Nucleic acids Zoology - I Unit III.pdf

# Genetic Material ## Before the Discovery of DNA - in 1953 James Watson and Francis Crick discovered the structure of DNA - Deoxyribose Nucleic Acid was considered the basic building block of DNA - many years of observations and experiments led to the idea that DNA is the primary genetic material...

# Genetic Material ## Before the Discovery of DNA - in 1953 James Watson and Francis Crick discovered the structure of DNA - Deoxyribose Nucleic Acid was considered the basic building block of DNA - many years of observations and experiments led to the idea that DNA is the primary genetic material ## Discovery of Transformations - 1928 - Frederick Griffith discovered the phenomenon of transformations in bacteria - **Transformation:** The modification of a genome by receiving external DNA from a cell of different genotype. - It can also be said that transformation is the conversion of normal higher eukaryotic cells in tissue culture to a cancer-like state of uncontrolled division. - Griffith experimented with the bacterium *Streptococcus pneumoniae* - He used 2 strains of bacterium: - **S-Strain:** Virulent (causes disease) - A polysaccharide capsule surrounds the cells - Colonies appear smooth - **R-Strain:** Non-virulent (does not cause disease) - No polysaccharide capsule - Colonies appear rough - Griffith's Experiment: - Step 1: Heat-killed S-strain bacteria injected into mice - Mice lived - Step 2: Live R-strain bacteria injected into mice - Mice lived - Step 3: Heat-killed S-strain and Live R-strain bacteria injected into mice - Mice died - Live S-strain bacteria were recovered from the dead mice. - The R-strain bacteria were transformed by the heat-killed S-strain bacteria - The process was called **transformation**. ## Discovery of the Nature of Transforming Principle - 1944 - Oswald Avery, Colin MacLeod, and Maclyn McCarty determined that DNA is the transforming principle - They separated different classes of molecules in a cell - They tested each for transforming capacity and found that only DNA was effective - Thus, they concluded that DNA was the transforming principle. ## Hershey-Chase Experiment on Hereditary Material - Alfred Hershey and Martha Chase provided further evidence for DNA being the genetic material (1952) - Their experiment used T2 phage, a bacterial virus that infects *Escherichia coli* - T2 phage is made up of protein and DNA - They used radioisotopes to label the phage: - 32P to label DNA - 35S to label protein - Phage infected the *E. coli* bacteria - Then, they separated the phage ghosts (empty outer shells) from the bacteria by centrifugation. - They found that the phage DNA entered the bacteria and was incorporated into the new phages. - But the protein remained outside the bacteria. - Their results confirmed Avery's results and finally established that DNA is the hereditary material. ## Chemical Composition and Structure of Nucleic Acids - Nucleic acids are macromolecules present in the nucleus. - They are polymers of nucleotides. - A nucleotide is composed of: - A nitrogenous base - A pentose sugar - A phosphoric acid - There are two types of nucleic acids: DNA and RNA. - They differ in sugar and one of their bases: - **DNA:** Deoxyribose, thymine - **RNA:** Ribose, uracil ### Nitrogenous Bases - **Purines:** Adenine (A) and Guanine (G) - **Pyrimidines:** Cytosine (C), Thymine (T), and Uracil (U) ### Nucleosides - A nucleoside is a nitrogenous base linked to a pentose sugar. - They are named according to the sugar: - Ribonucleosides - if the pentose is ribose - Deoxyribosides - if the pentose is deoxyribose ### Nucleotides - A nucleotide is a nucleoside linked to a phosphate group. - They are named according to the base: - Adenylic acid or adenosine monophosphate (AMP) - Guanylic acid or guanosine monophosphate (GMP) - Cytidylic acid or cytidine monophosphate (CMP) - Uridylic acid or uridine monophosphate (UMP) ## Polynucleotides and 3'-5' Phosphodiester Linkage - A polynucleotide is a large polymer of nucleotides that are joined together - The nucleotides are linked by **phosphodiester bonds** - The phosphate of one nucleotide links to the 3' carbon of the sugar of the next nucleotide - The 5' carbon of the sugar is linked to the phosphate group - The backbone consists of alternating phosphates and pentoses - This gives the nucleic acid an acidic character - The nitrogenous bases project inwards toward the centre of the helix. ## Watson-Crick Model of DNA - James Watson and Francis Crick proposed a double helix model for DNA - Their model was based on: - **X-ray diffraction data:** Provided by Rosalind Franklin and Maurice Wilkins - **Chemical data:** Regarding base composition, provided by Erwin Chargaff - The two strands of DNA are antiparallel - There are ten nucleotides per turn of the helix - The bases are paired according to **Chargaff's rules:** - Adenine (A) pairs with Thymine (T) - Guanine (G) pairs with Cytosine (C) - The two strands are linked by **hydrogen bonds** between the bases - A-T pairs have 2 hydrogen bonds - G-C pairs have 3 hydrogen bonds - The double helix structure has two grooves: - **Major groove:** Wider and deeper - **Minor groove:** Narrower and shallower ## Types of DNA - **A-DNA:** 11 base pairs per turn, right-handed helix - Less common - **B-DNA:** 10 base pairs per turn, right-handed helix - Most common in cells - **Z-DNA:** Left-handed helix, zig-zag sugar-phosphate backbone - Found in certain regions of DNA, and is rich in guanine and cytosine - **Circular DNA:** Found in bacteria and some viruses ## DNA in Prokaryotes - Prokaryotes are single-celled organisms without a nucleus. - They have a single circular DNA molecule in the nucleoid region. - They also have small circular DNA molecules called **plasmids**. - **Plasmids:** Autonomous genetic elements that replicate and are inherited independently of the chromosome - They can be transferred between bacteria - They often carry genes for antibiotic resistance and other advantageous traits ## Organelle DNA (Extranuclear DNA) - Some DNA is found in organelles like mitochondria and chloroplasts. - This DNA is circular and replicates independently of the nuclear DNA. ### Mitochondria - Mitochondria are organelles found in all eukaryotic cells that perform oxidative phosphorylation. - They have their own DNA, ribosomes, and protein synthesis machinery. - Mitochondrial DNA is circular and encodes some of the mitochondrial proteins. ### Chloroplasts - Chloroplasts are organelles found in plant cells that perform photosynthesis. -They have their own DNA, ribosomes, and protein synthesis machinery. - Chloroplast DNA is circular and encodes some of the chloroplast proteins. ## RNA - Ribonucleic acid (RNA) is a polymer of nucleotides that is usually single-stranded. - Three main types of RNA are involved in protein synthesis: - **Ribosomal RNA (rRNA):** Makes up part of ribosomes, which are the sites of protein synthesis. - **Messenger RNA (mRNA):** Copies the genetic code from DNA, carries it to ribosomes, and directs protein synthesis. - **Transfer RNA (tRNA):** Carries specific amino acids to ribosomes, matching them to codons in mRNA. ## Transcription - Transcription is the process of copying a DNA sequence into an RNA molecule. - The RNA polymerase enzyme catalyzes this reaction. - In eukaryotes, there are three main types of RNA polymerase: - RNA polymerase I: Transcribes rRNA genes - RNA polymerase II: Transcribes mRNA and some snRNA genes - RNA polymerase III: Transcribes tRNA, 5S rRNA, and other small RNA genes - **Steps of eukaryotic Transcription:** - Initiation: - The RNA polymerase II complex binds to the promoter region of the gene - Several transcription factors are involved in this process - Elongation: - RNA polymerase II moves along the DNA template, unwinding and copying the template strand. - Termination: - The RNA polymerase II complex detaches from the DNA template, stopping transcription. ## Post-Transcriptional Processing of mRNA - In eukaryotes, newly synthesized mRNA undergoes three main stages of processing: - **Capping:** A methylated guanine nucleotide is added to the 5' end of the mRNA. - **Splicing:** Introns, which are non-coding regions of the mRNA, are removed. - **Polyadenylation:** A poly(A) tail is added to the 3' end of the mRNA. ## Replication - Replication is the process of copying the DNA molecule. - It is essential for cell division and is based on the complementary nature of the two DNA strands. - **Steps of DNA Replication:** - **Initiation:** The process begins at specific origins of replication, where the DNA double helix is unwound. - **Elongation:** DNA polymerase enzymes add nucleotides to a new strand, using the old strand as a template according to the base-pairing rules (A with T, C with G). - **Termination:** The replication process stops when the entire chromosome has been copied. - **Semiconservative Replication:** - Each new DNA molecule is composed of one old strand and one new strand. ## Flow of Genetic Information in Eukaryotic Cells - The central dogma of molecular biology describes the flow of genetic information in cells: - **DNA → RNA → Protein** - **Replication:** DNA is copied to make more DNA. - **Transcription:** DNA is copied to make RNA. - **Translation:** RNA is used to make proteins. - These processes are regulated by complex mechanisms to ensure that the correct genes are expressed at the right time and place.