H-Evolution-003
Basic Concepts in Genetics
Cells are of two basic types: eukaryotic
and prokaryotic - Structurally, cells consist of two basic
types, although, evolutionarily, the story is more complex (above) Prokaryotic cells lack a nuclear
membrane and possess no membranebounded cell organelles, whereas eukaryotic
cells are more complex, possessing a nucleus and membranebounded organelles
such as chloroplasts and mitochondria.
The gene is the fundamental unit of
heredity - The precise way in which a gene is defined often varies. At the
simplest level, we can think of a gene as a unit of information that encodes a
genetic characteristic. We will enlarge this definition as we learn more about
what genes are and how they function.
Genes come in multiple forms called
alleles - A gene that specifies a characteristic may exist in several
forms, called alleles. For example, a gene for coat color in cats may exist in
alleles that encode either black or orange fur.
Genes encode phenotypes -
One of the most important concepts in genetics is the distinction between
traits and genes. Traits are not inherited directly. Rather, genes are
inherited and, along with environmental factors, determine the expression of
traits. The genetic information that an individual organism possesses is its
genotype; the trait is its phenotype. For example, the A blood type is a
phenotype; the genetic information that encodes the blood type A antigen is the
genotype.
Genetic information is carried in DNA and
RNA - Genetic information is encoded in the molecular structure of
nucleic acids, which come in two types: deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA). Nucleic acids are polymers consisting of repeating
units called nucleotides; each nucleotide consists of a sugar, a phosphate, and
a nitrogenous base. The nitrogenous bases in DNA are of four types (abbreviated
A, C, G, and T), and the sequence of these bases encodes genetic information.
Most organisms carry their genetic information in DNA, but a few viruses carry
it in RNA. The four nitrogenous bases of RNA are abbreviated A, C, G, and U.
Genes are located on chromosomes -
The vehicles of genetic information within the cell are chromosomes, which
consist of DNA and associated proteins. The cells of each species have a
characteristic number of chromosomes; for example, bacterial cells normally
possess a single chromosome; human cells possess 46; pigeon cells possess 80.
Each chromosome carries a large number of genes.
Chromosomes separate through the processes
of mitosis and meiosis - The processes of mitosis and meiosis
ensure that each daughter cell receives a complete set of an organism's
chromosomes. Mitosis is the separation of replicated chromosomes during the
division of somatic (nonsex) cells. Meiosis is the pairing and separation of
replicated chromosomes during the division of sex cells to produce gametes
(reproductive cells).
Genetic information is transferred from
DNA to RNA to protein - Many genes encode traits by specifying
the structure of proteins. Genetic information is first transcribed from DNA
into RNA, and then RNA is translated into the amino acid sequence of a protein.
Mutations are permanent, heritable changes
in genetic information - Gene mutations affect only the genetic
information of a single gene; chromosome mutations alter the number or the
structure of chromosomes and therefore usually affect many genes.
Some traits are affected by multiple
factors - Some traits are influenced by multiple genes that interact in
complex ways with environmental factors. Human height, for example, is affected
by hundreds of genes as well as environmental factors such as nutrition.
Evolution is genetic change -
Evolution can be viewed as a two-step process: first, genetic .variation arises
and, second, some genetic variants increase in frequency, whereas other
variants decrease in frequency.
Viruses are
processed by RNA.
Despite their tremendous diversity, all living organisms use the same genetic system. A complete set of genetic instructions for any organism is it’s Genome, and all genes are encoded in nucleic acids, either DNA or RNA. Genetic instructions are in the same format, the same words are identical. The process by which genetic information is copied and decoded is remarkably similar for all forms of life. All life on earth came from the same primordial ancestor that arose 3.5 to 4 billion years ago.
Cells
are of two basic types: eukaryotic and prokaryotic
DNA consists of two polynucleotide chains that are antiparallel and complementary
RNA consists of a single nucleotide chain
Plant and Animal Cells, similar but different
Most DNA is contained in the Nucleus Synthesis of mRNA > cytoplasm > synthesis of Protein
A few organelles , notably chloroplasts and mitochondrion , contain DNA. Each human mitochondrion contains about 15,000 nucleotides of DNA, encoding 37 genes. Compared with that of nuclear DNA, which contains some 3 billion nucleotides encoding perhaps 35,000 genes, the amount of mitochondrial DNA (mtDNA) is very small none the less mtDNA and chloroplast (cpDNA) (plant) genes encode some important characteristics.
The
first Genetic Code was probably RNA, not DNA
In 1981 Thomas
Cech and his colleagues discovered that RNA can serve as a biological catalyst.
They found that RNA from the protozoan Tetrahymena hermophila can excise 400
nucleotides from its RNA in the absence of any protein. Other examples of catalytic RNAs have now
been discovered in different types of cells. Called ribozymes, these RNA molecules
can cut out parts of their own sequences, connect some RNA molecules together,
replicate others, and even catalysze the formation of peptide bonds between
amino acids. The discovery of ribozymes complements other evidence suggesting
that the original genetic material was RNA.
Ribozymes that
were self-replicating probably first arose between 3.5 billion and 4 billion
years ago and may have begun the evolution of life on Earth. Early life was an
RNA world, with RNA molecules serving both as carriers of genetic information
and as catalysts that drove the chemical reactions needed to sustain and
perpetuate life. These catalytic RNA’s may have acquired the ability to
synthesize protein-based enzymes, which are more efficient catalysts; with
enzymes taking over more and more of the catalytic functions, RNA probably
became relegated to the role of information storage and transfer. DNA, with its
chemical stability and faithful replication, eventually replaced RNA as the
primary carrier of genetic information. In modern cells, RNA still plays a
vital role in both DNA replication and protein synthesis.
Transcription is the synthesis of RNA
molecules, with DNA as a template, and it is the first step in the transfer
genetic information from genotype to phenotype. The process is complex, and
requires a number of protein components. As we examine the stages of
transcription, try to keep all the detail in perspective; focus on
understanding how the details relate to the overall purpose of transcription
-the selective synthesis of an RNA molecule.
Single chromosome set of
chromosome
The gene is the fundamental unit of heredity ;Genes are located on chromosomes
Structure of a eukaryotic chromosome Removal of the tubulin subunits from microtubules at the kinetochore, are responsible for the poleward movement of chromosomes during anaphase
Cell division is essential to Growth of the Living plants or animals
What was simple cell division for prokaryotic became more complicated for eukaryotic cells.
The number of chromosomes and DNA molecules changes in he course of the cell cycle
Meiosis I
Prophase I Chromosomes condense, homologeus
chromosomes synapse, crossing over takes place, nuclear envelope breaks down,
and mitotic spindle forms.
Metaphase I Homologous p;airs of chromosomes line up on
the metaphase plate.
Anaphase I The two chromosomes (each with two
chromatids) of each homologous pair separate and move toward opposite poles.
Telophase I Chromosomes arrive at the spindle poles.
Cytokinesis The cytoplasm divides to produce two cells,
each having half the original number of chromosomes.
Interkinesis In some dells the spindle breaks down,
choromo9s9jes relax, and a nuclear envelope reforms, but no DNA synthesis takes
place.
Meiosis II
Prophae II Chromosomes condense, the spindle forms,
and the nuclear envelope disintegrates.
Metaap;hase II Individual chromosomes line upon the
metaphase plate.
Anaphanse II Sister chromoatids separate and migrate as
individual chromosomes toward the spindle poles.
Telophase II Chromosomes arrive at the spindle poles;
the spindle breaks down and a nuclear envelope reforms.
Cytokineses The cytoplasim divides.
Comparison of mitosis and meiosis
Crossing over takes place in meiosis and is responsible for recombination
left: Male and female gametes (sperm and egg) differ in size
right: The X & Y chromosomes in humans differ in size and genetic content.
Inheritance of sex in organisms with X &
Y chromosomes results in equal numbers of male & female offsprings
Powerful X-Rays can cause mutations
