Chapter 3
The Biological Basis
of Life
Chapter Outline
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The Cell
DNA Structure
DNA Replication
Protein Synthesis
Cell Division: Mitosis and Meiosis
New Frontiers
The Cell
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Cells are the basic units of life in all living
organisms.
Complex life forms, such as plants and
animals, are made up of billions of cells.
The cells of all living organisms share many
similarities as a result of their common
evolutionary past.
Cells
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Prokaryotic cells are single celled organisms,
such as bacteria and blue-green algae.
Life on earth can be traced back 3.7 billion
years in the form of prokaryotic cells.
Eukaryotic cells, structurally complex cells,
appeared 1.2 billion years ago.
Structure of a Eukaryotic Cell
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The outer boundary of a cell is the cell
membrane.
Organelles are structures found in the
cytoplasm:
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Mitochondria produce energy.
Ribosomes manufacture protein
The nucleus is surrounded by the cytoplasm
and contains chromosomes.
Two Types of Cells
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Somatic cells are the components of body
tissues.
Gametes are sex cells.
 Ova
are egg cells produced in female ovaries.
 Sperm are sex cells 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
depends on the structure and function of DNA.
DNA is composed of two chains of nucleotides.
A nucleotide consists of a sugar, a phosphate,
and one of four nitrogenous bases.
DNA Structure
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Nucleotides form long chains.
The two chains are held together by bonds
formed on their bases with their complement
on the other chain.
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Adenine (A) is the complement of Thymine(T)
Guanine(G) is the complement of Cytosine(C)
The DNA Replication Process
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Enzymes break the bonds between the DNA
molecule.
Two nucleotide chains serve as templates for
the formation of a new strand of nucleotides.
Unattached nucleotides pair with the
appropriate complementary nucleotide
The DNA Replication Process
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4.
The result is two newly formed strands of
DNA.
Each new strand is joined to one of the
original strands of DNA.
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.
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.
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.
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.
Gene Structure
The gene consists of exons and introns.
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Exons are DNA segments transcribed into
mRNA that code for specific amino acids.
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Introns are DNA sequences not expressed
during protein synthesis.
Universal Genetic Code
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The DNA code of all life on earth is composed
of the same molecules and carries on similar
functions.
The universality of the genetic code implies a
common ancestry for all life on the planet.
Organisms differ according to the arrangement
of the DNA.
Cell Division: Mitosis and Meiosis
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Cell division results in production of new cells.
During cell division:
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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.
– Homologous chromosomes carry genetic
information influencing the same traits.
– Homologous chromosomes 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.
– 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.
Steps in Mitosis
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The 46 chromosomes line up in the center of
the cell.
The chromosomes are pulled apart at the
centromere.
The strands separate and move to opposite
ends of the dividing cell.
The cell membrane pinches in and two new
cells exist.
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.
– 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.
Abnormal Numbers of
Chromosomes
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Down's syndrome occurs because of three
copies of chromosome #21.
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Problems include mental retardation, heart defects
and respiratory infections.
Nondisjunction may occur in the X and Y
chromosomes and result in sterility.