nucleotides and nucleic acids Flashcards
(26 cards)
what is an individual nucleotide made up of
- a pentose sugar (monosaccharide containing 5 carbons)
- a phosphate group
- a nitrogenous base (containing one or two carbon rings + nitrogen in its structure)
what are phosphodiester bonds between in nucleotides
phosphate group of one nucleotide forms a covalent bond with the hydroxyl (OH) group on the pentose sugar of an adjacent nucleotide
how are phosphodiester bonds broken
hydrolysis - the reverse of condensation, releasing the individual nucleotides
what does DNA stand for
deoxyribonucleic acid
what are pyrimidines
the smaller bases which contain single carbon ring structures
Thymine and Cytosine
what are purines
the larger bases which contain double carbon ring structures
Adenine and Guanine
what bond holds bases together
hydrogen
what does antiparallel mean in DNA
the two parallel strands are aranged so that they run in opposite directions
number of hydrogen bonds between pairs
A - T form two hydrogen bonds
C - G form three hydrogen bonds
known as complementary base pairing
what does RNA stand for
ribonucleic acids
how do RNA molecules differ to DNA
- shorter to fit out nuclear pores
- pentose sugar is ribose instead of deoxyribose
- thymine base is replaced by uracil
what does semi-conservative replication mean
- double helix structure unwinds
- free nucleotides pair with their complementary bases
- in this way, two new molecules of DNA are produced each one consists of one old strand of DNA and one new one
what is the role of helicase in DNA replication
- helicase travels along the DNA backbone, catalysing reactions that break the hydrogen bonds between compementary base pairs
- this unwinds and seperates the two strands of DNA which occurs before replication can take place
what is the role of DNA polymerase in DNA replication
it catalyses the formation of phosphodiester bonds between the free nucleotides and the newly exposed bases on the template strand
what is continuous replication
- DNA polymerase always moves along the template strand in the same direction. it can only bind to the 3’ (OH) end so travels in the direction of 3’ to 5’
- as DNA only unzips in one direction, DNA P has to replicate each of the template strands in opposite directions
- the strand that is unzipped from the 3’ end can be continuously replicated as the strands upzip
- this strand is said to be the leading strand and undergoes continuous replication
what is discontious replication
- the other strand is unzipped from the 5’ end so DNA P has to wait until a section of the strand has unzipped then work back along the strand
- this results in DNA being produced in sections which then have to be joined
- this strand is called the lagging strand and undergoes discontinous replication
what is the role of ligase in DNA replication
joins the DNA fragments (called Okazaki fragments) together
these are created in discontinous replication
what is mRNA a copy of
one of the strands of DNA that contains the code for the protein to be synthesised
this is called the sense strand or the coding strand
what is the template strand
the other strand (5’ to 3’) that is a complementary copy of the sense strand and does not code for a protein.
It acts as the template strand during transcription
what occurs in transcription
- DNA molecule unwinds (aided by helicase which breaks the hydrogen bonds)
- free RNA nucleotides will base pair with the complementary bases exposed on the antisense strand when DNA unzips
- Phosphodiester bonds are formed between the RNA nucleotides by the enzyme RNA polymerase
- the mRNA then detaches from the DNA template strand and leaves the nucleus through a nuclear pore and travels to a ribosome
what is the role of RNA polymerase in transcription
forms phosphodiester bonds between the RNA nucleotides.
sequence of translation
- the mRNA binds to the small unit of the ribosome at its start codon
- in the cytoplasm there are free molecules of tRNA
- the tRNA molecules bind with their specific amino acids and bring them to the mRNA molecule on the ribosome
- the triplet (anticodon) on each tRNA molecule pairs with complemtary triplet (codon) on the mRNA molecule
- two tRNA molecules fit onto the ribosome at any one time
- A peptide bond is then formed (via a condensation reaction) between the two amino acids
- This process continues until a ‘stop’ codon on the mRNA molecule is reached
- the amino acid chain then forms the final polypeptide
what are the important parts of tRNA which help it to carry out its function
at the top, it has a binding site for an amino acid
at the bottom, a triplet of bases called an anticodon
