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CHAPTER OUTLINE
25.1 DNA Structure
Deoxyribonucleic acid (DNA) is the genetic material of life, it is able to store information that
pertains to the development, structure, and metabolic activities of the cell or organism and is
stable so that it can be replicated with high accuracy during cell division and be transmitted from
generation to generation.
The Nature of the Genetic Material
Experiments by Griffith in the late 1920s led investigators to look for the transforming
substance in bacteria to determine the chemical nature of the genetic material. By the
1940s scientists recognized that genes are on chromosomes and that chromosomes
contain both proteins and nucleic acids. In 1944 Avery, MacLeod and McCarty
published a paper concluding that DNA is the genetic material based on their
experiments. An experiment by Hershey and Chase in the early 1950s firmly established
DNA as the genetic material.
Structure of DNA
The structure of DNA was determined by Watson and Crick in the early 1950s. DNA is a
chain of nucleotides, two strands of alternating phosphate and sugar molecules form a
double helix, which are held together by hydrogen bonding.
25.2 DNA Replication
The process of copying one DNA double helix into two identical double helices is called DNA
replication. DNA replication is termed semiconservative because a new double helix has one
conserved old strand and one new strand.
Messenger RNA
DNA serves as the template for messenger RNA (mRNA), which carries the information
from DNA to the ribosomes in the cytoplasm.
Transfer RNA
Transfer RNA (tRNA) transfers amino acids to the ribosomes to form proteins.
Ribosomal RNA
Ribosomal RNA (rRNA) joins with proteins made in the cytoplasm to form the subunits
of the ribosomes.
25.3 Gene Expression
The process of using a gene sequence to synthesize a protein is called gene expression. Gene
expression relies on several different forms of ribonucelic acid (RNA) and requires two
processes called transcription and translation.
Transcription
During transcription, a segment of the DNA called a gene serves as a template for the
production of an RNA molecule.
Messenger RNA
The purpose of messenger RNA (mRNA) is to carry genetic information from
the DNA to the ribosomes for protein synthesis. Transcription begins when the
enzyme RNA polymerase binds tightly to a promoter. The RNA polymerase
joins the RNA nucleotides to form an mRNA molecule.
Processing of mRNA
After the mRNA is transcribed in eukaryotic cells, it must be processed
before entering the cytoplasm. Introns are removed and the exons are
joined to form a mature mRNA molecule consisting of continuous exons.
Translation
Translation is the second process by which gene expression leads to protein synthesis.
The Genetic Code
The genetic code is a triplet code and each triplet of nucleotides is called a
codon. The genetic code is just about universal in all living organisms.
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Transfer RNA
Transfer RNA (tRNA) molecules bring amino acids to the ribosomes, the site of
protein synthesis. Each tRNA contains an amino acid on one end and an
anticodon complementary to a codon of mRNA.
Ribosomes and Ribosomal RNA
Ribosomes are composed of many proteins and several risbosomal RNAs
(rRNAs). A ribosome moves down the mRNA molecule producing a protein.
Translation Requires Three Steps
The codons of an mRNA base pair with the anticodons of tRNA molecules
carrying specific amino acids. The process of translation must be extremely
orderly so the amino acids are sequenced correctly.
Initiation
Initiation is the step that brings all the translation components together.
Elongation
Elongation is the protein synthesis step in which a polypeptide increases in
length one amino acid at a time.
Termination
During termination, the polypeptide and the assembled components that carried
out protein synthesis are separated from one another.
Review of Gene Expression
A gene is expressed when its protein product has been synthesized, which requires the
process of transcription and translation.
25.4 Control of Gene Expression
Only certain genes are active in cells that perform specialized functions, such as nerve, muscle,
gland, and blood cells. The activity of selected genes accounts for the specialization of cells.
Gene expression is controlled in a cell, and this control accounts for its specialization.
Control of Gene Expression in Prokaryotes
An operon is a cluster of genes usually coding for proteins related to a particular
metabolic pathway, along with the short DNA sequences that coordinately control their
transcription. The parts of an operon include a repressor and a regulator gene as well as
the structural genes.
Gene Expression in Eukaryotes
In contrast to the prokaryotes, eukaryotes employ a variety of mechanisms to regulate
gene expression.
Levels of Gene Control
Eukaryotic genes exhibit control of gene expression at five different levels.
Pretranscriptional Control
Eukaryotes use DNA methylation and chromatin packing as a way to
keep genes turned off.
Transcriptional Control
Transcriptional control is dependent on the interaction of proteins with
particular DNA sequences. Transcription factors are proteins that help
RNA polymerase bind to a promoter.
Posttranscriptional Control
The speed of transport of mRNA from the nucleus into the cytoplasm can
ultimately affect the amount of gene product following transcription.
Translational Control
The longer an mRNA remains in the cytoplasm before it is broken down,
the more gene product can be translated.
Posttranslational Control
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Some proteins are not active immediately after synthesis. Folding and
other modifications can affect the activity of a protein.
25.5 Gene Mutations
A gene mutation is a permanent change in the sequence of bases in DNA. Gene mutations can
lead to malfunctioning proteins in cells.
Causes of Mutations
Gene mutations may be caused by errors in replication, mutagens, and the activity of
transposons.
Errors in Replication
DNA replication errors are a rare source of mutations because DNA polymerase
proofreads the new strand.
Mutagens
Environmental influences called mutagens, which include radiation and certain
organic chemicals, cause mutations in humans.
Transposons
Transposons are specific DNA sequences that have the ability to move
within and between chromosomes, which can alter the expression of neighboring
genes.
Effects of Mutations on Protein Activity
Point mutations involve a change in a single DNA nucleotide, resulting in a possible
change in a specific amino acid. Frameshift mutations because one or more nucleotides
are either inserted or deleted from DNA, the result can be a completely new sequence of
codons and nonfunctional protein.
Nonfunctional Proteins
A single nonfunctioning protein can have a dramatic effect on phenotype,
because enzymes are often a part of metabolic pathways.
Mutations Can Cause Cancer
The development of cancer involves a series of accumulating mutations that can be
different for each type of cancer. Mutations in tumor suppressor or proto-oncogenes often
lead to cancer.
Characteristics of Cancer Cells
Cancer cells are genetically unstable, they do not correctly regulate the cell cycle, they
escape the signals for cell death and can survive and proliferate elsewhere in the body.
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