Transcript Chapter 3

Chapter 3
Human Genetics
RNA – Ribonucleic Acid
Differs from DNA in that:
1. Contains the sugar Ribose
2. Is mostly single stranded
3. Contains the base Uracil
instead of Thymine
Transcription
The synthesis of RNA on a DNA
template
(copying process that makes RNA)
Process of Transcription:
1: RNA polymerase binds with part of the DNA
strand
2: RNA polymerase unwinds a portion of the
double helix (separating the strands). RNA
nucleotides pair with complementary bases one
at a time
3: A continuous RNA strand is formed
4: The RNA molecules separate, polymerase
comes off, and the DNA strands rejoin
The product of transcription is RNA
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
Information flows from
DNA  RNA  Protein
Translation
(protein synthesis)
Why is mRNA made?
• DNA canNOT leave the nucleus
What does mRNA do?
• Carries DNA information to the ribosomes
• Is used to make protein
How does mRNA make proteins?
• At the ribosome, mRNA assembles amino
acids into chains
– Codons – group of 3 nitrogenous bases
• tRNA helps assemble the AA
• AA chains combine to form proteins
tRNA
• Carries anticodon and an amino acid
Figure 3.4
Figure 3.5 (1)
Figure 3.5 (2)
Figure 3.5 (3)
Translation uses all 3 forms of RNA:
• mRNA as the template
• tRNA to match a codon to an amino acid
• rRNA to form the platform where the process takes
place
Figure 3.2a
Mutations
Heritable changes in DNA sequence
• Can result from mistakes during DNA replication
• Are fixed by mechanisms in your body
• In somatic cells can affect individuals but not necessarily
the next generation
• In gametes may be passed on to the next generation
Point Mutations
• Single-base mutation
Eg. Substitution of 1 nitrogenous base for another
Frameshift Mutation
• 1 or 2 extra nitrogenous bases are inserted/deleted
into a DNA sequence
Chromosomal aberrations
(larger changes in genes)
Chromosomal fragment may:
1. Be duplicated
2. Become attached at new location
3. Be lost completely
4. Be inverted and reinserted (*most frequent)
5. Change in chromosome #
 Karyotype – an arrangement of homologous
chromosome pairs
 Autosomal chromosomes – same in both sexes
(22 of 23 pairs)
 Sex chromosomes – 23rd pair; have a role in
determining the sex of an individual
(most of the genes on the X chromo. are not on the Y chromo.)
Genes that are carried on sex
chromosomes determine sex and
sex-linked traits
Sex determination
Females
Males
XX
XY
female
male
Exceptions to the rule:
Not all females are XX and not all males are XY
A cross-over during meiosis can occur between an X
& Y chromosome and is followed by an exchange of
DNA pieces ---translocation
1/20,000 normal males is XX (chromosomally)
1 of X chromo. contains small translocated piece of Y chromo.
1/20,000 normal females is XY (chromosomally)
missing same small piece of the Y chromo.
Lead to the assumption that:
• An XY female has 99.8% of the Y chromosomal DNA;
therefore, the male-determining factor must be located
in the .2% portion of the chromo. she did not have
• The .2% of the chromo. has the sry gene – which is
thought to induce male development
www.biointeractive.org
Sex-linked traits
• Most genes are located on the X chromo.
• Genes that are on the X chromo. and not on
the Y chromo. are sex-linked genes
Females
Males
• Can be either
homozygous or
heterozygous for sexlinked traits (b/c they
have 2 X chromo. and 2
alleles of every sex-linked
gene)
• Because males only have
1 X chromo., they have 1
allele for each sex-linked
trait (which determines
his phenotype for the
trait)
• Hemizygous
e.g., red-green
colorblindness
Trait: red-green colorblindness
Genotype: (XCY x XcXc)
Phenotype: male is normal, female is colorblind
Male gametes
XC
Female
Xc
gametes
Xc
Y
Trait: red-green colorblindness
Genotype: (XCY x XCXc)
Phenotype: male is normal, female is carrier (normal)
Male gametes
XC
Female
XC
gametes
Xc
Y
Chromosomal variation
• Normal human –
22 pairs of autosomal chromo.
1 pair of sex chromo.
• Any difference in chromosomal # has consequences 
called syndromes
Variations in sex chromosomes:
Klinefelter’s syndrome
• XXY
• Male phenotype
• Sterile
• Some symptoms treated
with hormones
Turner’s syndrome
• XO
• Female phenotype
• Infertile (ovaries do not
produce female hormones,
therefore puberty isn’t
reached and gametes do not
develop)
• Some symptoms treated
with hormones
Klinefelter’s
Figure 2.15
• Klinefelter’s and Turner’s are thought to result from
nondisjunction
• Nondisjunction –
abnormal cell division of sex chromo.
Variations in autosomal chromosomes:
Down’s syndrome
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Trisomy 21
Distinct facial characteristics
Heart abnormalities
Variable amounts of mental retardation
Frequency of Down’s Syndrome
Variations in autosomal chromosomes:
Patau’s syndrome or Trisomy 13
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Small head, extra fingers and toes
Cleft lip, large triangular nose, wide-spaced eyes
Severe mental retardation
Death usually by 1 year
Trisomy 13 karyotype
Variations in autosomal chromosomes:
Cri du chat syndrome (Cry of the Cat)
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Deletion in chromosome 5
Small head, mental retardation, catlike cry
Short lifespan
1 in 50,000 births
Cri du chat karyotype
Other diseases and abnormalities that
are inherited in genes:
(not chromosomal aberrations)
• Albinism
– Single mutation that prevents the formation of
the pigment melanin (which blocks ultraviolet
light) in eyes, skin, hair and internal organs
– Affects all races of humans and other species
– In U.S. 1 in 17,000 people have some type of
albinism
Pedigrees
• Diagrams giving the pattern of mating and descent
of certain traits
– most basic methods for presenting genetic data
– are most useful when they span many generations
Pedigrees help determine if traits are:
1. Inherited
2. Dominant or recessive
3. Have a more complex genetic basis
Hereditary diseases associated with known
genes:
1. Alkaptonuria (error in a series of closely related metabloic
pathways)
2. Phenylketonuria (PKU)
3. Duchenne’s Muscular Dystrophy (X chromo.)
4. Huntington’s Disease (chromo. 4)
5. Cystic fibrosis (chromo. 7)
And have found linkage areas for:
6. Alzheimer’s Disease (chromo. 21)
7. 1 form of colon cancer (chromo. 2)
8. 2 forms of manic depression (chromo. 11 & X chromo.)
Human Genome Project
Proposed in 1986
Completed in 2003
Purpose:
1. Make a genetic map of the human genome
a. Including chromosomal location of all human
genes
2. Complete DNA sequence of the human genome
3. DNA sequence of each human is unique
a. People share same genes, but sequences vary
(different alleles)
Genome - Complete set of inherited genetic
information encoded in our DNA
• 3.1 billion nucleotide base pairs in each cell
• Only about 2% of the human genome contains
genes, which are the instructions for making
proteins
• Approx. 28,000 – 35,000 genes in the human
genome
• The human genome is nearly the same (99.9%)
in all people
Human Genome Project:
Legal Issues:
1.
2.
3.
4.
Who owns the human genome?
Who owns the sequence of a particular gene?
Can genes be patented?
Who should have access to the information?
a. Physicians?
b. Insurance companies?
Ethical Issues:
1. Genetic discrimination?
2. Misuse of genetic info. due to ignorance.
3. Who should be tested?
Those at higher risk, for ex.:
a. African Americans – sickle-cell anemia
b. Mediterranean or southeast Asian –thalassemia
c. Ashkenazi Jews –Tay-Sachs disease
d. European Americans – cystic fibrosis
4. Eugenics - Altering a gene pool
a. Positive eugenics – encouraging certain genotypes to breed in
greater numbers
b. Negative eugenics – preventing certain genotypes from breeding
• What is a favorable genotype?
• Hitler??? & the “Master Race”
Gene Therapy
• Introduction of genetically engineered (functional
genes) into cells altering cell’s genotype
– For curing a disease or genetic defect
Cloning
Asexually produced organism (or cells)
Procedure:
1. Nucleus from a fertilized egg (zygote) is removed
2. and replaced with the nucleus from a cell of a fully
developed individual
3. implanted in the womb
4. and brought to term
Now to the FUN stuff!!!!
Many genes are highly complex and involve the
interaction of many genes
(such as hair color, eye color, skin color, height, etc.)
Several traits are determined by only one gene, and
if you have the gene then you have the trait.
such as the following:
• Come to Class for Examples!! *see, it’s not all
right here 
Review Mitosis & Meiosis
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Mitosis
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Meiosis
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Review Cell Structure
Cytoplasm with
organelles (ribosomes)
nucleus
chromosomes
Review Cell Processes
• Replication
• Transcription
• Translation