it involves how mRNA forms and how RNA is getting converted into synthesis of proteins. it explains translation processes.

1. Introduction to genetics

2. • Genetics is a field of biology that studies how
traits are passed from parents to their
offspring. The passing of traits from parents to
offspring is known as heredity, therefore,
genetics is the study of heredity. This
introduction to genetics involves basic
components of genetics such as DNA, genes,
chromosomes and genetic inheritance.

3. • Genetics is built around molecules called DNA.
DNA molecules hold all the genetic
information for an organism. It provides cells
with the information they need to perform
tasks that allow an organism to grow, survive
and reproduce. A gene is one particular
section of a DNA molecule that tells a cell to
perform one specific task.

4. • Heredity is what makes children look like their
parents. During reproduction, DNA is
replicated and passed from a parent to their
offspring. This inheritance of genetic material
by offspring influences the appearance and
behavior of the offspring. The environment
that an organism lives in can also influence
how genes are expressed

5. DNA
• DNA is the cornerstone of genetics and is the
perfect place to start for an introduction to
genetics. DNA stands for deoxyribonucleic acid
and it is the molecule that holds the genetic
information for a cell and an organism.

6. A DNA molecule contains a code
that can be used by a cell to
express certain genes.
Specific sections of a DNA molecule
provides the information to build
specific proteins which can then be
used by a cell to express the
desired gene.

7. • A DNA molecule is a nucleic acid. It comes in
the form of a long, linear molecule referred to
as a strand. Each strand of DNA is bonded to a
second strand of DNA to form a DNA double
helix. In eukaryotic cells, DNA is found in the
nucleus as a tightly coiled double helix.

8. • DNA molecules are replicated during cell
division. When a cell divides, the two new
cells contain all the same DNA that the
original cell had.
• In sexual reproduction with two parents, half
of the DNA of the offspring is provided by
each of the parents. The genetic material of a
child is made from 50% of their mother’s DNA
and 50% their father’s DNA.

9. GENES
• A gene is a specific segment of a DNA molecule that
holds the information for one specific protein. DNA
molecules have a unique code for each gene which
codes for their specific protein. Some organisms can
have more than 100,000 different genes so they will
have 100,000 unique sequences of DNA ‘code’.
• Genes are the basic unit of heredity. The genes of an
individual are determined by their parent or parents. A
bacteria that is born by one parent cell splitting into
two cells and has the exact same genes as their one
parent cell.

10. • A human, on the other hand, has two copies
of each gene – one set from their mother and
a second set from their father. Different forms
of the same gene are called alleles. For each
gene, a human can have two different alleles
or two of the same alleles – one from each
parent.

14. CHROMOSOMES
• A chromosome is a structure
made from tightly packed
strands of DNA and proteins
called histones. Strands of
DNA are tightly wrapped
around the histone proteins
and form into long worm-
shaped structures called
‘chromatids’. Two
chromatids join together to
form a chromosome.

15. • Chromosomes are formed in the nucleus of a cell
when a cell is dividing. It is possible to see
chromosomes under an ordinary light microscope
if the cell is in the right stage of cell division.
• The number of chromosomes varies between
species. Humans have 46 chromosomes. Some
species can have many more than 100
chromosomes while others can have as little as
two.

16. GENETIC INHERITANCE
• Inheritance is the backbone of genetics. Long
before DNA had been discovered and the
word ‘genetics’ had been invented, people
were studying the inheritance of traits from
one generation to the next.
• Genetic inheritance occurs both in sexual
reproduction and asexual reproduction. In
sexual reproduction, two organisms
contribute DNA to produce a new organism.

17. • In asexual reproduction, one organism
provides all the DNA and produces a clone of
themselves. In either, genetic material is
passed from one generation to the next.
• Experiments performed by a monk named
Gregor Mendel provided the foundations of
our current understanding of how genetic
material is passed from parents to their
offspring.

18. Genetic pattern of inheritance
• Each human body cell has a full complement
of DNA stored in 23 pairs of chromosomes.
The image shows the pairs in a systematic
arrangement called a karyotype. Among these
is one pair of chromosomes, called the sex
chromosomes, that determines the sex of the
individual (XX in females, XY in males). The
remaining 22 chromosome pairs are
called autosomal chromosomes.

19. • Each of these chromosomes carries hundreds
or even thousands of genes, each of which
codes for the assembly of a particular
protein—that is, genes are “expressed” as
proteins. An individual’s complete genetic
makeup is referred to as his or her genotype.
The characteristics that the genes express,
whether they are physical, behavioral, or
biochemical, are a person’s phenotype.

20. Each pair of chromosomes contains hundreds to thousands of genes.
The banding patterns are nearly identical for the two chromosomes
within each pair, indicating the same organization of genes. As is
visible in this karyotype, the only exception to this is the XY sex
chromosome pair in males.

21. Mendel’s Theory of Inheritance
• Working in the mid-1800s, long before anyone
knew about genes or chromosomes, Gregor
Mendel discovered that garden peas transmit
their physical characteristics to subsequent
generations in a discrete and predictable fashion.
When he mated, or crossed, two pure-breeding
pea plants that differed by a certain
characteristic, the first-generation offspring all
looked like one of the parents. For instance, when
he crossed tall and dwarf pure-breeding pea
plants, all of the offspring were tall.

22. • Mendel called tallness dominant because it was
expressed in offspring when it was present in a
purebred parent. He called
dwarfism recessive because it was masked in the
offspring if one of the purebred parents
possessed the dominant characteristic. Note that
tallness and dwarfism are variations on the
characteristic of height. Mendel called such a
variation a trait. We now know that these traits
are the expression of different alleles of the gene
encoding height.

23. • In the language of genetics, Mendel’s theory
applied to humans says that if an individual
receives two dominant alleles, one from each
parent, the individual’s phenotype will express
the dominant trait. If an individual receives two
recessive alleles, then the recessive trait will be
expressed in the phenotype. Individuals who
have two identical alleles for a given gene,
whether dominant or recessive, are said to be
homozygous for that gene (homo- = “same”).

24. • Conversely, an individual who has one
dominant allele and one recessive allele is said
to be heterozygous for that gene (hetero- =
“different” or “other”). In this case, the
dominant trait will be expressed, and the
individual will be phenotypically identical to
an individual who possesses two dominant
alleles for the trait.

25. Mutations
• A mutation is a change in the sequence of
DNA nucleotides that may or may not affect a
person’s phenotype. Mutations can arise
spontaneously from errors during DNA
replication, or they can result from
environmental insults such as radiation,
certain viruses, or exposure to tobacco smoke
or other toxic chemicals.

26. • Because genes encode for the assembly of
proteins, a mutation in the nucleotide
sequence of a gene can change amino acid
sequence and, consequently, a protein’s
structure and function. Spontaneous
mutations occurring during meiosis are
thought to account for many spontaneous
abortions (miscarriages).

27. Chromosomal Disorders
• Sometimes a genetic disease is not caused by
a mutation in a gene, but by the presence of
an incorrect number of chromosomes.
• For example, Down syndrome is caused by
having three copies of chromosome 21. This is
known as trisomy 21.
• The most common cause of trisomy 21 is
chromosomal nondisjunction during meiosis.

29. • The frequency of nondisjunction events appears to
increase with age, so the frequency of bearing a child
with Down syndrome increases in women over 36.
• The age of the father matters less because
nondisjunction is much less likely to occur in a sperm
than in an egg.
• Whereas Down syndrome is caused by having three
copies of a chromosome, Turner syndrome is caused by
having just one copy of the X chromosome.
• This is known as monosomy. The affected child is
always female. Women with Turner syndrome are
sterile because their sexual organs do not mature.

31. Protein synthesis
• DNA, or deoxyribonucleic acid, contains the
genes that determine who you are.
• DNA contains instructions for all the proteins the
body makes.
• Proteins, in turn, determine the structure and
function of all cells.
• It begins with the sequence of amino acids that
make up the protein
• Instructions for making proteins with the correct
sequence of amino acids are encoded in DNA.

32. • DNA is found in chromosomes. In eukaryotic cells,
chromosomes always remain in the nucleus, but
proteins are made at ribosomes in the cytoplasm or on
the rough endoplasmic reticulum (RER).
• Another type of nucleic acid is responsible for this
synthesis. This nucleic acid is RNA or ribonucleic acid.
RNA is a small molecule that can squeeze through
pores in the nuclear membrane.
• It carries the information from DNA in the nucleus to
a ribosome in the cytoplasm and then helps assemble
the protein
• In short: DNA → RNA → Protein

33. • Discovering this sequence of events was a
major milestone in molecular biology. It is
called the central dogma of biology.
• The two processes involved in the central
dogma are transcription and translation.

35. Transcription
• Transcription is the first part of the central dogma of
molecular biology: DNA → RNA.
• It is the transfer of genetic instructions in DNA to mRNA.
• Transcription happens in the nucleus of the cell.
• During transcription, a strand of mRNA is made that is
complementary to a strand of DNA called a gene.
• A gene can easily be identified from the DNA sequence.
A gene contains the basic three regions, promoter, coding
sequence (reading frame), and terminator.
• There are more parts of a gene which are illustrated in
Figure.

36. The major components of a gene
. 1. promoter
2. transcription initiation
3. 5' upstream untranslated region,
4. translation start codon site,
5. protein -coding sequence,
6. translation stop codon region,
7. 3' downstream untranslated region, and 8. terminator.

37. Steps of Transcription
• Transcription takes place in three steps, called initiation, elongation,
and termination.
• Initiation is the beginning of transcription. It occurs when
the enzyme RNA polymerase binds to a region of a gene called
the promoter.
• This signals the DNA to unwind so the enzyme can “read” the bases
in one of the DNA strands. The enzyme is ready to make a strand of
mRNA with a complementary sequence of bases.
• The promoter is not part of the resulting mRNA.
• Elongation is the addition of nucleotides to the mRNA strand.
• Termination is the ending of transcription. As RNA polymerase
transcribes the terminator, it detaches from DNA. The mRNA strand
is complete after this step.

39. • Processing mRNA
• In eukaryotes, the new mRNA is not yet ready
for translation, At this stage, it is called pre-
mRNA, and it must go through more processing
before it leaves the nucleus as mature mRNA.
• The processing may include the addition of a 5'
cap, splicing, editing, and 3' polyadenylation
(poly-A) tail.
• These processes modify the mRNA in various
ways. Such modifications allow a single gene to
be used to make more than one protein.

40. • 5' cap protects mRNA in the cytoplasm and helps in the attachment of
mRNA with the ribosome for translation.
• Splicing removes introns from the protein-coding sequence of mRNA.
Introns are regions that do not code for the protein. The remaining
mRNA consists only of regions called exons that do code for
the protein.
• Editing changes some of the nucleotides in mRNA. For example, a
human protein called APOB, which helps transport lipids in the blood,
has two different forms because of editing. One form is smaller than
the other because editing adds an earlier stop signal in mRNA.
• Polyadenylation adds a “tail” to the mRNA. The tail consists of a string
of As (adenine bases). It signals the end of mRNA. It is also involved in
exporting mRNA from the nucleus, and it protects mRNA from
enzymes that might break it down.

41. Processing of mRNA

42. Translation
• The translation is the second part of the central dogma
of molecular biology: RNA --> Protein
• It is the process in which the genetic code in mRNA is
read to make a protein.
• After mRNA leaves the nucleus, it moves to
a ribosome, which consists of rRNA and proteins.
• Translation happens on the ribosomes floating in
the cytosol, or on the ribosomes attached to the rough
endoplasmic reticulum.
• The ribosome reads the sequence of codons in mRNA,
and molecules of tRNA bring amino acids to
the ribosome in the correct sequence.

43. • Each tRNA molecule has an anticodon for the amino acid it
carries. An anticodon is complementary to the codon for
an amino acid.
• For example, the amino acid lysine has the codon AAG, so
the anticodon is UUC. Therefore, lysine would be carried by
a tRNA molecule with the anticodon UUC. Wherever
the codon AAG appears in mRNA, a UUC anticodon of tRNA
temporarily binds.
• While bound to mRNA, tRNA gives up its amino acid.
• With the help of rRNA, bonds form between the amino
acids as they are brought one by one to the ribosome,
creating a polypeptide chain. The chain of amino acids
keeps growing until a stop codon is reached.

44. • Just as with mRNA synthesis, protein synthesis
can be divided into three phases: initiation,
elongation, and termination.
• In addition to the mRNA template, many other
molecules contribute to the process
of translation such as ribosomes, tRNAs, and
various enzymatic factors

45. • Translation Initiation: The small subunit binds
to a site upstream (on the 5' side) of the start
of the mRNA. It proceeds to scan the mRNA in
the 5'-->3' direction until it encounters the
START codon (AUG).

46. • Translation Elongation: The ribosome shifts one codon at a
time, catalyzing each process that occurs in the three sites.
With each step, a charged tRNA enters the complex,
the polypeptide becomes one amino acid longer, and an
uncharged tRNA departs.
• The energy for each bond between amino acids is derived
from GTP, a molecule similar to ATP.
• Briefly, the ribosomes interact with other RNA molecules to
make chains of amino acids called polypeptide chains, due to
the peptide bond that forms between individual amino acids.
• The E. coli translation apparatus takes only 0.05 seconds to
add each amino acid, meaning that a 200-amino acid
polypeptide could be translated in just 10 seconds.

47. • Translation Termination: Termination
of translation occurs when a stop codon (UAA,
UAG, or UGA) is encountered.
• When the ribosome encounters the stop codon,
the growing polypeptide is released with the help
of various releasing factors and the ribosome
subunits dissociate and leave the mRNA.
• After many ribosomes have
completed translation, the mRNA is degraded so
the nucleotides can be reused in another
transcription reaction.