Chapter 10.1
This diagram shows how DNA is packaged into highly condensed metaphase chromosomes. First, DNA is wrapped around histone proteins to form nucleosomes.
Then, the nucleosomes are compacted into chromatin fibers, which are coiled into looped domains. The looped domains are compacted, ultimately forming chromosomes.
How is the large discrepancy between DNA length and nucleus size addressed in eukaryotic cells?
DNA found within a nucleus is very long compared to the diameter of the nucleus. In order to address the large differences between chromosome length and nuclear dimensions, DNA molecules are packaged in a highly organized way in chromosomes.
A typical eukaryotic cell contains much more DNA than a bacterium does, and it is organized in the nucleus as multiple chromosomes that vary widely in size and number among different species. Although a human nucleus is about the size of a large bacterial cell, it contains more than 1000 times the amount of DNA found in E. coli.
The DNA content of a human sperm cell is about base pairs; stretched end to end, it would measure almost 1 m long. Remarkably, this DNA fits into a nucleus with a diameter of only 10 μm.
When a cell prepares to divide,
its chromosomes become thicker and shorter as their long chromatin fibers are compacted.
An organism may have thousands of genes. For example, humans have more than 20,000 genes that code for proteins. The concept of the gene has changed considerably since the science of genetics began, but our traditional definitions have always centered on the gene as an informational unit.
By providing information needed to carry out one or more specific cell functions, a gene affects some characteristic of the organism. For example, genes govern eye color in humans, wing length in flies, and seed color in peas. As you will learn in later chapters, these concepts are being expanded as scientists study the many ways information stored in DNA controls the workings of the cell.
How does a eukaryotic cell pack its DNA into the chromosomes?
Chromosome packaging is facilitated by certain proteins known as histones.Footnote Histones have a positive charge because they have a high proportion of amino acids with basic side chains (see Chapter 3). The positively charged histones associate with DNA, which has a negative charge because of its phosphate groups, to form structures called nucleosomes. The fundamental unit of each nucleosome consists of a beadlike structure with 146 base pairs of DNA wrapped around a disc-shaped core of eight histone molecules (two each of four different histone types) (Fig. 10-2). Although the nucleosome was originally defined as a bead plus a DNA segment that links it to an adjacent bead, today the term more commonly refers only to the bead itself (i.e., the eight histones and the DNA wrapped around them).
Chromosomes are made of chromatin, a material consisting of DNA and associated proteins. When a cell is not dividing, the chromosomes are present but in an extended, partially unraveled form. Chromatin consists of long, thin threads that are somewhat aggregated, giving them a granular appearance when viewed with the electron microscope (see Fig. 4-11a).
During cell division, the chromatin fibers condense and the chromosomes become visible as distinct structures (Fig. 10-1).
For which of the states of chromatin illustrated is histone H1 most responsible?
Histone H1 is involved in the packing of 10 nm nucleosomes, forming 30 nm packed nucleosome fibers.
Nucleosomes function like tiny spools, preventing DNA from becoming tangled. You can see the importance of this role in Figure 10-3, which illustrates the enormous length of DNA that unravels from a mouse chromosome after researchers have removed the histones. The role of histones is more than simply structural because their arrangement also affects the activity of the DNA with which they are associated.
Histones are increasingly viewed as an important part of the regulation of gene expression, that is, whether genes are turned off or on. We discuss gene regulation by histones in Chapter 14.
Prokaryotic and eukaryotic cells differ markedly in their DNA content as well as in the organization of DNA molecules. The bacterium Escherichia coli normally contains about base pairs (almost 1.35 mm) of DNA in its single, circular DNA molecule.
In fact, the total length of its DNA is about 1000 times as long as the length of the cell itself. Therefore, the DNA molecule is, with the help of proteins, twisted and folded compactly to fit inside the bacterial cell (see Fig. 25-2).
The major carriers of genetic information in eukaryotes are the chromosomes, which lie within the cell nucleus. Although chromosome means "colored body," chromosomes are virtually colorless; the term refers to their ability to be stained by certain dyes.
In the 1880s, light microscopes had been improved to the point that scientists such as German biologist Walther Fleming began to observe chromosomes during cell division. In 1903, American biologist Walter Sutton and German biologist Theodor Boveri noted independently that chromosomes were the physical carriers of genes, the genetic factors Gregor Mendel discovered in the 19th century (discussed in Chapter 11).
What are the informational units on chromosomes called? Of what do these informational units consist?
Informational units on chromosomes are called genes. Genes consist of sequences within DNA molecules that encode information needed to carry out one or more specific cell functions.
Other species have different chromosome numbers. A certain species of roundworm has only 2 chromosomes in each cell, whereas some crabs have as many as 200, and some ferns have more than 1000.
Most animal and plant species have between 8 and 50 chromosomes per somatic cell. Quantities above and below these numbers are uncommon. The number of chromosomes a species has does not indicate the species' complexity or its status within a particular domain or kingdom.
Every individual of a given species has a characteristic number of chromosomes in the nuclei of its somatic (body) cells. However, it is not the number of chromosomes that makes each species unique but the information the genes specify.
Most human somatic cells have exactly 46 chromosomes, but humans are not humans merely because we have 46 chromosomes. Some other species—the olive tree, for example—also have 46. Some humans have an abnormal chromosome composition with more or fewer than 46 (see Fig. 16-5).
The wrapping of DNA into nucleosomes represents the first level of chromosome structure. Figure 10-4 shows the higher-order structures of chromatin leading to the formation of a condensed chromosome. The nucleosomes themselves are 10 nm in diameter. The packed nucleosome state occurs when a fifth type of histone, known as histone H1, associates with the linker DNA, packing adjacent nucleosomes together to form a compacted 30 nm chromatin fiber. In extended chromatin these fibers form large, coiled loops held together by scaffolding proteins, nonhistone proteins that help maintain chromosome structure.
The loops then interact to form the condensed chromatin found in a chromosome. Cell biologists have identified a group of proteins, collectively called condensin, required for chromosome compaction. Condensin binds to DNA and wraps it into coiled loops that are compacted into a chromosome.