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G. Mendel Hereditary factors

W.Johannsen,

1909The gene – hereditary unit located in chromosomes

G.W.Beadle,

E.L.Tatum, 1945Hypotheses “One gene – one enzyme”

Ingram, 1957 Hypotheses “One gene – one polypeptide”

Actual conceptsThe gene – a sequence of DNA responsible for

synthesis of macromolecules

Evolution of knowledge about gene

DNA

Double stranded molecule;

Polynucleotide chains;

Contains information about RNAs and proteins.

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Genetic code - DNA:

Letters: A, G, C, T

Words (one codon one amino acid): AAG - lys

AGC - ser

GCA - ala

TTC – phe

TAG – stop

Phrases: 5' AAGAGCGCATTCTAG 3'

lys – ser – ala – phe – stop

Gene expression

DNA mRNA Protein

5’-ATTGCAAGATTACCATGT-3’ Coding strand (untranscribed)

3’-TAACGTTCTAATGGTACA-5’ Template strand (transcribed)

Transcription (RNA polymerase)

5’-AUUGCAAGAUUACCAUGU-3’ mRNA

Translation (tRNA, ribosomes)

Leu – Ala – Arg – Leu – Pro – Cys polypeptide

Definition:Gene – a fragment of polynucleotide chain

of DNA which contains information about

synthesis of:

one polypeptide or

several polypeptides or

a functional RNA (rRNA, tRNA, snRNA)

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Gene expression

DNA

rRNA

tRNA

mRNA Protein

Classification:

1st class genes

• encode 5,8S, 18S and 28S rRNA;

2nd class genes= structural

• encodemRNAproteins;

3rd class genes

• encode tRNA, 5S rRNA.

Gene’s localization:Genes are located in DNA molecules;

Genes consist of unique or repeated sequences;

The genes from one molecule of DNA are separated by non-coding sequences – spacers;

There are no morphological borders each gene has only functional frontiers;

The length of genes is different.

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- The dimensions of human genome – 3,164 x 109 bp

- 2% of human genome encode for proteins

- Number of genes - 30000-40000

- Chromosome 1 contains – 3380 genes

- Chromosome Y contains – 397 genes

- Known function – 50% human studied genes

- Average length of gene – 3000 bp

- gene for β-globin – 1,5 kb

- gene for insulin – 1,7 kb

- gene for catalase – 34 kb

- gene for dystrophin - 2,4 Mb

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Length, kb %

Up to 10 23,3

10-25 35,6

25-50 20,2

51-100 13,0

101-500 6,7

over 500 1,2

Distribution of human genes by length

General structure of the

transcription unit

Central region – coding region;

Regulatory regions:

proximal – PROMOTER

distal – TERMINATOR

± Modulation sequences

Functions:

Molecular level

• control of polypeptide’s synthesis functional protein

Cellular level

• production of a normal cellular structure, metabolic chain, signaling chain, etc.

Tissue level

• realization of a specific function (respiration, digestion, contraction, etc.)

Organismlevel

• a specific trait (character)

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Each cell contains a complete set of genes

(30-40000 pairs of genes in all 46 molecules of DNA)

Expression – only 10% of all genes

Permanent

expression

rRNA genes

tRNA genes

House keeping

genes

Temporaryexpression

depending on:

tissue;

ontogenetic period;

cell cycle period;

environment factors

No expression

pseudogenes

The 2nd class genes = structural(25% of nuclear DNA)

Encode one or several polypeptides;

Form monocistronic transcription units;

Have a mosaic structure (exon/intron);

Could be transcribed:

• In all cells (house keeping genes)

• Specific, depending on type of cell, age, factors;

Are transcribed by RNA-polymerase II in a primary transcript – pro-mRNA;

Are numerous, usually unique and heterogeneous;

May form repetitive or non-repetitive families of genes;

Present individual polymorphisms.

Types of structural genes

House keeping –genes that

encode indispensable cell proteins, active in

all cells, in all periods of life;

Tissue-specific –genes that encode for

proteins require for tissue

specialization;

Regulatory of ontogenesis;

Dependent on environment

factors.

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Gene families

Repetitive gene’s family: a family of identical genes

Non-repetitive gene’s family: a family of genes of related structure and usually related function

Peculiarities of the 2nd class genes structure

-10-20-30-40 +1 +10 +20 +30

TAFs

TFIID

TFIIA

TFIIB

RNA-polymerase II

TFIIF

TFIIE

Initiation of transcription of the 2nd class genes

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Regions of structural genes

Promoter:

• TATA box (-20, -30)

• CAAT box (-70, - 100)

• Tissue-specific boxes( - )

Coding region:

• site +1, leader sequence

• exon1/intron/exon2/intron/.../exonn

Terminator

• Site of polyadenilation

Enhancers and silencers

Promoter of the 2nd class genes

Controls the initiation of transcription:

• Activation of gene;

• Fixing of TF and RNA-polymerase II;

• Identification of (+1) and transcribed strand;

• Directing of RNA-polymerase II.

Is not transcribed;

In different genes promoters contain different specific boxes;

Mutations in promoter may induce gene inactivation.

Conservative boxes in structure of

eukaryotic promoters

Structure Sequence PositionLength of

bound DNATranscription

factors

TATA-box TATAAAA - 30 10 p.n. TBP

CAAT-box GGCCAATCT - 75 22 p.n. CTF/NF1

GC-box GGGCGG - 90 20 p.n. SP1

Octamer ATTTGCAT20 p.n. Oct1, Oct2

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Structure of promoter in structural genes in eukaryotes

(2nd class genes)

Structure of promoter in structural genes in prokaryotes

Interaction promoter-enhancer

Exons

Sequences of structural genes that encode polypeptide sequences;

Are found in pro-mRNA and mRNA;

Are transcribed and translated;

Each exon encodes a region of protein;

During alternative splicing some exons may be removed.

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Introns

Non-coding sequences of structural genes that separate

exons;

Are found in pro-mRNA but not in mRNA;

Are transcribed but nottranslated;

During splicing all introns are removed;

Structure of terminator

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Structure of transcription unit which contains rRNA

genes in eukaryotes (1st class genes)

• Promoter (-45 ... +20)

• Gene 18S

• Gene 5,8S

• Gene 28S

• Terminator

n

Structure of 5S rRNA genes

Organization of the 3rd class genes

•Promoter – A box (+ 55) and B box (+ 80)

•Genes for tRNA / rRNA 5S

•Terminator

n

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Structure of operon in prokaryotes

Human mitochondrial genom

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Mobile genetic elements =

TRANSPOSONS

Type Main geneType of

transpositionExemples

DNA

transposonsTransposase

Transposition through

excision or replication

Tn (bacterial)

P (Drosophila)

Retrovirus like

transposons

Revers-

transcriptase

(revertase)

Transposition through

RNA produced on the

basis of promoters

located in LTR

THE-1 (human)

Ty (yeast)

Retro-

transposons

Revers-

transcriptase

(revertase)

Transposition through

RNA produced on the

basis of neighbor

promoters

L1 - LINEs

Biological role of transposons

Site-specific recombination

Individual polymorphism

of DNA

Insertional mutagenesis

Genome instability

fragile sites in DNA

Evolution of genomes

Medical importance of transposons

Changes in structure / function of structural genes

genetic diseases (hemophilia B, epilepsy, retinita

pigmentosum, etc)

Variability of pathogen agents resistance to antibiotics

and immune system