Biology 12

246 MHR • Unit 3 Molecular Genetics

hundreds of thousands of copies of the same or

very similar genes. These families may be clustered

together on the same chromosome, or they may be

distributed among a number of different

chromosomes. In some cases, multi-gene families

code for products that are in very high demand in

the cell. (Examples include the genes that code for

histone proteins and RNA products.) In other cases,

however, the genome contains only a single copy of

a gene even though it codes for a product in high

demand. The existence of multi-gene families is

partly the result of the action of transposonsor

jumping genes, which are sequences of DNA that

are inserted and copied randomly throughout the

cell genome. You will examine the effect of

transposons more closely in Chapter 9.

Pseudogenes

In some multi-gene families, some copies of the

gene may have mutated to the point that they no

longer work as genes. These sequences are known

as pseudogenes, since they are nearly identical to

functional genes but are never expressed during

the life cycle of the cell. It is not currently known

whether such pseudogenes have a metabolic

function.

Repetitive Sequences

Along with introns and multi-gene families,

eukaryotic cells also contain long strands of

repetitive sequence DNA. These regions contain

short sequences of nucleotides repeated thousands

or millions of times. About 30 percent of mouse

DNA consists of repetitive sequences of about

10 nucleotides. While such structures do not have

a coding function, they can play an important role

in processes such as DNA replication. Telomeres

are an example of such a repetitive sequence. A

similar repetitive sequence occurs on chromosomes

where the centromere forms, suggesting that this

sequence helps in the attachment of spindles

during cell division.

Genome Size Versus

Organism Complexity

It was once thought that the human genome

contained approximately 100 000 genes. This figure

was based partly on the estimated number of

proteins produced by human cells, as well as on

estimates of the ratio of genes to the length of the

overall genome. The Human Genome Project,

however, found that the actual figure is much

smaller — about 30 000 to 35 000. This finding

indicates that the complexity of mammals is not

only the result of information stored in the genes

themselves. In fact, a wide range of different

mechanisms affect when and how genes are

translated into their protein products. The structures

and processes involved in moving from the

molecular sequence of DNA to functional proteins

in an organism are explored in more detail in

Chapter 8.

SECTION REVIEW

1. Explain the one gene-one enzyme theory
   proposed by Beadle and Tatum. Why was this theory
   later modified first to one gene-one protein and then
   to one gene-one polypeptide?


2. Describe the arrangement of genes in the
   genome of organisms. Account for the large amount
   of DNA that does not code for genetic or regulatory
   sequences.


3. Explain the differences in the number of introns
   and exons in organisms of differing complexity. What
   is the significance of introns in protein production?


4. What is a pseudogene?


5. In small groups, debate whether eukaryotic cells
   enjoy selective advantages because they contain long
   stretches of repetitive DNA sequences. If so, what
   might these advantages be?
   6. How might humans be affected if our genome
   really did contain 100 000 genes? What if it
   contained only 10 000 genes? Evaluate some of the
   possibilities, and give reasons for your conclusions.
   7. Do research to discover some of the nucleotide
   sequences that code for specific characteristics in
   humans or other eukaryotic organisms. Using everyday
   materials, build one or more standing models of the
   DNA double helix, each incorporating one of these
   sequences on one strand and its corresponding
   sequence on the other strand. Do the corresponding
   sequences also encode useful information? Explain
   your answer in a summary paper.

Find out what, if any, gene is associated with the cancer

you are studying. Try to find this gene on a map of the

human genome.

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