G. Mendel Hereditary factors

W.Johannsen,

1909

The gene – hereditary unit located in

chromosomes

G.W.Beadle,

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

Ingram, 1957Hypotheses “One gene – onepolypeptide”

Actual conceptsThe gene – a sequence of DNA resopnsable

for synthesis of macromolecules

Evolution of knowledge about geneEvolution of knowledge about gene

DNA Double stranded molecule;

Polynucleotide chains;

Contains information about RNAs and proteins.

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 expressionGene expression

DNA mRNA ProteinDNA mRNA Protein

55’’--ATTGCAATTGCAAAGATTACCATGTGATTACCATGT--3’ 3’ Coding strandCoding strand ((untranscribeduntranscribed))

3’3’--TAACGTTCTAATGGTACATAACGTTCTAATGGTACA--5’ 5’ Template strandTemplate strand ((transcribedtranscribed))

Transcription (RNA polymerase)

55’’--AUUAUUGCAGCAAAGAGAUUAUUACCACCAUGUUGU--3’3’ mRNAmRNA

Translation (tRNA, ribosomes)

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

Definition:Gene – a fragment of polynucleotide chain

of DNA which contains information about

synthesis of:

one polypeptide or

some polypeptides or

a functional RNA (rRNA, tRNA, snRNA)

Gene expressionGene expression

DNADNA

rRNArRNA

tRNAtRNA

mRNAmRNA ProteinProtein

Classification:

Ist class genes – encode 5,8S, 18S and 28S rRNA;

IInd class genes = structural – encode mRNA

proteins;

IIIrd 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.

- 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 – 2610 genes

- Chromosome Y contains – 255 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

LengthLength, , kbkb %%

Up toUp to 1010 23,323,3

1010--2525 35,635,6

2525--5050 20,220,2

5151--100100 13,013,0

101101--500500 6,76,7

overover 500500 1,21,2

Distribution of human genes by lengthDistribution of human genes by length

General structure of the gene

Central region – coding region;

Regulatory regions:

proximal – PROMOTER

distal – TERMINATOR

Modulation sequences

Functions: At molecular level – control of polypeptide’s synthesis

functional protein

At cellular level – production of a normal cellular

structure, metabolic chain, signaling chain, etc.

At tissue level – realization of a specific function

(respiration, digestion, contraction, etc.)

At organism level – a specific trait (character)

IInd class genes = structural(25% of nuclear DNA)

Encode one or some polypeptides;

Form monocistronic transcription units;

Have a mosaic structure (exon/intron);

May 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.

Each cell contains a complete set of genes Each cell contains a complete set of genes

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

Expression Expression –– only only 10% 10% of all genesof all genes

Permanent Permanent

expressionexpression

rRNA genesrRNA genes

tRNA genestRNA genes

HHouse keepingouse keeping

genesgenes

TemporaryTemporaryexpressionexpression

depending ondepending on::

-- tissuetissue;;

-- ontogenetic ontogenetic periodperiod;;

-- cell cycle cell cycle periodperiod;;

-- environment environment factorsfactors

Absence of Absence of

expressionexpression

pseudogenespseudogenes

Types of structural genes House keeping – genes that encode for

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.

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 IInd class genes structure

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

TAFs

TFIID

TFIIA

TFIIB

RNA-polymerase II

TFIIF

TFIIE

Initiation of transcription of IIInitiation of transcription of IIndnd class genesclass genes

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 IInd 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 Conservative boxes in structure of

eukaryotic promoterseukaryotic promoters

StructureStructure SequenceSequence PositionPositionLength of Length of

bound DNAbound DNA

Transcription Transcription

factorsfactors

TATATATA--boxbox TATAAAATATAAAA -- 3030 1010 pp..nn.. TBPTBP

CAATCAAT--boxbox GGCCAATCTGGCCAATCT -- 7575 2222 pp..nn.. CTF/NFCTF/NF11

GCGC--boxbox GGGCGGGGGCGG -- 9090 2020 pp..nn.. SPSP11

OctamerOctamer ATTTGCATATTTGCAT2020 pp..nn.. OctOct11,, OctOct22

Structure of promoter in structural genes in eukaryotes

(IInd 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 exones may be

removed.

Introns

Non-coding sequences of structural genes that

separate exons;

Are found in pro-mRNA but not in mRNA;

Are transcribed but not translated;

During splicing all introns are removed;

Structure of terminatorStructure of terminator

Structure of transcription unit which contains rRNA

genes in eukaryotes (Ist class genes)

• Promoter (-45 ... +20)

• Gene 18S

• Gene 5,8S

• Gene 28S

• Terminator

n

Structure of 5S rRNA genes

Organization of IIIrd class genes

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

•Genes for tRNA / rRNA 5S

•Terminator

n

Structure of operon in prokaryotes

Human mitochondrial genom

Mobile genetic elements =

TRANSPOSONES

TypeType Main geneMain geneType of Type of

transpositiontranspositionExemplesExemples

DNA DNA

transposonestransposonesTransposaseTransposase

Transposition through Transposition through

excision or replicationexcision or replication

Tn (Tn (bacterialbacterial))

P (P (DrosophilaDrosophila))

Retrovirus like Retrovirus like

transposonestransposones

ReversRevers--

transcriptase transcriptase

(revertase)(revertase)

Transposition through Transposition through

RNA produced on the RNA produced on the

basis of promoters basis of promoters

located in located in LTRLTR

THETHE--1 (1 (humanhuman))

Tz (Tz (yeastyeast))

RetroRetro--

transposones transposones

ReversRevers--

transcriptase transcriptase

(revertase)(revertase)

Transposition through Transposition through

RNA produced on the RNA produced on the

basis of neighbor basis of neighbor

promoters promoters

L1 L1 -- LINEsLINEs

Biological role of transposones

Site-specific recombination

Individual polymorphism of DNA

Insertional mutagenesis

Genome instability fragile sites in DNA

Evolution of genomes

Medical importance of transposones

Changes in structure / function of structural genes

genetic diseases (hemophilia B, epilepsy, retinita

pigmentosum, etc)

Variability of pathogen agents resistance against

antibiotics and immune system