Introduction to Molecular Genetics Studiju materiāli / MolekularasBiologijas / Ievads MolGen / EN

MUTATION – a permanent (heritable) change in the DNA sequence Genetic STABILITY Genetic CHANGES SIN o temporary o permanent o somatic o germline o spontaneous (background) o induced o DNA replication o chemical factors o physical factors o transposons, viruses o neutral o functional - coding sequences - regulatory regions - loss-of-function - gain-of-function - bad and lethal - adaptive

Single base changes (point mutations) transitions transversions Molecular Biology of the Gene, 5th Edition The most frequent types of mutations Insertions / Deletions Changes in the number of simple sequence repeats Vogel and Motulsky’s Human Genetics, 4th Edition; modified.

Cells make large investments in DNA repair Molecular Biology of the Gene, 5th Edition

Replication is highly accurate, but not perfect Molecular Biology of the Gene, 5th Edition

Eukaryotes MISMATCH REPAIR corrects replication errors

Some replication errors still escape detection: MUTATION Molecular Biology of the Gene, 5th Edition MUTATION REPLICATION ERROR (still can be repaired) (a permanent change; can not be repaired)

hydrolatic attack oxidative damage uncontrolled methylation DNA frequently suffers from spontaneous alterations and chemical damage

Depurination and deamination are the most frequent spontaneous chemical reactions causing DNA damage

UV induces the formation of pyrimidine dimers

If uncorrected, these changes may result in mutations

Molecular Biology of the Gene, 5th Edition Some damages can be repaired through simple reversal, eg, photoreactivation of pyrimidine dimers by DNA photolyase

Base Excision Repair and Nucleotide Excision Repair are the two major pathways for repairing DNA damage

Two distinct mechanisms repair double-strand breaks

Molecular Biology of the Cell, 5th Edition (modified)

CANCER is a disease of genetic changes

RECOMBINATION Homologous (or general) –between two homologous DNA sequences Site-Specific –between two defined DNA sequence elements Transposition –between specific DNA element and any DNA site

Homologous recombination repairs strand breaks

Homologous recombination is crucial for meiosis Molecular Biology of the cell, 5th Edition

Homologous recombination is crucial for meiosis and creates new combinations of alleles IZOMERIZATION OF A HOLLIDAY JUNCTION

Homologous recombination in meiosis may result in Gene Conversion

Gene conversion by mismatch repair

Modern Genetic Analysis Bacteria have no sex, yet they have several ways for receiving genes from other bacteria

Bacterial viruses (bacteriophages) reproduce by a lytic or a lysogenic cycle Phage – temperate Bacteria – lysogenic Life The Science of Biology, 7th Edition

Lysogenic cycle involves integration of phage into the host chromosome by SITE-SPECIFIC RECOMBINATION Molecular Biology of the Gene, 5th Edition

The insertion of bacteriophage genome into the host DNA (recombination sites) (recombinase)

Site-specific recombination is mediated by specific sequences Molecular Biology of the Gene, 5th Edition

Types of site-specific recombination Molecular Biology of the Gene, 5th Edition the darker red and blue boxes – the recombinase recognition sequences the black arrows – the crossover regions recombination sites

Unlike elements moving by site-specific recombination, TRANSPOSONS can insert into any DNA sequence Molecular Biology of the Gene, 5th Edition (transposon)

Molecular Biology of the Gene, 5th Edition Transposons can move with or without duplication

There are three principal classes of transposable elements Molecular Biology of the Gene, 5th Edition (LTR retrotransposons or elements) P - promoter LTR - long terminal repeat RT - reverse transcriptase UTR – untranslated region ORF - open reading frame (nonviral retrotransposons) ORF 2 = RT and endonuclease (DNA-only transposons)

The ‘cut-and-paste’ transposition of DNA transposons (synaptic complex)

Bacterial transposons often carry antibiotic resistance genes

RETROTRANSPOSITION involves RNA intermediate Genomes, 2nd Edition

Molecular Biology of the Gene, 5th Edition Retrotransposition of LTR elements cDNA – copy DNA – DNA that has been made by reverse transcription from RNA

The life cycle of a retrovirus

Content of transposons differs among different organisms Molecular Biology of the Gene, 5th Edition

Transposons (mainly – retrotransposons) occupy nearly half of the human genome

The genetic basis of antibody diversity

Antibody diversity is based on unusual composition of immunoglobulin genes and the V(D)J recombination

The V(D)J recombination pathway Molecular Biology of the Gene, 6th Edition recombinases

We have arrived at the GENE EXPRESSION GENE EXPRESSION Molecular Biology of the Gene, 6th Edition