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Genetics
Factors responsible for character traits among living organisms
•Members of the same species of animals look very
similar. Within a bird species, the shape of their beaks,
their feathers and the position of their eyes are very much
alike.
•Even humans have many characteristics in common, for
example: the position of our eyes, nose, ears and mouth.
•We also differ in many ways, making us unique (eye and
hair colour). The similarities and differences are called
character traits.
Chromosomes
•Most eukaryotic cells contain a nucleus holding a person’s
basic genetic information.
•The main component of the nucleus is chromatin, made up
of a molecule of DNA, combined with proteins.
•When the cells are about to divide, chromatin contracts
forming visible rods called chromosomes.
Humans have 46 chromosomes in the cells; 23 chromosomes in
the sex cells (eggs and sperm).
Other species have differing numbers of chromosomes; the
number of chromosomes does not determine the complexity of
the organism.
If you classify the chromosomes according to size and distinctive
features, each chromosome is observed to have a partner of the
same size and shape. The pair is called homologous
chromosomes.
Humans have 23 pairs of homologous chromosomes.
An ordered representation of these pairs (an individual’s
chromosomes) is known as a karyotype.
Normally, only one pair can contain chromosomes of different
sizes, the pair of sex chromosomes.
As with most animals, two sex chromosomes are the same for
females (XX pair), while in the male, one of the two
chromosomes is smaller (XY pair).
DNA and Genes
Each chromosome is made up of DNA.
DNA is shaped like a twisted ladder (double helix). It consists
of a series of chemical units called nucleotides.
Nucleotides consist of:
•A sugar: deoxyribose (sides of the ladder)
•A phosphate group (sides of the ladder)
•One of 4 nitrogenous bases: adenine(A), thymine(T),
guanine (G) or cytosine (C) (rungs of ladder)
Nitrogenous bases pair accordingly:
Adenine with thymine:
A-T or T-A
Cytosine with guanine:
C-G or G-C
A gene (a segment of DNA) is a particular sequence of
bases. They differ from one another in their sequence of
nitrogenous bases. Genes contain the information for protein
making.
Proteins determine character traits and body functioning.
A genome consists of all the genes we receive from our
fathers and mothers.
The difference between a human and a chimpanzee lies in
only one percent of their genes. Nearly 99% of the two
genomes are nearly identical.
The human body contains more than 100 000 proteins.
The sequence of amino acids in a chain determines the
function of the protein.
Protein structure
Amino acid: A molecule that can combine with other amino
acids to form proteins.
Some amino acid chains are made of 600 amino acids.
Most, however, contain between 100 and 200 amino acids.
Protein Synthesis
Usually carried out by the ribosomes, it is the creation of
proteins by the cells.
Since DNA cannot leave the nucleus, a messenger must carry
the information from the DNA to the ribosomes: RNA
An RNA molecule is very similar to a DNA molecule with a few
important differences:
RNA
DNA
Single strand
Double strand
RNA
DNA
Uracil base pair
Thymine base pair
RNA
DNA
Sugar is ribose
Sugar is deoyxribose
Process of Protein synthesis
The are two major types of RNA:
Messenger RNA (mRNA): It acts as a messenger for
carrying instructions about the gene to the ribosomes.
Transfer RNA (tRNA):
It transfers the amino acids in the
cell’s cytoplasm to the ribosomes in order to make proteins.
Principles of heredity
Heredity:
The transmission of parent’s character traits to
their offspring.
When a trait is said to be hereditary, this means it can be
passed on from parents to offspring. Ex. Eye colour in
humans, or the colour of pea blossoms are hereditary traits.
A scientist names Gregor Mendel was the first to study and
establish the fundamental laws of heredity (Mendel’s Laws).
The founding father of genetics published these results in
1865 but only gained significance by 1900.
Mendel’s experiments
Mendel created a Pure Line through self-pollination of the pea
plant. This means a flower from a pea plant can be pollinated
with pollen from the same flower. The results are particular traits
that are passed on from generation to generation without
variation.
He chose purple flowered pea plants and white flowered pea
plants. To prevent self pollination he removed the stamens (male
organs which produce pollen). He then used a paintbrush to
deposit pollen from the white flowers to the carpal (female organ
containing ovules). This is Crossbreeding.
The offspring of this cross were allowed to reproduce again.
Mendel did not know of genes or chromosomes. He knew there
was some type of information unit present (he called Hereditary
factor).
He also knew that there must be several alternatives for each of
these traits. We call them alleles of a gene. Today we know that
the various alleles are due to differences in the nucleotide
sequences.
Homozygote and Heterozygote
Homozygote: An individual with two identical alleles
for a given character trait.
Heterozygote: An individual with two different
alleles for a given character trait.
Dominate and Recessive Alleles
Dominant allele: An allele that is expressed when an individual
carries two different alleles for a given gene.
It can be heterozygous dominant or
homozygous dominant.
Recessive: An allele that is not expressed when an individual
carries two different alleles for a given gene.
It is considered homozygous recessive
Genotypes and Phenotypes
Genotype:
An individual’s genetic inheritance. It describes all
of an individual’s alleles for specific genes.
Phenotype: The way in which a genotype expresses itself. It
describes the appearance or state of the
individual for one or more character traits.
To describe a genotype:
•Each allele is represented by a letter.
•The dominant from of the allele is a capitol letter while the
recessive allele gets a lower case letter.
The Law Of Segregation Of Alleles
Gametes are formed through a process called meiosis. This type
of cell division, unlike mitosis, results in cells with half the number
of chromosomes normally present in the cells. Instead of carrying
pairs of chromosomes, gametes contain only one chromosome
from each pair. Thus, only one allele is present instead of two.
Law of Segregation of Alleles:
The two alleles for a particular character trait separate when
gametes are formed. Half of the gametes receive one of the two
alleles and the other half receive the second allele.
Punnett Squares
It is a tool used to predict the genotypes and phenotypes of
offspring.
Four steps must be followed to use this tool properly:
1. Determine the genotypes of the two parents.
2. Find the possible genotypes for the gametes and place
them in circles.
3. Place the possible gametes of one parent at the top of the
Punnett square and the other gametes on the left side.
4. Indicate all of the possible gamete combinations. This
provides all of the genotypes and phenotypes.
The law of independent assortment of character traits.
The different pairs of alleles separate into gametes,
independently of each other.
There are two natural methods of reproduction:
Sexual reproduction: It involves the combination of genes
of two parents which result in genetically unique offspring.
(except identical twins).
Asexual reproduction: It involves only one parent resulting
in offspring genetically identical the parent. (clones)
Cloning
It is the reproduction of an individual, part of that
individual or one of its genes in order to obtain an exact
copy.
We will look at:
•Natural cloning
•Artificial plant cloning
•Animal cloning
•Human cloning
•Molecular cloning
Natural cloning:
It produces genetically identical individuals
through asexual reproduction.
Artificial plant cloning
Plant cuttings or in vitro methods. People often take a cutting
form a plant and place it in water unit it grows roots. It is then
transferred to a pot. In vitro methods are shown below. Since
both methods involve human participation, that are called artificial
plant cloning.
Animal cloning
1. Remove a cell from the individual to be cloned.
2. Take an ovum from another individual and remove the
nucleus.
3. Combining the cell and the ovum (no nucleus) results in an
embryo with the same genetic material as the individual.
4. Implant the embryo into the uterus of a surrogate mother.
Human cloning
Human cloning has two forms:
1. Reproductive cloning: The application of cloning techniques to
obtain a new individual genetically identical to the one being
cloned.
2. Therapeutic cloning: The application of cloning techniques
to obtain tissues or organs genetically identical to those of a
person in need of a transplant or medical grafting.
Molecular cloning: The production of multiple copies of
the same gene.
Many illnesses have genetic causes, which means they are due to
defective genes. Scientists may want to make multiple copies of
these defective genes to study them more closely.