HUMAN BIOLOGY

422 Chapter 21

expLoreon your own

In some parts of the world about 140 million children under the age of 6 suf-

fer blindness and other serious health problems due to a vitamin A deficiency.

Eating 1 cup a day of genetically modified, or GM, Golden Rice (right) can correct the deficiency.

This GM rice has been engineered to contain beta carotene, a pigment formed in some plants

that is a precursor for the formation of vitamin A. Rice is a cheap staple food in many regions

where poor people suffer the most from vitamin A deficiency. For them, Golden Rice might make

the difference between relative good health and a life of misery. Even so, opponents of GM food

crops have campaigned strongly against allowing human consumption of Golden Rice.

To explore this topic further, do some Web research on GM foods. Do you buy products that

are likely to contain material from genetically modified plants? If you live in an agricultural area,

do farmers there plant GM crops?

summary

section 21.1 A gene is a sequence of

nucleotide bases in DNA. These bases are

adenine, thymine, guanine, and cytosine (A,

T, G, and C). The nucleotide sequence of

most genes codes for the sequence of amino

acids in a protein (polypeptide chain).

section 21.2 A DNA molecule consists of two

strands of nucleotides twisted together in a double

helix. DNA is copied (duplicated) by semiconservative

replication. One strand in each new DNA molecule

is new and one is from the parent molecule. DNA

polymerases and other enzymes unwind the existing

DNA molecule, keep the strands apart, and assemble a

new strand on each one.

A gene mutation is a change in the DNA nucleotide

sequence. Mutations may be harmful, neutral, or helpful.

Mutations in germ cells (cells that produce gametes) can be

passed to the next generation.

section 21.3 Converting genetic

information into a protein requires steps

called transcription and translation,

which are summarized in Figure 21.24. In

transcription, DNA instructions guide the formation of RNA

from nucleotides in the cell. The double-stranded DNA is

unwound at a gene region, then RNA polymerases use the

exposed bases as a template to build a corresponding strand of

RNA. Base-pairing rules govern which bases pair up. In RNA,

guanine pairs with cytosine, and uracil (not thymine) pairs

with adenine.

DNA encodes three kinds of RNA molecules. Messenger

RNA (mRNA) carries instructions for building proteins.

Ribosomal RNA (rRNA) forms the subunits of ribosomes,

the structures on which amino acids are assembled into

polypeptide chains. Different kinds of transfer RNA (tRNA)

pick up amino acids and deliver them to ribosomes in the

order specified by mRNAs.

A new mRNA transcript consists of introns (nucleotide

sequences that do not code for proteins) and exons. Exons are

the mRNA sequences that carry protein-building instructions.

Regulatory proteins stimulate or suppress gene transcription

and so control gene activity.

section 21.4 In translation, RNAs link

amino acids in the sequence required to produce

a specific polypeptide chain.

Translation follows the genetic code, a set of

sixty-four base triplets—nucleotide bases that

ribosome proteins “read” three at a time.

Golden Rice

© Rice Humanitarian Board

UAU

UAC

UAA

UAG

tyr

stop

stop

GCAU

CGTA

pre-mRNA

mRNA

DNA

Transcription

mRNA processing

Polypeptide

Translation

Cell

Cytoplasm

Nucleus

Figure 21.24 This diagram is a summary of transcription

and translation, the two steps leading to protein synthesis

in cells. DNA is transcribed into RNA in the nucleus. RNA is

translated into proteins in the cytoplasm. (© Cengage Learning)

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