Instant Notes: Plant Biology

Section B – Structure

B5 Nucleus and genome

Each living plant cell contains a nucleus separated from the cytoplasm and other

organelles by a double membrane, the nuclear envelope (Topic B4). The size

and prominence of the nucleus varies depending on the major functions of the

cell. Cells in meristems (Topic C1), which are to undergo cell division, have a

large nucleus. Other cells, such as those of the parenchyma (Topic C3), have a

smaller nucleus, with most of the cell occupied by the vacuole (Topic B4). The

nuclear envelope surrounds the fluid nucleoplasm. Movement from nucleo-

plasm to cytoplasm is possible through complex protein structures, the nuclear

pores, which span both membranes. Nuclear pores are complex multi-protein

structures that regulate movement into and out of the nucleus; their most

important function is to permit messenger RNA (mRNA) molecules and ribo-

somes to leave the nucleus. Within the nucleus, the DNA is entwined with

proteins, the DNA–protein complex being termed chromatin. In interphase, the

chromatin appears to be spread throughout the nucleoplasm; in mitosis,

however, it condenses to form distinct chromosomes (Topic B6).

Structure of the
nucleus

Key Notes

The nucleus is bounded by a double membrane, the nuclear envelope,

which is perforated by nuclear pores that permit the movement of

materials from the interior of the nucleus (nucleoplasm) to the cytoplasm.

The genetic material (DNA) is present as a DNA–protein complex known

as chromatin. The nucleolus is the site of synthesis of the ribosome

subunits.

The DNA double helix is entwined around histone proteins in units

called nucleosomes, packed together to give a 30-nm fiber of chromatin.

Genetic information is contained within the DNA as codons.Genes are

made up of a structural region encoding the protein to be synthesized

and promoter regions that control gene activity. In transcription, the

DNA untwines and RNA polymerase II copies it as an RNA strand. Non-

transcribed introns are removed giving messenger RNA (mRNA) which

is translated into protein on ribosomes.

Chromosomes are made up of tightly packed chromatin. Chromosomes

in dividing cells have two chromatids joined at a centromere.

Endoreduplication results in many copies of each chromosome, while

gene amplification results in multiple copies of a few highly used genes.

Polyploid cells have more than the usual number of chromosomes;

normally only polyploids with an even number of chromosome copies

are fertile as they can undergo chromosome pairing in meiosis.

Related topics Membranes (B4) Features of growth and

Cell division (B6) development (F1)

Plant chromosomes

Structure of the

nucleus

Structure and

function of

chromatin