Page 137 - Algae Anatomy, Biochemistry, and Biotechnology

120 Algae: Anatomy, Biochemistry, and Biotechnology

sequences of unknown function that are removed in the primary mRNA transcript. Exons are the
DNA sequences that code for amino acids).
In the nucleus of eukaryotic algae, long, linear, and unbranched molecules of DNA are associ-
ated with proteins and small amount of RNA. Two types of proteins are found: the relatively
uniform histones (about 20), involved in the structural organization of the DNA, and the very vari-
able proteins involved in gene activity regulation, such as DNA and RNA polymerases. The DNA–
protein complex, made up of repeating units termed nucleosomes, is known as chromatin, which is
usually highly dispersed in the interphase nucleus. During mitosis and meiosis metaphases the
DNA–protein complexes are more helically condensed around the nucleosome and form the
chromosomes, each chromosome consisting of a single DNA molecule.
The nucleus is surrounded by a two-membrane envelope continuous with the endoplasmic reti-
culum. Between the two membranes there is a narrow perinuclear space about 20 nm wide. The
nuclear envelope is perforated by numerous pores 60–100 nm in diameter. Nuclear pores have a
complicated superstructure; they are not simply free openings, but are gateways in nuclear envelope
through which the controlled transport of macromolecules (RNA, proteins) takes place. Pores can
be arranged in straight lines as in Bumilleria (Xanthophyceae, Heterokontophyta), in closely hex-
agonal groups as in Prorocentrum (Dinophyta), or can be randomly distributed as in Glenodinium
(Dinophyta).
All algal nuclei possess nucleoli that vary in shape, size, and number in the different algal div-
isions. Nucleoli are dense concentrations of ribonucleoprotein-rich material, which are intimately
associated with the specific region of the chromosomal DNA coding for ribosomal RNA. Nucleoli
can be single and central as in Cryptomonas (Cryptophyta) or be more than 20 scattered in the
nucleus as in Euglena acus (Euglenophyta).
In eukaryotic algae, the features of the interphase nucleus, DNA replication, and processes of
transcription and translation in the expression of genetic information and cytokinesis are compar-
able with those of all the other eukaryotes. Exceptions to this rule will be described.

Rhodophyta
The basic features of mitosis and cytokinesis are the same throughout the division, though some vari-
ation may occur. Unlike other groups of eukaryotic algae, where the mitotic spindle is formed
between two pairs of centrioles, one at each spindle pole, in red algae the poles of the spindle are
marked by ring-shaped structures named polar rings. The absence of centrioles reflects the complete
absence of flagella in the red algae. The chromosomal and interzonal microtubules do not converge
towards the polar rings, so that the spindle poles are very broad. The nuclear envelope does not break
down, though it is perforated by large holes, hence the mitosis is closed. The spindle persists for some
time at early telophase, holding daughter nuclei apart; it collapses only at late telophase, when daugh-
ter nuclei are kept separated by a vacuole. Cytokinesis is by furrowing, but is typically incomplete;
the furrow impinges upon the vacuole but leaves open a cytoplasmic connection between sibling
cells. This connection is then filled and blocked by a proteinaceous stopper, named pit plug. The sim-
plest type of pit plug consists only of a proteinaceous core; two- or one-layered caps partly composed
of polysaccharides can border the core of both sides with different thickness. A cap membrane can be
present between the two layers of the cap or bounding the plug core.

Cryptophyta
These algae possess a peculiar organelle termed nucleomorph located in the space between the
chloroplast endoplasmic reticulum and the chloroplast envelope. This organelle has a double mem-
brane around it, pierced by pores and contains both DNA and a nucleolus-like structure where rRNA
genes are transcribed. The DNA is organized in three small chromosomes encoding genes for its
own maintenance. It possesses the ability to self-replicate, without forming a spindle during
mitosis. It is considered a vestigial nucleus belonging to a photosynthetic eukaryotic symbiont.