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3.2 Genome organization and packaging

(a)

(b)

Eukaryotic cell

Prokaryotic cell

10 — 100 µm

1 µm

Nucleus

Small circular ds DNA plasmids

Single circular ds DNA chromosome

Linear ds DNA chromosomes

Mitochondria contain multiple copies of their small circular ds DNA genome.

Figure 3.10 Organization of the genome in prokaryotic and eukaryotic cells. All cellular genomes consist of double-stranded (ds) DNA. (a) Prokaryotic cells each contain a single circular genome, which is highly folded and compacted to fit it in the small space. They may also contain small double-stranded circular plasmids that can replicate independently. (b) The genome of eukaryotic cells is contained in the nucleus and is organized as linear double-stranded chromosomes, the number and sizes of which vary between species. Eukaryotic cells contain homologous pairs of chromosomes, denoted here by colour coding. We explain the significance of homologous pairs later in Chapter 4. The eukaryotic cell contains mitochondria, which have multiple copies of their own double-stranded circular genome. In plant cells, chloroplasts also have their own genome.

function and may therefore be considered an integral part of the gene. The regulatory sequences of genes are also considered fur ther in Chapter 5. Eukaryotes contain split genes consisting of introns and exons In prokaryotes the protein-coding region of a gene is continuous, but in eukaryotes the coding region is typically interrupted by seg ments of DNA that do not code for amino acids. These ‘interven ing’ sequences are termed introns, while the coding or ‘expressed’ sequences are termed exons. In the human genome exons total only about 1.6 per cent of the DNA, while the transcribed sequences of genes, including introns, make up around 25 per cent of the genome. When split genes are transcribed into RNA the sections of the primary transcripts that correspond to the introns are removed and the remaining sequences are joined together to make a con tinuous messenger RNA, a process known as splicing . We discuss splicing in more detail in Chapter 5. The human genome contains transposable elements and other repetitive DNA sequences Figure 3.11 illustrates the composition of the human genome and shows that about half of our DNA is made up of repetitive sequences. A surprisingly large proportion of these derive from sequences

Genome sizes vary between species The size of an organism’s genome (and hence the number and length of its chromosomes) generally, although not invariably, reflects the complexity of the organism. Genome sizes of a range of organisms are shown in Table 3.2. Table 3.2 also indicates the number of protein-coding genes found in the genomes of these organisms. Like genome size, the number of genes an organism contains in its genome generally increases with complexity. However, the genomes of many eukar yotes (including humans) contain much more DNA than can be accounted for by their genes alone. The nature of these intergenic sequences in the human genome is discussed later in this section, but before this we consider two additional points regarding the genes themselves, firstly the question of what actually constitutes a gene, and secondly, the existence of ‘split genes’ in eukaryotes. What is a gene? It is surprisingly hard to define exactly what we mean by a ‘gene’. For many purposes it is sufficient to think of the DNA sequence that encodes a particular protein. However, some genes encode RNA sequences that are not translated into proteins, for example tRNAs and rRNAs (discussed further in Chapter 5). Additionally, DNA sequences that flank coding sequences but are not them selves transcribed into RNA are involved in regulating gene

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