BASES OF NUCLEIC ACIDS
- PYRIMIDINE: single ring with two nitrogen atoms
- PURINE: two rings each with two nitrogen atoms.
BASIC because the nitrogen atoms are H+ acceptors.
- PHOSPHODIESTER BOND: the bond between the 3' OH of a sugar (nucleotide) and a phosphate group attached to the 5' C of another sugar.
- HYDROGEN BOND: Weak, noncovalent linkages with favorable electrostatic interactions.
- N-GLYCOSIDIC BOND: In DNA, refers to the N-C linkage between the 9' N of purine bases or 1' N of pyrimidine bases and the 1' C of the sugar group.
TYPES OF RNA
There are 3 major types of RNA:
- messenger RNA (mRNA - 5%): carries genetic info copied from DNA to the ribosomes --> TRANSCRIPTION phase.
- transfer RNA (tRNA - 15%): translates the genetic info in mRNA into the AA sequence for the protein --> TRANSLATION phase.
-ribosomal RNA (rRNA - 80%): most abundant type of RNA; combined with proteins to form ribosomes.
RNA & TRANSCRIPTION
The three types of RNA differ by function in the cell:
- ribosomal RNA makes up most of the structure of the ribosomes,
- messenger RNA carries genetic information from the DNA to the ribosomes
-transfer RNA places the correct amino acids in a growing peptide chain.
TRANSCRIPTION is the process by which RNA polymerase produces mRNA from one strand of DNA.
- Transcription factors at the promoter region bind RNA polymerase to DNA, which activates the transcription of a gene.
- The bases in the mRNA are complementary to the DNA, except A
in DNA is paired with U in RNA.
- The production of mRNA occurs when certain proteins are needed
in the cell.
Genetic Code & Protein Synthesis
The genetic code consists of a series of codons (sequences of 3 bases that specify the order for the amino acids in a protein).
- AUG codons signals the start of transcription
- codons UAG, UGA, and UAA signal it to stop the transcription.
Proteins are synthesized at the ribosomes in a translation process that includes 3 steps: initiation, chain elongation, and termination.
- During TRANSLATION, tRNAs bring the appropriate amino acids to the ribosome, and peptide bonds form to join the amino acids in a peptide chain.
- When the polypeptide is released, it takes on its secondary and tertiary structures and becomes a functional protein in the cell.
PYRAMIDINE & PURINE BASES
DNA contains the bases A, G, C, and T
RNA contains A, G, C, and U.
-Template Strand (ANTI-SENSE): info that codes for genes; contains anti-codons.
-Coding Strand (SENSE): complementary to anti-sense; contains codons (decoded by tRNA, which interacts with ribosome-bound mRNA).
-mRNA: made from template strand; has the same info as coding strand except T is replaced by U.
TYPES OF MUTATIONS
- POINT MUTATION: the replacement of one base in the template strand of DNA with another, causing a change in AA.
- SILENT MUTATION: when a point mutation does not change the amino acid.
- DELETION MUTATION: a base is deleted from codon, changing all that follow and producing a different sequence of AA.
- INSERTION MUTATION: a base is inserted into the normal order of bases, changing all the codons that follow and producing a different sequence of AA.
In reverse transcription:
- a retrovirus, which contains viral RNA but no viral DNA, enters a cell.
- the viral RNA uses reverse transcriptase to produce a viral DNA strand.
- the viral DNA strand forms a complementary DNA strand.
- the new DNA uses the nucleotides and enzymes in the host cell to synthesize new virus particles.
NUCLEOSIDES VS NUCLEOTIDES
NucleoSide = Base + Sugar
NucleoTide = Base + Sugar + Phosphate group(s)
In nucleoside, the nitrogen atom of the base is connected by a b-N-glycosidic bond to the C1 of the sugar.
A nucleotide forms a phosphoester bond with the C5 OH group of the sugar.
Pentose sugars in Nucleic Acids
- RNA: ribose
- DNA: deoxyribose; no hydroxyl group in C2
-helicase unwinds the parent DNA at several sections.
-DNA polymerase catalyzes the replication process at each of the open DNA sections called replication forks.
-polymerase moves in the 3′–5′direction, catalyzing the formation of new phosphodiester linkages.
-lagging strand (growing in the 5′–3′ direction) is synthesized in short sections called Okazaki fragments.
-DNA ligase joins the Okazaki fragments.
- During DNA replication, DNA polymerase makes new DNA strands along each of the original DNA strands that serve as templates.
- Complementary base pairing ensures the correct pairing of bases to give identical copies of the original DNA.
After a retrovirus injects its viral RNA into a cell, it forms a DNA strand by reverse transcription. The single-stranded DNA forms a double-stranded DNA called a provirus, which is incorporated into the host cell DNA. When the cell replicates, the provirus produces the viral RNA needed to produce more virus