genetics- gene expression and regulations

PRESENTED BY GOPALASATHEESKUMAR

1ST M.PHARM., PHARMACOLOGY

7/12/2017 KMCH COLLEGE OF PHARMACY 1

GENETICS

“Genetics is the study of heredity, the processin which a parent passes certain genes ontotheir children.”

Children inherit their biological parents’ genesthat express specific traits, such as somephysical characteristics, natural talents, andgenetic disorders.

The number of human genes is about 20,000-25,000. Different genes can vary in length andcover thousands of bases.


• If all 46 chromosomes were combined and arrangedlengthwise, the total length would be 1.8 meters.

• If all the chromosomes from all the nuclei in the humanbody (1014 cells) were to be arranged lengthwise, it wouldmeasure around 180 000 million kilometres.

• For a better understanding, compare it to the distance fromthe Earth to the Sun which measures 150 millionkilometres. The length of the DNA would thus be onethousand times greater.


Human Traits

• Many human traits, such as height and blood

sugar, show a similar pattern, and these traits

can also be inherited (e.g., tall parents tend to

have tall children).

• Diseases such as heart disease, cancer, and

diabetes are complex too. Genetically,

complex traits are caused by many genes and

environmental factors.


Genetic Variation

• Every person is born with genetic differences, calledvariation. Variation is why each individual is unique atthe level of genes and traits.

• Most variation is harmless, but some causes disease.Genetic and trait variation allows populations to adaptmore readily to different environmental challenges.

• In fact, variation in populations is necessary for theevolution of species. Because many traits andconditions are the result of combinations of genes andenvironment, we see a wide range of variation for mosttraits in the population.


Genes and Environment

• Our genetic makeup is constant throughout

life, our genes alone do not DETERMINE our

future.

• All genes work in the context of environment.

Changes in the environment, such as diet,

exercise, exposure to toxic agents, or

medications can all influence our genes and

traits.



Hippocrates 460 – 370 BC Gregor Mendel 1830Hugo de Vries 1870Hugo de Vries in the 1890sKathleen Rubins 2016

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Genetic Information

• Gene – basic unit of geneticinformation. Genes determine theinherited characters.

• Genome – the collection of geneticinformation. 2% formation ofprotiens

• Chromosomes – storage units ofgenes.

• DNA - is a nucleic acid that containsthe genetic instructions specifying thebiological development of all cellularforms of life

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Types of Genes

House Keeping Genes (Constitutive Genes):

They are those genes which are constantly expressing themselves in acell because their products are required for the normal cellularactivities, e.g., genes for glycolysis, ATP-ase

Non-constitutive Genes (Luxury Genes):

The genes are not always expressing themselves in a cell. They areswitched on or off according to the requirement of cellular activities,e.g., gene for nitrate reductase in plants, lactose system in Escherichiacoli. Non- constitutive genes are of further two types, inducible andrepressible.

Inducible Genes:

The genes are switched on in response to the presence of a chemicalsubstance or inducer which is required for the functioning of theproduct of gene activity, e.g., nitrate for nitrate reductase.


Repressible Genes

They are those genes which continue to express themselves till a

chemical inhibits or represses their activity. Inhibition by an end

product is known as feedback repression

Multigenes (Multiple Gene Family)

It is a group of similar or nearly similar genes for meeting

requirement of time and tissue specific products, e.g., globin gene

family (e, 5, (3, у on chromosome 11, oc and 8 on chromosome

16).

Repeated Genes

The genes occur in multiple copies because their products are

required in larger quantity, e.g., histone genes, tRNA genes,

rRNA genes, actin genes.


Single Copy Genes

The genes are present in single copies (occasionally 2—3times), e.g., protein coding genes. They form 60—70% ofthe functional genes. Duplications, mutations and exonreshuffling can form new genes.

Pseudogenes

They are genes which have homology to functional genesbut are unable to produce functional products due tointervening nonsense codons, insertions, deletions andinactivation of promoter regions, e.g., several of snRNAgenes.

Processed Genes

They are eukaryotic genes which lack introns. Processedgenes have been formed probably due to reversetranscription or retroviruses. Processed genes aregenerally non-functional as they lack promoters.


Transposons (Jumping Genes)

They are segments of DNA that can jump or move from one place inthe genome to another.

Structural Genes

Structural genes are those genes which have encoded information forthe synthesis of chemical substances required for cellular machinery.

Polypeptides for the formation of structural proteins (e.g., colloidalcomplex of protoplasm, cell membranes, elastin of ligaments,collagen of tendons or cartilage, actin of muscles, tubulin ofmicrotubules, etc.).

Polypeptides for the synthesis of enzymes

Transport proteins like haemoglobin of erythrocytes, lipidtransporting proteins, carrier proteins of cell membranes, etc.

Proteinaceous hormones, e.g., insulin, growth hormone, parathyroidhormone

Antibodies, antigens, certain toxins, blood coagulation factors, etc.

Non-translated RNAs like tRNAs, rRNA. Broadly speaking,structural genes either produce mRNAs for synthesis ofpolypeptides/proteins/enzymes or noncoding RNAs.


Regulatory Genes

Regulatory genes do not transcribe RNAs for

controlling structure and functioning of the cells.

Instead, they control the functions of structural

genes.

Tissue Specific Genes:

They are genes which are expressed only in

certain specific tissues and not in others.


Three Important Points to Remember

Chromosomes are made of DNA

Segments of DNA code for a protein

Protein in turn, relates to a trait (eye color,

enzymes, hormones..)


How Proteins are Synthesized from DNA

DNA is transcribed into mRNA (messenger RNA)

mRNA leaves the nucleus and travels to the

cytoplasm

Ribosomes in the cytoplasm use the code on

mRNA to translate it into amino acids

Amino acids form a chain - a protein


Properties of the Genetic Code

Three bases in mRNA make a “codon” , also called a“triplet”

Each codon specifies a single amino acid

Most amino acids have more than one codon (helps preventeffects of DNA mutations)

It is unambiguous (each codon has only one meaning)

The code has start and stop signals. (one start, three stops)

The code is universal – all living things share it (commonevolutionary ancestors)

We can move genes from one organism to another (usebacteria to make insulin) because it is universal.


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Chromosome Logical Structure

• Locus – location of a gene/marker on the chromosome.

• Allele – one variant form of a gene/marker at a particular locus.

Locus1

Possible Alleles: A1,A2

Locus2

Possible Alleles: B1,B2,B3


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Human GenomeChromatin: DNA, RNA & proteins

that make up chromosme

Chromatids: one of the two identical parts of the chromosome.

Centromere: the point where two chromatids attach

Most human cells contain 46 chromosomes:

• 2 sex chromosomes (X,Y):XY – in males.XX – in females.

• 22 pairs of chromosomes named autosomes.


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Genotypes Phenotypes

• At each locus (except for sex chromosomes) there

are 2 genes. These constitute the individual’s

genotype at the locus.

• The expression of a genotype is termed a phenotype.

For example, hair color, weight, or the presence or

absence of a disease.


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Genotypes Phenotypes (example)

• Eb- dominant allele.

• Ew- recessive allele.

genotypes

phenotypes

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Dominant vs. Recessive

A dominant allele is

expressed even if it is

paired with a recessive

allele.

A recessive allele is only

visible when paired with

another recessive allele.

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GENE EXPRESSION

Process by which information from a gene is used in the

synthesis of a functional gene product.

These products are often proteins, but in non-protein coding

genes such as transfer RNA (tRNA) or small nuclear RNA

(snRNA) genes, the product is a functional RNA.

The process of gene expression is used by all known life—

eukaryotes (including multicellular organisms), prokaryotes

(bacteria and archaea), and utilized by viruses—to generate the

macromolecular machinery for life.


Genetic materials (chemistry)

Bases

Sugars

Phosphates

Bonds

Nucleosides

Nucleotides


STEPS INVOLVED

Transcription

Occurs in nucleus

Translation

Occurs in cytosol


Transcription

Particular gene or group of genes

RNA is produced

mRNA – code carrier

tRNA - read the information

rRNA – associate with the synthetic machinery


Main components in transcription

RNA polymerase

Ribo-nucleosides

5’ tri-Phosphate

Mg2+

RNA Polymerase

RNA Polymerase I - synthesis rRNA

RNA Polymerase II – synthesis mRNA

RNA Polymerase III – synthesis tRNA


The production of the RNA copy of the DNA is called

transcription.

This RNA is complementary to the template 3' → 5'

DNA strand, which is itself complementary to the

coding 5' → 3' DNA strand.

Therefore, the resulting 5' → 3' RNA strand is identical

to the coding DNA strand with the exception that

thymines (T) are replaced with uracils (U) in the RNA.

A coding DNA strand reading "ATG" is indirectly

transcribed through the non-coding strand as "AUG" in

RNA.


In eukaryotes most mature RNA must be exported

to the cytoplasm from the nucleus.

While some RNAs function in the nucleus, many

RNAs are transported through the nuclear pores

and into the cytosol.

Notably this includes all RNA types involved in

protein synthesis.

In some cases RNAs are additionally transported

to a specific part of the cytoplasm, such as a

synapse; they are then towed by motor proteins

that bind through linker proteins to specific

sequences (called "zipcodes") on the RNA


TRANSLATIONmRNA decoded into a.a

Energy provided by GTP

tRNA contains 75-85 nucleotides

Ribosomes

Prokaryotic 70S (svedberg unit)

Small-30S(16S rRNA,21 dif. Protein)

Large- 50S(5S rRNA, 23S rRNA, 32)

Eukaryotic 80S

Small- 40S (18S rRNA, 30 P)

Large – 60S (20S rRNA, 5.8S rRNA, 5S rRNA, 40p)


Each subunit having two tRNA binding sites

Site A- Amino acyl site

Site B- Peptide site

During protein synthesis more ribosomes

attached to single mRNA to form polysome


Steps involved in protein synthesis

Activation of a.a and tRNA charging

Initiation

Elongation

Termination

Post translational modification


Activation of amino acid and tRNA

charging


INIT

IAT

ION


ELONGATION


TERMINATION


Post translation modification

Protein folding- tertiary structure formation

Proteolytic cleavage- chain is cut short by

protease

Chemical modification- individual a.a are

modified

Intron splicing- extra a.a sequence are removed


Chemical process

Acetylation of amino terminal residues

Loss of signal seq.

Removal or modification of carboxyl terminal

residue

Modification of individual a.a

Attachment of carbohydrate side chain

Formation of di-sulphide cross linking


GENETIC CODETriplet- codon

Universal- code for an a.a remain the same for allplants and animals

Commaless- each codon is followed by the next codonleaving no space b/w the nxt

Linear arrangement- codons are linear arrange in themRNA

Degeneracy- some a.a are represented by more than onecodon

Initiation codon – AUG – beginings of all polypeptidechains in both pro and eukaryotes

Termination codons- UAA, UAG, UGA- nonsensecodons(amber, ochre, opal)


Growth curve in microbes


OPERON

LAC OPERON

activated by allolactose

TRP OPERON

inhibited by a chemical (tryptophan)


Regulation

Epigenetic mechanism

Post transcriptional modification

mRNA degradation

Translational regulation

Post translation


Epigenetics machanism

The Greek prefix epi- (Greek: επί- over, outside

of, around) in epigenetics implies features that

are "on top of" or "in addition to" the traditional

genetic basis for inheritance.

DNA methylation

Histone tail modification

Epigenetic factor


Post-transcriptional modification

process in eukaryotic cells where primary

transcript RNA is converted into mature RNA.

A notable example is the conversion of

precursor messenger RNA into mature

messenger RNA (mRNA) that occurs prior to

protein translation


Post-translational modification (PTM) refers to

the covalent and generally enzymatic

modification of proteins during or after protein

biosynthesis.

Proteins are synthesized by ribosomes

translating mRNA into polypeptide chains,

which may then undergo PTM to form the

mature protein product.

PTMs are important components in cell

signaling.


STEPS


CAPPING

Capping of the pre-mRNA involves the addition

of 7-methylguanosine (m7G) to the 5' end.

To achieve this, the terminal 5' phosphate requires

removal, which is done with the aid of a

phosphatase enzyme.

The pre-mRNA processing at the 3' end of the

RNA molecule involves cleavage of its 3' end and

then the addition of about 250 adenine residues to

form a poly(A) tail.


RNA splicing

RNA splicing is the process by which introns,

regions of RNA that do not code for proteins,

are removed from the pre-mRNA and the

remaining exons connected to re-form a single

continuous molecule.

exons (coding or important sequences

involved in translation), and introns (non-

coding sequences)


mRNA degradation


Different mRNAs within the same cell have

distinct lifetimes (stabilities).

In bacterial cells, individual mRNAs can survive

from seconds to more than an hour; in

mammalian cells, mRNA lifetimes range from

several minutes to days.

The greater the stability of an mRNA the more

protein may be produced from that mRNA.

The limited lifetime of mRNA enables a cell to

alter protein synthesis rapidly in response to its

changing needs.


Prokaryotic mRNA degradation

In general, in prokaryotes the lifetime ofmRNA is much shorter than in eukaryotes.

Prokaryotes degrade messages by using acombination of ribonucleases, includingendonucleases, 3' exonucleases, and 5'exonucleases.

Eukaryotic mRNA turnover

Inside eukaryotic cells, there is a balancebetween the processes of translation andmRNA decay.


AU-rich element decay

Eukaryotic messages are subject to surveillance by

nonsense mediated decay (NMD), which checks for the

presence of premature stop codons (nonsense codons) in the

message.

Small interfering RNA (siRNA)

In metazoans, small interfering RNAs (siRNAs) processed

by Dicer are incorporated into a complex known as the

RNA-induced silencing complex or RISC.

This complex contains an endonuclease that cleaves

perfectly complementary messages to which the siRNA

binds. The resulting mRNA fragments are then destroyed by

exonucleases


MicroRNA (miRNA)

MicroRNAs (miRNAs) are small RNAs thattypically are partially complementary tosequences in metazoan messenger RNAs.

Binding of a miRNA to a message can represstranslation of that message and accelerate poly(A)tail removal, thereby hastening mRNAdegradation.

Other decay mechanisms

There are other ways by which messages can bedegraded, including non-stop decay and silencingby Piwi-interacting RNA (piRNA)


Translational regulation Translational regulation refers to the control of the levels of

protein synthesized from its mRNA.

The corresponding mechanisms are primarily targeted on

the control of ribosome recruitment on the initiation codon,

but can also involve modulation of the elongation or

termination of protein synthesis.

In most cases, translational regulation involves specific

RNA secondary structures on the mRNA.

An example of regulation at the level of initiation is the

phosphorylation of the translation initiation factor eIF2.

An example of regulation at the level of termination is

functional translational read through of the lactate

dehydrogenase gene LDHB.


Translational Regulation in Plants

Translation in plants is tightly regulated as in animals,however, it is not as well understood as transcriptionalregulation.

There are several levels of regulation includingtranslation initiation, mRNA turnover and ribosomeloading.

Recent studies have shown that translation is also underthe control of the circadian clock. Like transcription,the translation state of numerous mRNAs changes overthe diel cycle (day night period).


REFERENCES

Applied genetics recent trends and techniques

by C.Emmanuel

Gene expression system by Joseph

M.Pernander

Medical genetics by Dorian J.Pritchard


genetics- gene expression and regulations

Health & Medicine