Download The Biological Basis of Life

Necessities of Life
The Biological Basis of Life:
• Reproduction so that life can continue
generation after generation
– Facilitated by molecules from the chemical
family of Nucleic Acids
Molecules and Cells
• Maintenance of essential physiological
functions of life (metabolism)
– Facilitated by molecules from the chemical
family known as Proteins
Reproduction: DNA
• Reproduction is based on the accurate
duplication of DNA
• The precise and accurate duplication of a
DNA molecule, known as Replication, is
made possible by the chemical structure of
the DNA molecule
DNA Structure
• Ladder or lattice-like molecule composed of
smaller building blocks: Nucleotides
• Nucleotides are composed of three subunits:
– Sugar (deoxyribose for DNA)
– Phosphate
– Nitrogenous base--the key to the functioning of
DNA
• Purines (2 N/C rings): Adenine, Guanine
• Pyrimidines (1 N/C ring): Thymine, Cytosine
DNA
• The sides of the DNA “ladder” are
composed of bonded sugar and phosphate,
the “rungs” are based on hydrogen bonds
between complementary nitrogenous bases
• The key to DNA function is the
complementarity of the bases: Adenine
bonds only with Thymine and Guanine only
with Cytosine.
1
Replication
Maintenance
• Life functions are regulated by proteins:
– Structural proteins like the muscle cell building
block, myosin
– Regulating proteins, including enzymes and
hormones, like the enzyme that breaks down
starch molecules, salivary amylase
– Transporter proteins like the oxygen
transporter, hemoglobin
Protein Structure and Function
• Function of different proteins is based on
structure
• Structure is determined by the number and
type of building blocks, called Amino Acids
• Amino acids are assembled into chains called
polypeptides
• A functional protein may include several
polypeptides
Protein Synthesis
• The sequence of amino acids in a
polypeptide is determined by the sequence
of nitrogenous bases in the DNA unit (or
gene) coding for that polypeptide.
• Protein synthesis is a two-step process:
– Transcription: copying the DNA to RNA
– Translation: using the RNA to assemble the
polypeptide
2
Hemoglobin
How does this work in Humans?
• The Hemoglobin molecule is a complex
protein structure that carries oxygen and
carbon dioxide through the blood stream
– It consists of four polypeptides: 2 alpha and 2
beta chains
– Each of these polypeptides has a separate
section of DNA carrying the code for the
appropriate sequence of amino acids
• Each alpha chain consists of 141 Amino Acids,
requiring a sequence of 423 nucleotides in the DNA
• Each beta chain consists of 146 Amino Acids,
requiring a sequence of 438 nucleotides in the DNA
Mutation
Sickle Cell Anemia
• Even small changes to the sequence of
nucleotides in the DNA can have significant
repercussions in terms of protein structure
and function
• Changes can involve single nucleotides or
large groups of nucleotides
• The result of mutation is determined by
what it does to the protein structure
• Sickle cell anemia is a genetic condition
caused by a point mutation: the change in
one nucleotide within the sequence of 438
bases coding for the hemoglobin beta chain
• The shift in the 17th nucleotide from a
Thymine base to an Adenine base causes a
shift in the 6th amino acid from glutamic
acid to valine
Sickle Cell Anemia
Point
Mutation
• The change of one amino acid results in
hemoglobin that has a tendency to clump
together and destroy the Red Blood Cells
that hold the molecules
• This produces a life-threatening disease that
has only come under some control by
modern medicine in the last few decades
3
Chromosomes
Principles of Inheritance
How Traits are Transmitted within
Families
• Chromosomes are the complex DNA and
Protein units that carry the genetic code in
all cells with nuclei
• In sexually-reproducing organisms,
chromosomes come in homologous pairs
– Each member of the pair contains information
on how to build the same protein products
– One member of each pair comes from the
mother and one comes from the father
Human Karyotype
Karyotype
• A Karyotype is a photomicrograph of the
chromosomal complement of an individual
• The chromosomes are arranged according to
size, and numbered, with the first pair being
the largest chromosomes and the twentysecond pair being the smallest in humans,
except for the Y (male-determining)
chromosome
Human Genome
• Humans have 23 pairs of chromosomes
– 22 pairs of autosomal chromosomes affecting
almost all aspects of the individual other than
sex
– 1 set of sex chromosomes
• A pair of X chromosomes for Females
• One X and one Y chromosome for Males
• Approximately 30,000 genetic loci on the
23 pairs of chromosomes
Locus
• The position of a gene on an homologous
chromosome pair is known as a Locus
– The locus of the beta gene for the Hemoglobin
molecule is near the tip of the short arm of
chromosome number 11
– The locus of the alpha gene is near the tip of the
short arm of chromosome number 16
4
Alleles
• Many genes have different forms
• We discussed two forms of the gene for
Hemoglobin, the normal form called type
A, and the mutant variety that results in
sickle cell, type S
• These variants of a particular gene are
called Alleles
Genotype vs. Phenotype
• Genotype is the genetic makeup of an
individual
– This usually refers to what alleles an individual
has at a specific locus
• e.g., at the ABO locus, one A allele, one O allele
• Phenotype is the observable expression of
the genotype
– The phenotype for the above genotype would
be Blood Type A.
Homozygous
• If an individual has two of the same alleles
at a particular locus, he is said to be
homozygous (or is a homozygote)
– A person is homozygous if he inherits a
Hemoglobin S allele from both his mother and
father
– Genotype: HbS/ Hb S
– Phenotype: Sickle Cell Anemia
Dominant and Recessive
• Alleles are said to be dominant or recessive
depending upon whether they are expressed
(dominant) or hidden (recessive) in
heterozygotes
– In the ABO system, A and B alleles are
dominant over O, and co-dominant with each
other (Blood type AB)
– O is recessive to both A and B
Heterozygous
• If an individual has two different alleles at a
particular locus, he is said to be
heterozygous (or is a heterozygote)
– A person is heterozygous if he inherits a
Hemoglobin S allele from his mother and a
Hemoglobin A allele from his father
– Genotype: Hb S/ Hb A
– Phenotype: Sickle Cell Trait (Carrier)
SO HOW DOES IT WORK?
GAMETES
Mother
A
Mother
O
Father
A
Father
O
Blood Type A
AA
3out of 4=75%
AO
AO
Blood Type O
1 out OO
of 4=25%
5
Independent Assortment
• The ABO locus is on chromosome 9, the
Rhesus (Rh) locus in on chromosome 1
• These two factors are transmitted
independent of one another
– Phenotype (Blood type): A+
– Genotype: A/O +/– Gametes: A/+ A/- O/+ O/- in equal numbers
GAMETES
Father
A+
Father
A-
Father
O+
Father
O-
Mother
A+
AA++
AA+-
AO++
AO+-
Mother
A-
AA+-
AA--
AO+-
AO--
Mother
O+
AO++
AO+-
OO++
OO+-
Mother
O-
A+
• If two different genes have loci on the same
homologous chromosome pair, they are said
to be Linked
– The locus for insulin and the locus for tissue
compatibility (affects transplant rejections) are
both found on the sixth chromosome pair in
man
– These two genes are linked
A-
O+
AO+-
Genetic Linkage
AO--
OO+-
O-
OO--
Autosomal Recessive: Phenylketonuria
Autosomal Dominant: Huntington’s Disease
Sex-Linked Recessive: Hemophilia A
6