DNA and RNA: Instructions for Life

Nucleic Acids

  • The genetic code in cells is made up of DNA.
  • DNA specifies the construction of proteins and the generation of RNA.
  • DNA and RNA are nucleotide polymers.
Structure of nucleic acids
  • Nucleic acids are information molecules in cells.
  • Deoxyribonucleic acid is the most famous nucleic acid.
  • Ribonucleic acid is less known and very important in cells.
  • Nucleic acid polymers are made up of repeating nucleotide monomers.
  • The structure of nucleic acid is complex because of the following reasons:
    • The nucleic acid monomers are nucleotides. Nucleotides contain 3 components which makes them complicated before the formation of chains.
    • Nucleic acids can be double-stranded or single-stranded.  Joining nucleotides together leads to the formation of polynucleotide chains.
    • RNA has a single polynucleotide chain while DNA has two double -helix polynucleotide chains.


  • Nucleotides are built from 3 components;
    • Pentose sugar (5 carbon) – DNA has deoxyribose sugar while RNA has a ribose sugar
    • Nitrogenous base – It is a double ring structure or a single ring structure containing many nitrogen atoms
    • Phosphate group – Contains a phosphorus atom that is surrounded by atoms of oxygen. They have a negative charge which makes nucleic acids negative in charge.
  • Carbon atoms on nucleotides  are numbered using the following system:
    • The sugar carbon atoms are numbered in a sequence from one to five.
    • The numbered carbon atoms are awarded a prime designation resulting in 1′, 2′, 3′,4′ and 5′.
    • Oneprime carbon is the first carbon.
    • All other carbon atoms are numbered consecutively.
The naming of nucleotide bases
  • There are five nitrogenous bases found on nucleotides. This includes the following:
    • Adenine (A)
    • Guanine (G)
    • Cytosine (C)
    • Thymine (T)
    • Uracil(U)
  • Nucleotides vary depending on the nitrogenous base attached to it.
  • Nitrogenous bases fall into two categories:
    • Pyrimidine-  Contain a single ring structure and two nitrogen atoms.
    • Purines- contain 2 fused rings with each ring containing two nitrogen atoms.

DNA and RNA synthesis

  • Nucleotides are joined together by a condensation reaction.
  • Repeating the process leads to the formation of a polynucleotide.
  • A primary structure is formed by the type and number of nucleotides.
Differences between RNA and DNA
  • DNA has thymine nitrogenous base while RNA has uracil base.
  • DNA nucleotides have a deoxyribose sugar while RNA nucleotides have a ribose sugar.
  • RNA is single-stranded while DNA is double-stranded.

The DNA double helix  

  • The DNA secondary structure is a double helix.
  • Two chains of polynucleotides join to form a molecule with a twisted ladder-like shape.
  • The sides of the ladder represent the Sugar phosphate backbones.
  • Nitrogenous bases project from the sugar backbone.
  • The two double helix chains are antiparallel.
  • The antiparallel strands are joined by hydrogen bonds between bases.
    • Adenine (A) joins to Thymine (T) by hydrogen bonds.
    • Cytosine (C) joins Guanine (G) by hydrogen bonds.

The function of DNA and RNA

  • Protein structure; the structure of proteins is determined by the DNA nucleotide sequence. Protein is generated from the process of transcription and translation.
  • RNA structure; the structure of RNA id determined by the DNA nucleotide sequence. The process of transcription generates nucleotides required to build RNA.
  • DNA regulation; it happens through the interaction of DNA binding proteins with special DNA regulatory sequences.

Types of RNA and their function

  • Messenger RNA (mRNA) carries a protein structure code from DNA to ribosomes where proteins are generated.
  • Transfer RNA (tRNA) decodes the mRNA message by allocating an amino acid that matches to the mRNA code.
  • Ribosomal RNA (rRNA) is part of the structure of the ribosome.
RNARibonucleic acid
Central DogmaDNA➞mRNA➞Protein
Structure of RNASingle strand of nucleotides, ribose sugar, uracil (instead of thymine)
Adenine pairs with…Uracil
Uracil pairs with…Adenine
Guanine pairs with…Cytosine
Cytosine pairs with…Guanine
Function of mRNACarries genetic information from DNA in the nucleus to the cytosol in eukaryotic cells
Structure of mRNASingle uncoiled chain
Function of tRNAAmino acids bind to it and it carries the amino acid to the ribosome for protein synthesis
Shape of tRNASingle chain (about 80 nucleotides) in hairpin shape
Number of tRNA varieties45
mRNAMessenger RNA
tRNATransfer RNA
Most abundant form of RNArRNA
Function of rRNASite of protein synthesis
Structure of rRNAGlobular shape composed of rRNA and proteins
TranscriptionTransfer of genetic information from DNA to RNA
Location of transcriptionIn the nucleus
DNA separationAt the beginning of the gene
Enzyme used in transcriptionRNA Polymerase
Role of RNA Polymerase in transcriptionBinds to promoter region and causes DNA to separate
Promoter RegionNucleotide sequence at the beginning of a gene, shows RNA Polymerase where to bind
TATA BoxSeries of Thymine and Adenine on the DNA template
Template strands used in transcriptionOne strand of DNA
Termination SignalSpecific sequence of nucleotides on the DNA template that signals the end of transcription
Transcription stopWhen RNA Polymerase hits the termination signal
RNA Polymerase action at termination signalBreaks off from the DNA template and the mRNA
Reusability of DNA templateYes, it can be reused to make more mRNA
Product of transcriptionmRNA (called transcript)
Destination of mRNA after transcriptionOut of the nucleus (through nuclear pores) and into the cytosol where it binds to a ribosome
CodonEvery 3 mRNA nucleotides
Codon’s functionCodes for a particular amino acid
Determinant of amino acid sequence in the proteinmRNA
Number of codons64
AUGCodon that signals the ribosome to start translation of mRNA
AUG codes forMethionine
Stop codonsUAA, UAG, and UGA
TranslationProcess of assembling polypeptides from the coded message of mRNA
Requirements for translationRibosome and tRNA
Ribosome’s role in translationHolds mRNA in place
tRNA’s role in translationTransports amino acids to mRNA
Composition of a ribosomerRNA and proteins
Number of binding sites on a ribosomeThree
Function of the first binding siteHolds mRNA transcript in place
Function of the second and third binding sitesAllow tRNA to bind to them
tRNA compositionA region where the amino acid binds and the opposite side contains an anticodon
AnticodonThe complement of a codon triplet on mRNA
Start of polypeptide assemblyRibosome attaches to the start codon (AUG)

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What are nucleic acids made up of?Nucleotides
What is the function of nucleic acids?Carry genetic information and instructions for cell functioning
What are the two main types of nucleic acids?DNA and RNA
Where is DNA found in eukaryotes?Nucleus, chloroplasts, and mitochondria
Is DNA enclosed in a nucleus in prokaryotes?No
What is the entire genetic content of a cell called?Genome
What forms eukaryotic chromosomes?DNA and histone proteins (chromatin)
What is the function of DNA?Control cellular activities by turning genes “on” or “off”
What is the function of RNA?Involved in protein synthesis and regulation
What are the nitrogenous bases in DNA?Adenine (A), guanine (G), cytosine (C), and thymine (T)
What are the nitrogenous bases in RNA?Adenine (A), guanine (G), cytosine (C), and uracil (U)
What is the sugar in RNA?Ribose
What is the sugar in DNA?Deoxyribose
What are the three components of a nucleotide?Nitrogenous base, pentose sugar, and phosphate group
What type of bonds link nucleotides in a nucleic acid?Phosphodiester linkages
What type of bonding holds the two strands of DNA together?Hydrogen bonds
What is the rule for base pairing in DNA?A can only pair with T, and G can only pair with C
How is DNA able to replicate?Complementary strands allow for each strand to serve as a template for the synthesis of a new strand
How is DNA compacted in prokaryotes?Supercoiling
How is DNA compacted in eukaryotes?Wrapping around histones to form nucleosomes and further compaction into 30 nm fibers
What are the two regions of eukaryotic chromosomes in interphase?Heterochromatin and euchromatin
What are the four major types of RNA?mRNA, rRNA, tRNA, and miRNA
What is the function of mRNA?Carries genetic code from DNA and serves as a template for protein synthesis
What is the function of rRNA?Major component of ribosomes for protein synthesis
What is the function of tRNA?Carries correct amino acids to the site of protein synthesis
What is the function of miRNA?Regulates gene expression
What is the Central Dogma of Molecular Biology?DNA transcribed into mRNA, which is then translated into protein

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