GENE EXPRESSION

AND CONTROL

7.2 The Nature of Genetic Information

DNA carries all the genetic information needed to build a

new individual

Genetic information consists of base sequences

Genes are subunits of that sequence

Gene

Part of a DNA base sequence

Specifies structure of an RNA or protein product

From Gene to RNA to Protein

Gene expression involves transcription (DNA to RNA),

and translation (mRNA, or messenger RNA, to protein)

Gene expression

Process by which the information in a gene becomes converted to

an RNA or protein product

Transcription

A genes nucleotide base sequence encodes instructions for building an RNA or protein product

A cell transcribes the base sequence of a gene into mRNA

mRNA carries a protein-building message

Transcription

Transcription

Process by which an RNA is assembled from nucleotides using the

base sequence of a gene as a template

Messenger RNA (mRNA)

Type of RNA that has a protein-building message

Translation

Translation requires the participation of tRNA (transfer

RNA) and rRNA (ribosomal RNA)

Translation

Process by which a polypeptide chain is assembled from amino

acids in the order specified by an mRNA

RNA and DNA Nucleotides

Fig. 7-2a, p. 117

Fig. 7-2b, p. 117

7.3 Transcription: DNA to RNA

Base-pairing rules in DNA replication also apply to RNA

synthesis in transcription, but RNA uses uracil in place of

thymine

The Process of Transcription

In transcription, RNA polymerase binds to a promoter in

the DNA near a gene

RNA polymerase

Enzyme that carries out transcription

Promoter

In DNA, a sequence to which RNA polymerase binds

The Process of Transcription

Polymerase moves along the DNA, unwinding the DNA so

it can read the base sequence

RNA polymerase assembles a strand of RNA by linking

RNA nucleotides in the order determined by the base

sequence of the gene

The new mRNA is a copy of the gene from which it was

transcribed

mRNA and the Genetic Code

The information in mRNA consists of sets of three

nucleotides (codons) that form words spelled with the four bases A, C, G, and U

Codon

In mRNA, a nucleotide base triplet that codes for an amino acid or

stop signal during translation

mRNA and the Genetic Code

Sixty-four codons, most of which specify amino acids,

constitute the genetic code

20 amino acids in proteins; most have more than one codon

Genetic code

Sixty-four mRNA codons; each specifies an amino acid or a signal

to start or stop translation

The Genetic Code

Translating mRNA to Amino Acids

7.5 Translating the Code: RNA to Protein

Translation, the second part of protein synthesis, occurs

in the cytoplasm of all cells

Translation is an energy-requiring process that converts

the protein-building information carried by an mRNA into a

polypeptide

Three Stages of Translation

Initiation mRNA joins with an initiator tRNA and two ribosomal subunits

Elongation Ribosome joins amino acids delivered by tRNAs in the order

specified by mRNA codons

Termination Polymerase encounters a stop codon; mRNA and polypeptide are

released; ribosome disassembles

ARE YOU GRATEFUL YOU

ARE NORMAL?

7.6 Mutated Genes and Their Products

Mutations are permanent changes in the nucleotide

sequence of DNA, which may alter a gene product

A mutation that changes a genes product may have harmful effects

Example: Mutations that affect the proteins in hemoglobin reduce

bloods ability to carry oxygen

Types of Mutations

Deletion

Mutation in which one or more base pairs are lost

Insertion

Mutation in which one or more base pairs become inserted into

DNA

Base-pair substitution

Type of mutation in which a single base-pair changes

What Causes Mutations?

Most mutations result from unrepaired DNA polymerase

errors during DNA replication

Some result from transposable element activity, or from

exposure to radiation or chemicals

Transposable element

Small segment of DNA that can spontaneously move to a new

location in a chromosome

Ionizing Radiation Damage

Ionizing radiation (x-rays) breaks chromosomes and

produces free radicals

Nonionizing Radiation Damage

Nonionizing radiation (UV light) results in thymine dimers,

which lead to skin cancer

Environmental Damage

Some natural and synthetic chemicals cause mutations in

DNA

Example: Cigarette smoke transfers small hydrocarbon

groups to bases in DNA, causing mispairing during

replication

7.7 Examples of

Eukaryotic Gene Controls

All cells in your body carry the same DNA

Some genes are transcribed by all cells, but most cells

are specialized (differentiated) to use only certain genes

Which genes are expressed at a given time depends on

the type of cell and conditions

Cell Differentiation

Cells differentiate when they start expressing a unique

subset of their genes controls over gene expression are the basis of differentiation

Differentiation

The process by which cells become specialized

Occurs as different cell lineages begin to express different subsets

of their genes

Controlling Gene Expression

Controlling gene expression is critical for normal

development and function of a eukaryotic body

All steps between transcription and delivery of gene

product are regulated

Transcription factor

Protein that influences transcription by binding to DNA

Homeotic Genes

Homeotic gene

Type of master gene that controls formation of specific body parts

during development

Master gene

Gene encoding a product that affects the expression of many other

genes

Controls an intricate task such as eye formation

Homeodomains

All homeotic genes encode transcription factors with a

homeodomain a region of about 60 amino acids that can bind to a promoter or some other DNA sequence

Sex Chromosome Genes

In mammals, males have only one X chromosome females have two, but one is tightly condensed into a Barr

body and inactive

Dosage compensation

Theory that X chromosome inactivation equalizes gene expression

between males and females

X Chromosome Inactivation

Female cells have Barr bodies, male cells do not

The Y Chromosome

The SRY gene, found on the Y chromosome, is the

master gene for male sex determination

Triggers formation of testes

Testosterone produced by testes controls formation of male

secondary traits

Absence of SRY gene in females triggers development of

ovaries, female characteristics

Development of

Human Reproductive Organs

Cancer: Gene Expression Out of

Control

Many gene expression controls regulate cell growth and

division mutations that disrupt normal controls can cause cancer

Cancer

Disease that occurs when a malignant neoplasm physically and

metabolically disrupts body tissues

Tumors

Tumor

Abnormally growing and dividing mass of cells

Metastasis

A process of cancer in which tumor cells lose membrane

recognition proteins, break free, and establish themselves in other

parts of the body

Cancer and Mutations

Cancer begins with a mutation in a gene whose product

controls cell growth and division

A mutation that causes cancer may be inherited or be

caused by environmental agents

Tumors are more likely to occur when mutations occur in

tumor suppressor genes, such as BRCA1 and BRCA2

Fig. 7-15b, p. 130