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3 Information: genes and genomes

different-length products depending on the number of repeats present. (PCR is covered in more detail in Chapter 8.) The individual whose DNA is being analysed in Figure 1 has inherited a chromosome containing seven GATA repeats at Locus 1 from one of their parents, and a chromosome containing two repeats at Locus 1 from the other parent. As the locus is highly polymorphic, a different person could have chromosomes containing 2, 3, 4, 5, 6, or 7 GATA repeats at that point. In reality, at least 10 different loci at different positions around the

genome are analysed in any one test, though results from only three loci are shown here. If multiple highly variable loci are analysed, the odds that another individual will have the same number of repeats at all loci as the individual whose DNA is being tested, become vanishingly small. If the DNA has been collected from a crime scene it can be matched with DNA from a suspect, and so long as the profiling has been carried out under strictly controlled conditions it gives evidence of identity that is accepta ble in court.

staining regions known as heterochromatin . Coding regions of the genome, which are potentially transcriptionally active, are generally less compacted, while repetitive DNA sequences in regions of the chromosomes that have no coding function, such as the centromeres and telomeres, remain compacted. We explore the eukaryotic cell cycle in more detail in Chapter 10. The exact degree of packing of transcriptionally active chroma tin is highly dynamic, perhaps alternating between 10 nm and 30 nm fibre structures with short sequences of the DNA becoming transiently free of nucleosomes to allow access by other factors. Chromatin structure and organization is thus of great importance for regulating eukaryotic gene transcription, as discussed further in Chapter 5. 3.3 DNA replication Before a cell divides its DNA must be replicated to provide a dupli cate genome. DNA replication must be extremely rapid but also highly accurate as the base sequence of the genome must be pre served to prevent harmful mutations arising. We will first discuss the basic principles of DNA replication, which are relatively sim ple, even though the details of the process are complex. DNA replication is semi-conservative The discovery of the structure of DNA (Section 3.1) was particu larly satisfying because it immediately suggested how DNA could act as the genetic material that would allow transmission of infor mation from one generation to the next. The specific base pair ing properties of nucleotides could allow one strand of the double helix to act as a template on which a second complementary strand could be built. Experimental evidence soon confirmed this model and showed that both strands of the helix act simultaneously as templates, generating two identical DNA molecules, each of which retains one strand of the original DNA base paired with a newly synthesized strand. This process is termed semi-conservative rep lication and is illustrated in Figure 3.14. Check your understanding of the concepts covered in this section by answering the questions in the e-book.

The DNA genome is packaged with proteins One of the most intriguing problems in biology is that of packaging the long DNA genome into the microscopic cell. For example, the human cell nucleus contains about 2 m of chromosomal DNA, and this is packed into a sphere about 10 μm in diameter. To put this in perspective, if the nucleus were the size of a tennis ball, the genome would be the length of 500 tennis courts! In both prokaryotic and eukaryotic cells, DNA is bound by positively charged proteins that counteract the negatively charged phosphate groups of the DNA, allowing it to be compacted into a small space. DNA in eukaryotic cells exists as chromatin —a DNA–protein complex. The main proteins of chromatin are histones , small pro teins that are rich in the amino acids arginine and/or lysine, giv ing them positive charges. The structure of chromatin is shown in Figure 3.12. Eight histone proteins form an ‘ octamer ’ complex around which are wrapped 146 base pairs of DNA. The octamer and its associated DNA form a unit called a nucleosome , which has the shape of a disc. Short linker sequences of about 30–40 base pairs of DNA join successive nucleosomes, arranged like beads on a string. Histone H1 binds the DNA as it enters and leaves the nucleosome, further condensing the nucleosomes together into a fibre 30 nm in diam eter. Further packaging of chromatin takes place through the for mation of long loops that are attached to a central chromosomal non-histone protein scaffolding. The tightness of DNA packaging changes during the eukar yotic cell cycle. As shown in Figure 3.13, it is especially tight during cell division ( mitosis and meiosis ) when the chromo somes must be separated without tangling. During cell division the looped structure of chromatin is further folded and coiled, achieving a 10,000-fold compaction of the length of the origi nal DNA. During interphase , when cell division is not taking place, chromatin is less tightly packed to allow enzymes and regulatory molecules to access the DNA for transcription and DNA replication. Therefore, mitotic chromosomes are vis ible using the light microscope, while individual chromosomes are not distinguishable in interphase nuclei. However, during interphase, chromatin packing varies in different regions of the genome, with less compact chromatin, which is observed to stain lightly in microscopy studies and is termed euchromatin, con trasting with tightly compacted chromatin identifiable as dark

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