Cell Nucleus Biogenesis, Structure and Function
379
Gene Expression
Genes are decoded into RNA, which is translated into protein.
DNA Replication
DNA is copied by enzymes into 2 identical molecules.
DNA Repair
DNA damage is corrected to maintain genetic integrity.
Cell Cycle
Sequence of events that yields 2 daughter cells from a single mother cell.
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The nucleus is an organelle that is found in eukaryotes but not in prokaryotes.
Nuclei contain the genetic information that deFnes the appearance and behaviour of
an organism. The genetic material - DNA - is packaged as a DNA-protein complex,
called chromatin, into units of manageable size, called chromosomes. In multi-
cellular eukaryotes, different types of cells are deFned by the different genes that
they express. Patterns of gene expression arise during cell differentiation and reflect
a cells position in the organism during development. Gene expression is regulated
at many levels in each cell. The major regulatory step occurs during transcription,
when the gene is copied by transcription into messenger RNA. Messenger RNAs
transfer genetic information to the cytoplasm and provide the template for protein
synthesis during translation. Higher eukaryotes contain very large numbers of cells
that are derived from a single precursor - a fertilised egg. The genetic material
of this cell is derived from the egg and sperm of the female and male parents.
During development, cells must perform many cycles of DNA duplication and
cell proliferation. The genetic information must be faithfully copied during DNA
replication and any damaged corrected by DNA repair, so that the same genetic
information is passed to all cells of the organism. This chapter describes these
fundamental biological processes and explains how they are regulated within the
nucleus of eukaryotic cells.
1
Introduction
Eukaryotes are deFned by the presence
of a nucleus in their cells. In most
eukaryotes,
the
nucleus
is
the
most
prominent organelle. It is the cellular
compartment where the vast majority of
genetic information is held (mitochondria
and chloroplasts contain very small DNA
molecules). The genetic code is held
within the DNA. Each human cell has
roughly 6
×
10
9
bp DNA. This is far too
much to be held in a single molecule
within the cell, as in bacteria, so the
DNA is divided between a number of
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