Biology Now, 2e

Pigs to the Rescue ■ 161

instructions behave the same. In humans, viral

DNA was integrated into the genome a long,

long time ago, and although it remains in the

human genome, it is no longer active. In pigs,

viruses entered and wove their DNA into the pig

Double or Nothing

DNA from viruses is actually present in all

mammalian genomes that we know of, includ-

ing the human genome, yet not all these viral

1

3

5

2

4

6

The chromatin

fiber folds and

loops, further

compacting

itself.

A nucleosome

consists of

DNA wound

around histone

proteins.

The loops coil

and coil until a

chromosome

is formed.

In the beads-

on-a-string

DNA structure,

nucleosomes

are the beads,

and adjoining

lengths of DNA

are the strings.

DNA is a

double-

stranded

nucleic acid.

The beads-

on-a-string

structures coil

around each

other to form a

chromatin fiber.

Figure 9.7

Chromosomes are meticulously organized DNA-protein complexes

The DNA double helix is continuously coiled and packaged around proteins until it is compacted into a

chromosome.

Q1: What are the structures that result from the first level of coiling around proteins called?

Q2: What makes up a “bead” and what makes up the “string” in the beads-on-a-string structure

of DNA?

Q3: What is the name for the structure that is more compact than the beads-on-a-string

structure but less compact than an actual chromosome?