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DNAStructur
e
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DNA
Deoxyribonucleic acid
Hereditary material passed from parent to child
In the nucleus in one of two forms: Chromatin – loose molecule with DNA and
proteins. Chromosomes – compact, X-shaped
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DNA Structure
Made of two long chains that are arranged into a ladder-like structure called a Double Helix.
A molecule of DNA is made up of millions of tiny subunits called Nucleotides.
Each nucleotide consists of:1. Phosphate group2. Pentose sugar3. Nitrogenous base
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Nucleotide Structure
Phosphate
Pentose
Sugar
Nitrogenous
Base
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Arrangement of sugar-phosphate backbone
The phosphate and sugar form the backbone
The bases form the “rungs”
There are four types of nitrogenous bases.
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Arrangement of the bases
Each base will only bond with one other specific base.
Adenine (A) Thymine (T)
Cytosine (C) Guanine (G)
Form a base pair.
Form a base pair.
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Complementary base pairing and DNA Replication
Because of complementary (matching) base pairing the order of the bases in one strand
determine the order of the bases in the other strand.
DNA is able to make accurate copies of itself
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If D.N.A. could not copy itself, how could new cells ever be made?
A
C
T
G
C
A
T
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Genes
A gene is a section of DNA that codes for a protein
Each gene has a unique sequence of bases.
In each cell, only specific genes are “read” to produce certain proteins
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Chromosomes Compact and X-shaped (formed when
chromatin compacts)
Formed when chromatin coils into compact X-shaped structures
Different organisms have different number of chromosomes Ex: Humans 46, butterflies 80, cows 60
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Human Chromosomes Humans have 46
chromosomes Chromosomes occur in
pairs Each parent contributes
23 chromosomes The 23rd pair is the sex
chromosomes: XX: female XY: male
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Homologous Chromosomes
Chromosomes of the same type are said to be homologous chromosomes They have the same length They have genes controlling
the same trait at the same position
One came from the father & the other from the mother
When stained, they show similar banding patterns
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Homologous Pairs ofChromosomes
The precise location where the gene is found on the chromosome is referred to as the locus
Many genes exist in several variant forms (alleles), but only 2 forms are present in an organism 2 identical alleles for a specific gene on both
homologs (homozygous for the trait) 2 different alleles for a specific gene on both
homologs (heterozygous for the trait) A dominant allele is expressed phenotypically
in the heterozygote and the homozygote A recessive allele is only expressed
phenotypically in the homozygote
Dominant vs Recessive Alleles
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Lack of pigment deposition in the human body is an abnormal recessive trait called ''albinism.''
Using A to represent the dominant (pigment-producing) allele and a to represent the recessive (albino) allele, we can describe the three genotypes and two phenotypes that are possible:
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Punnett Square
Table listing all possible genotypes resulting from a cross
All possible sperm genotypes are lined up on one side
All possible egg genotypes are lined up on the other side
Every possible zygote genotypes are placed within the squares
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Punnett Square ShowingEarlobe Inheritance Patterns
Sickle cell frequency
High frequency of heterozygotes 1 in 5 in Central Africans = HbHs
unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs
usually die before reproductive age
Why is the Hs allele maintained at such high levels in African populations?Why is the Hs allele maintained at such high levels in African populations?
Suggests some selective advantage of being heterozygous…Suggests some selective advantage of being heterozygous…
Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
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Heterozygote Advantage In tropical Africa, where malaria is common:
homozygous dominant (normal) die of malaria: HbHb
homozygous recessive die of sickle cell anemia: HsHs
heterozygote carriers are relatively free of both: HbHs
survive more, more common in populationHypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.
Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.
Frequency of sickle cell allele & distribution of malaria
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Human Genetic Disorders
Autosome - Any chromosome other than a sex chromosome
Genetic disorders caused by genes on autosomes are called autosomal disorders Some genetic disorders are autosomal dominant
An individual with AA has the disorder An individual with Aa has the disorder An individual with aa does NOT have disorder
Other genetic disorders are autosomal recessive An individual with AA does NOT have disorder An individual with Aa does NOT have disorder, but is a
carrier An individual with aa DOES have the disorder
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Autosomal Recessive Pedigree Chart
Look for: Skips in
generation Unaffected
parents can have affected children
Affected person must be homozygous
Males and females affected equally
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Autosomal Dominant Pedigree Chart
Look for: Trait in every
generation Once leaves the
pedigree does not return
Every person with the trait must have a parent with the trait
Males and females equally affected
Incomplete dominance
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Co-dominance
2 alleles are present in the genotype and both are fully observed in the phenotype
ABO blood types
The alleles: IA = A antigen on red cells, anti-B antibody in plasma IB = B antigen on red cells, anti-A antibody in plasma I = Neither A nor B antigens, both anti-A & anti-B antibodies
Phenotype(Blood Type) Genotype
A (actually AA or AO) IAIA or IAiB (actually BB or BO) IBIB or IBi
AB IAIB
O (actually OO) ii
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Inheritance of Blood Type
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Polygenic Inheritance
Occurs when a trait is governed by two or more genes having different alleles
Each dominant allele has a quantitative effect on the phenotype
These effects are additive
Result in continuous variation of phenotypes
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Height in Human Beings
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Frequency Distributions inPolygenic Inheritance
