A1.2: Nucleic Acids Flashcards
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
What is the problem of induction?
Inductive scientific method :
-> scientist making observation and collecting raw data
-> After data analysis
-> hypothesis formed
-> tested by suitable investigation
-> general conclusions being drawn based on observations
-> data from past used to create general predictions about the future (assume future will be the same)
Therefore:
Possible to prove a hypothesis generated by inductive reasoning as true
Cannot be sure observations made in past = observations in the future
= problem of induction
-> why scientific theories -> tentative
Even if many investigation support hypothesis -> can be proven incorrect in future
For this reason -> Karl Popper (philosopher) suggest new scientific knowledge in not gained by inductive steps but rather by the falsification of existing hypotheses
Explain the falsification of the tetranucleotide hypothesis
the biochemistPhoebus Levenediscovered thepentose sugarsof DNA and RNA in the early 1900’s
-> suggested that the structure of nucleic acid was arepeating tetramer unitwhich he called a nucleotide
-> this was called thetetranucleotide hypothesis
At the time of his research -> limitations to analytic techniques available -> difficult to determine relative amount of nucleotides present
Tetra nucleotide hypothesis -> falsified by Chargaffs data (late 1940s)
Further disproven when structure of DNA determined in 1950s
