Chapter 12 DNA and RNA
To understand genetics, biologists had to learn the chemical
makeup of the gene. Scientists discovered that genes are made of
DNA. Scientists also found that DNA stores and transmits the
genetic information from one generation of an organism to the
next. Scientists began studying DNA structure to find out how it
carries information, decides traits, and replicates itself.
• DNAis a long molecule made up of units called nucleotides.
Each nucleotide is made up of a 5-carbon sugar, a phosphate
group, and a nitrogen-containing base.
• There are four kinds of bases: adenine (A), guanine (G),
cytosine (C), and thymine (T).
Watson and Crick made a three-dimensional model of DNA.
Their model was a double helix, in which two strands were
wound around each other. A double helix is like a twisted ladder.
Sugar and phosphates make up the sides of the ladder. Hydrogen
bonds between the bases hold the strands together. Bonds form
only between certain base pairs: between adenine and thymine,
and between guanine and cytosine. This is called base pairing.

12–2 Chromosomes and DNA Replication
Most prokaryotes have one large DNA molecule in their
cytoplasm. Eukaryotes have DNA in chromosomes in their
nuclei. Before a cell divides, it copies its DNA in a process called
replication. During DNA replication,
• the DNA molecule separates into two strands. Each strand
of the DNA molecule serves as a model for the new strand.
• Following the rules of base pairing, new bases are added to
each strand. For example, if the base on the original strand is
adenine, thymine is added to the newly forming strand.
Likewise, cytosine is always added to guanine.
• The end result is two identical strands.
Summary

12–3 RNA and Protein Synthesis
For a gene to work, the genetic instructions in the DNA molecule
must be decoded. The first step is to copy the DNA sequence into
RNA. RNA is a molecule which contains instructions for making
proteins. RNA is similar to DNA, except for three differences:
• The sugar in RNA is ribose instead of deoxyribose.
• RNA is single-stranded.
• RNA has uracil in place of thymine.
Most RNA molecules are involved in making proteins. There
are three main kinds of RNA:
• Messenger RNA has the instructions for joining amino acids
to make a protein.
• Proteins are assembled on ribosomes. Ribosomes are made
up of proteins and ribosomal RNA.
• Transfer RNA carries each amino acid to the ribosome
according to the coded message in messenger RNA.
RNA is copied from DNA in a process called transcription.
During transcription:
• The enzyme RNA polymerase binds to DNA and separates
the two DNA strands.
• RNA polymerase builds a strand of RNA using one strand
of DNA as the template.
• The DNA is transcribed into RNA following base-pairing
rules except that uracil binds to adenine.
The directions for making proteins are in the order of the four
nitrogenous bases. This code is read three letters at a time. Each
codon, or group of three nucleotides, stands for an amino acid.
Some amino acids are specified by more than one codon. One
codon is a start signal for translation. Three codons signal the end
of a protein.
Translation is the process in which the cell uses information
from messenger RNA to make proteins. Translation takes place
on ribosomes.
• Before translation can begin, messenger RNA is transcribed
from DNA.
• The messenger RNA moves into the cytoplasm and attaches
to a ribosome.
• As each codon of the messenger RNA moves through the
ribosome, the proper amino acid is brought into the ribosome
by transfer RNA. The ribosome joins together each
amino acid. In this way, the protein chain grows.
• When the ribosome reaches a stop codon, it releases the
newly formed polypeptide and the process of translation
is complete.

12–4 Mutations
Mutations are mistakes made when cells copy their own DNA.
Mutations are changes in the genetic material of a cell.
• Gene mutations are changes in a single gene. A point
mutation occurs at a single point in the DNA sequence
of a gene. When a point mutation causes one base to replace
another, only one amino acid is affected. If a nucleotide is
added or removed, it causes a frameshift mutation. All the
groupings of codons are changed. This can cause the gene to
make a completely different protein.
• In a chromosome mutation, there is a change in the number
or the structure of chromosomes. There are four kinds of
chromosomal mutations: deletions, duplications, inversions,
and translocations.

12–5 Gene Regulation
Genes can be turned on and off as different proteins are needed. In
prokaryotes, some genes are turned on and off by a chromosome
section called an operon. An operon is a group of genes that work,
or operate, together. In bacteria, one operon controls whether the
organism can use the sugar lactose as food. It is called the lac
operon. The lac genes are turned off by repressors and turned on
by the presence of lactose. Operators and promoters are DNA
sequences in the operon that control when genes are turned on
and off.
• When the cell needs a certain protein, RNA polymerase
attaches to the promoter and makes a messenger RNA that is
translated into the needed protein.
• When the cell no longer needs the protein, it makes another
protein called the repressor. The repressor attaches to the
operator. This blocks the promoter so RNA polymerase cannot
attach to it. This turns the genes of the operon off.
Most eukaryotic genes are controlled individually and have
regulatory sequences that are much more complex than those
of the lac operon. In eukaryotes, genes are regulated by enhancer
sequences located before the point at which transcription begins.
Some proteins can bind directly to these DNA sequences. Ways in
which these proteins affect transcription include:
• increasing the transcription of certain genes
• attracting RNA polymerase
• blocking access to genes
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