Chapter 3
The Biological Basis of Life
Chapter Outline
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The Cell
DNA Structure
DNA Replication
Protein Synthesis
What is a Gene?
Cell Division: Mitosis and Meiosis
Cells
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Basic units of life.
Complex life forms have billions of cells.
 An adult human= as many as 1,000
billion cells.
Life on earth can be traced back 3.7 b.y.a.
 Prokaryotic cells (i.e. bacteria and bluegreen algae).
 Eukaryotic cells (1.2 billion years ago).
Types of Cells
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Somatic cells are components of body
tissue.
Gametes are sex cells.
 Ova are produced in female ovaries.
 Sperm are produced in male testes.
 A zygote is the union between a
sperm and an ovum.
DNA Structure
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Cellular function and an organism’s inheritance.
DNA is composed of two chains of nucleotides.
A nucleotide consists of a sugar, a phosphate, and one
of four nitrogenous bases.
The two chains are held together by bonds formed on
their bases with their complement on the other chain.
 Adenine (A) is the complement of Thymine(T)
 Guanine(G) is the complement of Cytosine(C)
Proteins
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The major structural components of tissue.
Enzymes are proteins that serve as catalysts,
initiating chemical reactions in the body.
Amino acids are the building blocks of protein.
Proteins differ according to number of amino
acids and the sequence in which they are
arranged.
Protein Synthesis
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Ribosomes help convert the genetic
message from the DNA into proteins.
Messenger RNA (mRNA) carries the
genetic message from the cell nucleus to
the ribosome.
Transfer RNA (tRNA),found in the
cytoplasm, binds to one specific amino
acid.
RNA and DNA
RNA differs from DNA in three important
ways:
1. It’s usually single-stranded.
2. It contains a different type of sugar.
3. It contains the base uracil as a substitute
for the DNA base thymine.
Protein Synthesis:
Transcription
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The process of coding a genetic message
for proteins by formation of mRNA.
A portion of the DNA unwinds and serves
as a template for the formation of a mRNA
strand.
Transcription
Protein Synthesis:
Translation
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The mRNA travels through the nuclear
membrane to the ribosome.
tRNAs arrive at the ribosome carrying their
specific amino acids.
The base triplets on the tRNA match up with the
codons on the mRNA.
As each tRNA line up in the sequence of
mRNA codons their amino acids link to form a
protein.
Assembly of an Amino Acid
Chain in Protein Synthesis
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As the ribosome
binds to the mRNA,
tRNA brings a
particular amino acid,
specified by the
mRNA codon, to the
ribosome.
Assembly of an Amino Acid
Chain in Protein Synthesis
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The tRNA binds to
the first codon while a
second tRNA–amino
acid complex arrives
at the ribosome.
Assembly of an Amino Acid
Chain in Protein Synthesis
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The ribosome moves
down the mRNA,
allowing a third amino
acid to be brought
into position by
another tRNA
molecule. Note that
the first two amino
acids are now joined
together.
Genes
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A gene is the entire sequence of DNA bases
responsible for the synthesis of a protein.
A mutation occurs when the sequence of
bases in a gene is altered.
Mutations may interfere with an organisms
ability to produce vital protein and may lead to a
new variety within the species, hence,
evolution.
Cell Division: Mitosis and
Meiosis
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Cell division results in production of new
cells.
During cell division:
 Cells are involved with normal cellular
and metabolic processes.
 The cell’s DNA becomes tightly coiled.
 DNA is visible under a microscope as
chromosomes.
Chromosome Structure
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A chromosome is composed of a DNA molecule
and associated proteins.
During normal cell functions, chromosomes
exist as single-stranded structures.
During cell division, chromosomes consist of
two strands of DNA joined at the centromere.
Since the DNA molecules have replicated, one
strand of a chromosome is an exact copy of the
other.
Chromosomes and Genetics
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Each species is characterized by a
specific number of chromosomes.
 Humans have 46 chromosomes.
Chromosome pairs are called
homologus:
 These carry genetic information that
influences the same traits.
 They are not genetically identical.
Types of Chromosomes
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Autosomes - govern all physical
characteristics except sex determination.
Sex chromosomes - X and Y
chromosome.
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Mammal females have two X
chromosomes.
Mammal males have one X and
one Y chromosome.
Mitosis
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Mitosis is cell division in somatic cells.
Mitosis occurs during growth and
repair/replacement of tissues.
The result of mitosis is two identical
daughter cells that are genetically
identical to the original cell.
Meiosis
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Production of gametes (sex cells).
2 divisions result in 4 daughter cells.
Each daughter cell contains 23 chromosomes.
Resulting gamete may unite with another
gamete to create a zygote.
The zygote inherits the DNA, half from each
parent, to develop and function normally.
Evolutionary
Significance of Meiosis
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Meiosis and sexual reproduction are
highly important evolutionary innovations.
Meiosis increases genetic variation at a
faster rate than mutation.
Offspring in sexually reproducing species
represent the combination of genetic
information from two parents.
Problems With Meiosis
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Errors in meiosis may lead to miscarriage.
Nondisjunction occurs when chromosomes don’t
separate during meiosis.
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A gamete containing one less chromosome
that fuses with a normal gamete will produce
a zygote containing 45 chromosomes.
A gamete containing one extra chromosome
that fuses with a normal gamete will produce
a zygote containing 47 chromosomes.
Mitosis and Meiosis
Compared
Quick Quiz
1. The entire sequence of DNA bases on
the chromosome that code for a
particular polypeptide chain is a(n)
a) ribosome.
b) amino acid.
c) gene.
d) polypeptide chain.
Answer: c
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The entire sequence of DNA bases on
the chromosome that code for a
particular polypeptide chain is a gene.
2. Which of the following is NOT true about
RNA?
a) It is single stranded.
b) Some forms of RNA are involved with
protein synthesis.
c) It has a different type of sugar than
DNA has.
d) It contains the base thymine.
Answer : d
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The following is NOT true about RNA.
 It contains the base thymine.
3. Which of the following is true for meiosis?
a) It has only one division that duplicates
the parent cell exactly.
b) It produces gametes.
c) When a mutation occurs it affects only
the individual.
d) It has no effect on evolution.
Answer: b
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The following is true for meiosis:
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It produces gametes.
4. Chromosomes that carry genes for the
same traits are _________________
pairs.
Answer: homologous
• Chromosomes that carry genes for the
same traits are homologous pairs.
5. Somatic cell division is termed
_________________ .
Answer: mitosis
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Somatic cell division is termed mitosis.