Genetics notes part 2
Gene linkage- chromosomes assort independently genes do not.
RNA and DNA
12-2
Deoxyribonucleic acid and Ribonucleic acid
Double helix = twisted ladder shape
Rungs of the latter formed by pares of nucleotide bases
Adenine – Thymine
Cytosine- Guanine
Replication = duplicating DNA
Dna unzips and new strands are formed with the help of DNA polymerase which links
news nucleotides and proofreads new strands.
RNA 12-3
Ribose sugar not deoxyribose.
Single strand
Uracil in place of Thymine
3 types of RNA
mRNA messenger RNA a copy of DNA from nucleus that is transcribed in the nucleus a
copy of one set of instructions.
rRNA Ribosomal RNA, Helps make up ribosomes, protein factories.
TRNA transfer RNA, Carry or transfer amino acids to the ribosome using information in
the mRNA to make protein.
This process is called translation.
There are 20 different amino acids
mRNA is read by ribosome three base pairs at a time (called a codon)
H. MUTATIONS. A mutation is any change or mistake in the genes or chromosomes
of an organism that can be inherited. These changes usually produce new
characteristics. To be inherited, a change in genetic makeup must occur in the gamete
(sex cell) As a result of fertilization, the changed gene or chromosome is passed on to the
new organism. Most mutations are harmful because they upset normal cell functions.
Fortunately, since mutations are usually recessive, they do not affect the organism
as long as the dominant gene is present. A beneficial mutation results in traits that
make an organism better adapted to its environment.
Mutations may occur naturally within the cell or they may be caused by forces
outside the cell called mutagenic agents. X rays, ultraviolet light, radioactive
substances, cosmic rays, and chemicals, such as formaldehyde and benzene, are
mutagenic agents. Asbestos fibers and drugs, such as LSD, marijuana and alcohol,
are also known to cause changes in genes and chromosomes. Unborn children can be
injured when their pregnant mothers are exposed to mutagenic agents. The chemical
colchicine causes mutations in plants.
Gene Mutations. A change in the genetic code of DNA is called a gene mutation.
Seedless oranges, albinism (lack of pigments), hemophilia, and sickle-cell anemia, are
examples of gene mutations.
• Chromosome Mutations. Chromosome mutations occur when there is a change in the
number or structure of chromosomes: Some chromosome mutations and their causes are
shown in Table 25-2.
MUTATION
CAUSE
Crossing-over
Chromatids break, exchange segments, and rejoin during
meiosis. Linked genes are separated resulting in variation
among offspring.
Pairs of homologous chromosomes fail to separate during
meiosis. Results in gametes containing 1 chromosome more
or less than the monoploid chromosome number.
Nondisjunction of human chromosome #21. Offspring has an
extra chromosome.
Nondisjunction
Down’s syndrome
Polyploidy
An entire set of chromosomes fails to separate during meiosis.
The resulting gamete contains the diploid (2n) chromosome
number. Fatal in animal offspring. In plants, offspring often
larger or more vigorous than normal diploid plants.
A chromosome segment is lost.
Deletions
Translocation
A chromosome breaks off and becomes reattached to a
nonhomologous chromosome.
A chromosome segment breaks off and becomes reattached at
a new point on the original chromosome.
Inversion
I. HUMAN INHERITANCE. Human inheritance follows the same genetic principles as
inheritance in other organisms. However, scientists who study human heredity, face many
GENETIC
DISEASE
CAUSE
DESCRIPTION
DETECTION&TREATMENT
Pheny
Absence of an
enzyme needed
to metabolize
an amino acid.
Mental retardation.
Detected by urine analysis of
newborn infants; mental
retardation can be avoided by
treatment with a special diet.
Formation of
abnormal
hemoglobin,
which makes
Cells with the sickle
shape tend to block
small blood vessels,
causing much pain.
Detected by blood screening.
Individuals who are carriers
(heterozygous) have some
sickle-shaped cells;
(PKU)
SickIe.CelI
Anemia
the red blood
cells fragile and
gives them a
sickle (quartermoon) shape.
The abnormal
hemoglobin cannot
carry enough oxygen
for the body cells; is
found most frequently
among people of
African descent.
homozygous people have
severe sickle-cell anemia.
Analysis of the amniotic fluid
can detect the condition in a
fetus.
Tay-Sachs
Disease
Accumulation
of fatty material
due to an
inability to
synthesize a
specific
enzyme.
Fatal disease
characterized by an
erosion of nervous
tissue; occurs most
frequently among
Jewish people of
Central European
descent.
Carriers of Tay-Sacs can be
detected by blood screening;
chemical analysis of amniotic
fluid can detect the condition
in a fetus.
Cystic
Fibrosis
Nonsecretion of
digestive
enzymes.
Life expectancy 12- 16
years (some live
longer); exists in
different degrees of
severity, thick mucus
interferes with lung
clearance; common in
persons of northern
European extraction.
Detected by examination of
amniotic fluid; symptoms are
treated with digestive enzyme
replacement and control of
respiratory infections.
K. HEREDITY AND THE ENVIRONMENT. Most characteristics of an organism
result from the interaction between the organism’s genes and the environment. Thus, the
development of an organism is controlled by its genetic makeup but may be influenced
by the environment. For example, in plants, genes control the production of chlorophyll,
but light must be present for the genes to produce chlorophyll. In this way, the
environment (light) influences the expression of the genes.
Another example of the effect of environment on the expression of genes is fur color in
Himalayan rabbits. In Himalayan rabbits, fur color is affected by temperature. The
gene for black fur is active at low temperatures. If white fur on the rabbit’s back is
shaved and the area covered with an ice pack, the fur grows in black. Also, the rabbit
normally has black fur on the tips of its ears. When the ears are kept warm, the fur grows
in white (Figure 25-6).
Studies with human identical twins have been done to learn about the relative effects of
environment and heredity. Such studies involve twins raised together in the same
environment and those separated at birth. The separated twins, who ate different amounts
of food, weren’t always the same size. The twin who ate more food was usually larger
than the twin who ate less food.
L. GENETIC ENGINEERING. Today, scientists can deliberately remove genes from one
organism and add them to the genetic material of another organism. This process, known
as genetic engineering, alters (changes) the DNA of a cell. The changed DNA is called
recombinant DNA. The cell that receives the recombinant DNA receives new traits,
such as, the ability to prevent a certain disease. Genetic engineering has greatly increased
our ability to treat genetically related disorders. For example, the biochemical insulin has
been synthetically manufactured for the treatment of diabetes. In addition, cancer
scientists have already used genetic engineering techniques to show the genetic link to
certain types of cancer. By removing small sections of particular genes, they have been
able to pinpoint the location of genes that have the potential to cause cancer. Cancercausing genes are known as oncogenes. With continued research into the use of
recombinant DNA, scientists hope to cure other conditions caused by genetic defects and
to develop plants and animals with desirable traits. There is, however, concern that
genetic engineering may lead to the reproduction of new forms of life potentially
dangerous to humans and other organisms.
1. What is genetic engineering?
Scientists deliberately remove genes from one organism and add them to the genetic
material of another organism.
2. DNA, which has been changed by genetic engineering, is called recombinant DNA
3. Explain how has genetic engineering helped humans and state an example?
Genetic engineering has greatly increased our ability to treat genetically related disorders.
For example, the biochemical insulin has been synthetically manufactured for the
treatment of diabetes.
4. What is an oncogene?
Cancer-causing genes are known as oncogenes