotic cells. Fungi are also eukaryotic;
some fungi are multicellular (such as mushrooms) others are unicellular (such as
baker’s and brewer’s yeast, Saccharomyces cerevisiae). Protozoans, most of which (such
as amebas) are unicellular, are the other main group of eukaryotes. Bacteria and
Archaea are the two kinds of life in which the cells are prokaryotic.
Within a eukaryotic cell nucleus, the DNA is physically carried in structures called
chromosomes. Chromosomes can be seen through a light microscope at certain stages in
the cell cycle. Individuals of different species characteristically have different numbers
of chromosomes a each individual human has 46, for example, whereas a fruitfly
Drosophila melanogaster has eight, and other species have other numbers. The finer
structure of the DNA is too small to be seen directly, but it can be inferred by the
method of X-ray diffraction. The molecular structure of DNA was worked out by
Watson and Crick in 1953.
Non-cellulosic cell wall
Cell membrane
Nucleoid
Small ribosomes
Flagellum
Prokaryote
Golgi body
Large
ribosome
Endoplasmic
reticulum
Chromatin
Eukaryote
Mitochondria Plastid
Kinetosome (9 + 0)
Kinetochores
Thylakoids
Plastid inner membrane
Plastid outer membrane
Nucleus
Nucleolus
Nuclear membrane
Cell membrane
Cell wall (cellulose)
Cilia (9 + 2)
Cell membrane
..