Introduction To Molecular Biology

Introduction To Molecular Biology Salwa Hassan Teama

Molecular Biology

Molecular biology ; the study of gene structure and functions at the molecular level to understand the molecular basis of hereditary, genetic variation, and the expression patterns of genes.

The Genome

T he totality of genetic information and is encoded in the DNA (or, for some viruses, RNA ).

All living things are grouped into three domain:

Eukaryotes;

Prokaryotes and

Archaea.

The Cell

The cell is the smallest living unit, the basic structural and functional unit of all living things. Some organisms, such as most bacteria, are unicellular consist of a single cell. Other organisms, such as humans, are multicellular.

Eukaryotic Cell

Eukaryotes are generally more advanced than prokaryotes .

Eukaryotic cells are found in animals; plants; fungi and protists cell.

Cell with a true nucleus , where the genetic material is surrounded by a membrane;

Eukaryotic genome is more complex than that of prokaryotes and distributed among multiple chromosomes;

Eukaryotic DNA is linear ;

Eukaryotic DNA is complexed with proteins called histones ;

Numerous membrane - bound organelles;

Complex internal structure ;

Cell division by mitosis.

Eukaryotic Cell

Prokaryotic Cell

Unicellular organisms, found in all environments . These include bacteria and archaea.

Without a nucleus ; no nuclear membrane ( genetic material dispersed throughout cytoplasm;

No membrane - bound organelles ;

Cell contains only one circular DNA molecule contained in the cytoplasm;

DNA is naked (no histone);

Simple internal structure ; and

Cell division by simple binary fission .

Archaea are prokaryotes; organisms without nucleus but some aspect of their molecular biology are more similar to those of eukaryotes.

Archaea

Deoxyribonucleic Acid (DNA), the genetic material of all cellular organisms and most viruses, the gigantic molecule which is used to encode genetic information for all life on Earth.

The Chromosome

The Chromosome, the storage place for all genetic information, the number of chromosomes varies from one species to another.

In normal human cell DNA contained in the nucleus, arranged in 23 pairs of chromosomes.

22 pairs of chromosomes (autosomes); the 23 chromosome pair determines the sex of individual and is composed of either two (x) chromosomes (female) or an (x) and (y) chromosome (male).

Genes located on chromosome on it's place or locus .

Central Dogma of Molecular Biology

General Structure of Nucleic Acid

DNA and RNA are long chain polymers of small compound called nucleotides. Each nucleotide is composed of a base ; sugar (ribose in RNA or deoxyribose in DNA ) and a phosphate group . The phosphate joins the sugars in a DNA or RNA chain through their 5` and 3` hydroxyl group by phosphodiester bonds.

Genomic DNA Organization

Eukaryotic genes: DNA molecules complexed with other proteins especially basic proteins called histones, to form a substance known as chromatin . A human cell contains about 2 meters of DNA . DNA in body could stretch to the sun and back almost 100 times. So it is tightly packed.

Eukaryotic chromatin is folded in several ways. The first order of folding involves structures called nucleosomes , which have a core of histones, around which the DNA winds ( four pairs of core histones H2A, H2B,H3 and H4 in a wedge shaped disc, around it wrapped a stretch of 147 bp of DNA ). The second order of chromatin folding is the 30 nm fiber. The third order of chromatin condensation appears to involve formation of a radial loop structure in eukaryotic chromosomes.

Genomic DNA Organization

DNA Replication

DNA Replication: The DNA (all gene duplication; the transfer the genetic information from a parent to a daughter cell ; the DNA base sequence are precisely copied).

Semiconservative replication describes the method by which DNA is replicated in all known cells. This method of replication was one of three proposed models of DNA replication

DNA Forms

A Form

B Form

Z Form

The Genetic Code

The purine and pyrmidine bases of the DNA molecule are the letters or alphabet of the genetic code. All information contained in DNA represented by four letters: A,T,C,G.

Three nucleotides of DNA (1st, 2nd and 3rd) form triplet codons, there are 64 possible codons, most amino acids have more than one possible codon. Out of the 64 possible 3-base codons, 61 specify amino acids; the other three are stop signals (UAG, UAA, or UGA).

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Series of codons in part of a mRNA molecule. Each codon consists of three nucleotides , representing a single amino acid .

The sequence of codons in the mRNA defines the primary structure of the final protein.

The RNA

Three major classes of RNA : messenger (mRNA), transfer (tRNA) and ribosomal (rRNA). Minor classes of RNA include small nuclear RNA ; small nucleolar RNA;………..

Ribosomal RNA (rRNA) is the central component of the ribosome, the function of the rRNA is to provide a mechanism for decoding mRNA into amino acids and to interact with the tRNAs during translation by providing peptidyl transferase activity.

mRNA molecules represent transcripts of structural genes that encode all the information necessary for the synthesis of a single type polypeptide of protein.

mRNA ; intermediate carrier of genetic information; deliver genetic information to the cytoplasm where protein synthesis take place.

All tRNAs share a common secondary structure represented by a coverleaf. They have four- base paired stems defining three stem loops (the D loop, anticodon loop, and T loop) and the acceptor stem to which amino acids are added in the charging step.

RNA molecules that carry amino acids to the growing polypeptide.

The Protein

Proteins are the basic building materials of a cell, made by cell itself; the final product of most genes.

Proteins are chain like polymers of a few or many thousands of amino acids. Amino acids are represented by codons, which are 3-nucleotide RNA sequences. Amino acids joined together by peptide bonds ( polypeptide) . Proteins can be composed of one or more polypeptide chains.

Proteins have many functions: provide structure that help cells integrity and shape (e.g. collagen in bone); serve as enzymes and hormones; bind and carry substance and control of activities of genes….

Four Level of Protein Structure

Ribosomes

Ribosomes; Factory for protein synthesis; are composed of ribosomal RNA and ribosomal proteins (known as a Ribonucleoproteinor RNP). They translate messenger RNA (mRNA) to build polypeptide chains using amino acids delivered by transfer RNA (tRNA) .

Eukaryotic ribosomes are larger. They consist of two subunits; a 60S subunit holds (three rRNAs 5S, 5.8S, 28S and about 40 proteins) and a 40S subunit contains ( an18S rRNA and about 30 proteins) , which come together to form an 80S particle compared with prokaryotic 70S ribosome .

Most mRNA are translated by more than one ribosome at a time; the result, a structure in which many ribosomes translate an mRNA in tandem, is called a polysomes.

Common Tools of Molecular Biology