A1.2: Nucleic Acids Flashcards
(26 cards)
What are nucleic acids?
Polymers
-> monomers: nucleotides
Used for storage/transfer of information
Two main functions:
Pass info between generations through DNA replication
Code for protein production
What are the 2 main types of nucleic acids?
DNA -> deoxyribnucleic acid
-> pass info
-> code for RNA
RNA -> ribonuceleic acid
-> code for proteins
-> 3 types: mRNA, rRNA, tRNA
Both polynucleotides -> long chains of nucleotides
DNA/RNA as the genetic material of living organisms
Carries genetic code in all living organisms -> universal -> all forms of life
Mainly found in nucleus in chromosomes
-> also in chloroplasts and mitochondria
RNA -> main component = ribosomes
-> role in protein synthesis
-> little RNA in nucleus and cytoplasm
Certain virus -> RNA instead of DNA
Structure of a nucleotide
RNA and DNA -> polymers made of repeated nucleotide units
Pentose sugar
-> ribose or deoxyribose
Nitrogenous base
- Purine:
-> Adenine and guanine
- Pyrimidine:
-> Cytosine
-> Thymine (in DNA) and Uracil (in RNA)
A phosphate group
-> acidic
-> neg charge
What are the main functions of nucleotides?
Formation of DNA and RNA
Form parts of molecules needed for metabolism
-> adenine -> adenosine triphosphate (ATP)
What type of bonds are formed within and between nucleotides?
Between: covalent bond
Within:
- base and sugar join with a glycosidic bond
- phosphate and sugar join with an ester bond
-> both require a condensation reaction
How do nucleotides bond with each other?
Condensation reaction:
Phosphate group of one nucleotide + pentose sugar of the next one -> covalent bond
Forms sugar-phosphate backbone
-> repeated and forms polymer -> nucleic acids/polynucleotides -> strand
-> DNA double strand
-> RNA single strand
4 different bases that can be join in any combination/sequence
How do the bases bond together?
Purine -> double ringed structure
Pyrimidine -> single ringed structure
Complementary base pairing:
Purine + pyrimidine -> base pair
-> A to T (DNA) / A to U (RNA)
-> 2 H bonds
-> C to G
-> 3 H bonds
RNA characteristics
Nucleotides -> ribose
Bases -> Uracil (x thymine)
Single stranded
Short molecule
-> length of 100s - 1000s nucleotides
Three types:
-> rRNA -> ribosomal
-> tRNA -> transfer (translation to transport specific AA coded in mRNA to ribosome -> protein synthesis)
-> mRNA -> messenger (formed by transcription, copy of DNA, read by ribosome -> polypeptide)
Adjacent RNA nucleotides -> linked via condensation reactions -> phosphodiester bonds
What is the shape/structure of DNA?
Double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs
Each DNA nucleotide -> 3’ and 5’ end
-> one strand 3-5
-> other strand 5-3
Bases towards interior
-> H bonds between bases hold together double helix
What are the difference between DNA and RNA?
DNA: thymine
RNA: uracil
DNA: deoxyribose
RNA: ribose
DNA: double strands
RNA: single stranded
DNA: long chains
RNA: short chains
T or F: complementary base pairing plays no role in the replication and expression of genetic information
False
Complementary base pairing ALLOWS DNA to be copied precisely during replication/copied accurately -> expressed accurately
Diversity of possible DNA base sequences
Despite only 4 bases -> can form large range of DNA base sequences
DNA -> limitless capacity for storing information
How can genetic information storage capacity be measured?
Number of gene contained within the DNA of an organism
Ex: humans 20,000
Dog 19,000
Rice plant 41,500
Number of base pairs contain within the genome of an organism
-> based pairs -> DNA with about 2m length -> in nucleus
-> basically DNA well packaged
-> enormous capacity for storing genetic data
Genetic code - forming a polypeptide
DNA -> carry genetic code as sequence of nitrogenous bases in nucleotides
1 strand DNA -> base sequence read by enzymes -> coding strand
Sequence of bases that form genes on coding strand -> determine order of AA in protein
Code read in triplets of bases (codons) -> 3 bases = 1 AA
Sequences of AA -> determine shape and function of protein
Universality of genetic code: evidence for common ancestor
Genetic code=universal -> almost all organisms use the same code
Same codon = same AA -> genetic code transferable between species
-> genetic engineering
Evidence for universal common ancestor
-> mutations led to changed in some of the base sueqences
Base sequences -> genome of organisms
-> split into coding sequences and non-coding sequences
-> Some of these sequences remain unchanged between all organisms -> conserved sequences
High conserved sequences -> genes that code for protein involved with transcription/translation/histone protein
TLDR:
Similarity of sequences -> living organisms shared a universal ancestor
Explain directionality of DNA
Nucleotides linked together -> phosphate group form bridge carbon 5 of the original sugar and the carbon 3 of the sugar its connecting to
DNA -> antiparallel -> 3-5 and 5-3
Directionally of DNA important in:
DNA replication
Transcription
Translation
How does the directionality of DNA play a role in transcription?
Genetic code on one of DNA strands -> transcribed into strand of mRNA
-> read in 3-5 direction by enzymes -> synthesize mRNA into 5-3 direction
mRNA -> cytoplasm -> ribosomes (translation) -> transcribe mRNA in 5-3
Base sequence of DNA -> determine the order of AA in the polypeptide chain
Directionality of RNA/DNA -> important to ensure genetic code is correctly copied/transcribed, translated
How did the purine+pyrimidine bonding become discovered?
How does the purine+pyrimidine help the stability of the double helix?
Francis Crick + James Watson worked to establish the double helix structure of DNA in 1953
Trial and error -> build model of DNA double helix where the different base pairs fit together correctly
A-T and C-G -> same length
DNA helix -> same 3D structure regardless of base sequence
Stability of double helix -> increased by hydrogen bond between base pairs
What is a nucleosome?
Only DNA -> DNA associated with histone protein to form chromatin
Strand of DNA coiled around a core of 8 histone proteins (octamer) -> bead like structure
Nucleosomes can be tagged with proteins -> promote or suppress transcription
Prophase (of mito or mei) -> condense even more -> supercoil into chromosomes -> prevent tangling/damage
DNA un-packaging -> transcription/replication require DNA to temporarily release from histone -> return to compact state
What is the structure of nucleosomes?
DNA takes 2 turns around histone core
Held in place by additional histone protein -> attached to linker DNA
-> Histone -> Pos charge
-> DNA (bc of phosphate group) -> neg charge
= bind tightly via electromagnetic tension
DNA molecules repeatedly wound around -> series of nucleosome
-> nucleosomes supercoil the DNA -> compact structure
-> also protect DNA
-> facilitate movement of chromosomes during cell division
Explain the Hershey-Chase experiment
DNA identified in 1869 -> but scientist still thought protein was the heritable material
1950s: Alfred Hershey and Martha Chase -> DNA (not protein) is a factor of heredity responsible for carrying genetic info from gen to gen
Used virus that infect bacteria -> used in experiment
-> used bc they were only DNA with protein coat
-> allowed bio molecule of heredity to be easily determined
What was the procedure of the Hershey-Chase experiment?
They took advantage of chemical difference:
DNA -> phosphorous
Protein -> sulfur
virus -> S-35 labelled protein coat or p-32 labelled DNA interior
Unlabelled bacteria -> infected separately with either type of virus
-> bacteria expected to contain heritable material following infection (S or P)
-> virus inject its genetic material into bacteria
Blender -> used to remove attached virus from bacteria
Centrifugation -> isolate bacteria
-> virus small so remained in supernatant
-> bacteria larger -> pellet
Only bacteria infected with P-32 labelled virus -> radioactive
Suggested that: DNA was transferred to bacteria and is the hereditary genetic material
Explains Chargaffs data
Erwin Chargaffanalysed theDNA composition of different organismsduring the 1930s and 1940s and made the following discoveries:
Number of purine bases = number of pyrimidine bases
Number of adenine bases = number of thymine bases
Number of guanine bases = number of cytosine bases
Meaning that:
Purine base can only pair with pyrimidine base (due to their sizes)
This forms foundation of complementary base pairing in DNA
