U3: AOS 1- nucleic acids and proteins Flashcards
(32 cards)
what is the monomer of nucleic acids?
nucleotides
role of nucleic acids?
encode instructions for the synthesis of proteins (DNA, 3 forms of RNA)
where is DNA found?
nucleus on chromosomes
what is RNA formed from?
a template strand of DNA
large …. polymers form when monomers bond together
*linear polymers..
(monomers/sub-units: nucleotides)
describe the structure of nucleotides
- 5 carbon pentose sugar
- negatively charged phosphate group
- nitrogenous base
describe nucleotide pairing
adenine/thymine/uracil pair to form 2 hydrogen bonds
guanine/cytosine form 3 hydrogen bonds
nucleotides in sugar-phosphate chain are joined by phosphodiester bonds (strong covalent bond)
describe the structure of DNA in detail
- nitrogenous base attached to 1st carbon in the ring- 1’
- phosphate group attached to carbon 5 of the sugar
- phosphate group binds with the hydroxyl group at carbon 3’ of the next nucleotide
*this bond creates a *phosphodiester bond**
*H2O molecule also released
describe condensation polymerisation reaction *
- 2 nucleotides join together to form a dinucleotide
- hydroxyl group on C5 joins to C5 of the other nucleotide to form water
***free nucleotides can be continuously added to the 3’ END, forming the polynucleotide
why does DNA replication occur in a 5’ to 3’ direction? *
nucleotides are added to the exposed hydroxyl group at the 3’ END (3rd carbon) of the existing nucleotide
describe the difference between the 2 antiparallel strands of DNA *
- phosphate group projects from 5’ end, as it’s attached to the 5’ carbon in the sugar
- hydroxyl group projects from the 3’ end
differentiate purines from pyrimidines
PURINES: adenine, guanine
-> 2 carbon rings
PYRIMIDINES: thymine, cytosine
-> 1 carbon ring
state 3 RNA types and provide definition for each
- mRNA: copy of DNA strand in nucleus, moves through nuclear pores to free ribosome (in cytoplasm) for protein synthesis
- rRNA: forms the ribosomal unit that synthesises the protein as coded by the mRNA
- tRNA: carries amino acids to the ribosome- contains an anticodon that binds to its complementary codon on the mRNA
(process repeats until polypeptide chain is complete)
describe the steps of transcription
- RNA polymerase attaches to the promoter sequence (upstream region) and the DNA double helix unwinds, exposing the template strand
- RNA polymerase moves along the strand in a 3’-5’ direction while complementary nucleotides (U REPLACES T) are added to the 3’ end (gradually forming mRNA strand)
- once the RNA polymerase has moved past the coding region, transcription stops and the pre-mRNA strand is released
describe the steps in RNA processing
-
capping: a methyl guanosine (methyl-G cap) is added onto the 5’ end
-> protects the mRNA degradation by exonuclease and provides further stability -
polyadenylation: a chain of adenine nucleotides (poly-A tail) is attached to the 3’ end
-> provides further stability and facilitates mRNA export from the nucleus -
splicing: spliceosomes remove introns (non-coding regions) and then fuse exons together, resulting in a mature mRNA strand
-> recognise specific base sequences at the ends of the introns: GU at the 5′ end and AG at the 3′ end.
*the mature mRNA moves through pores in the nuclear membrane to the cytosol, where it travels to the ribosome for translation
describe the process of translation
*various amino acids are present in solution in the cytosol.
- in the cytosol, the start codon (AUG) of mRNA binds to the small ribosomal subunit, and then a large ribosomal subunit binds to the small subunit.
- anticodons on tRNA molecules align opposite complementary mRNA codons, and each tRNA carries an amino acid
- ribosomes catalyse the formation of peptide bonds between adjacent amino acids.
- the ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon
- translation stops and the polypeptide chain is released
*polypeptide chain will either fold on its own or with other polypeptide chains, to become a functional protein.
define alternative splicing *
- the process of removing different exons from the same mRNA strand, thus creating multiple mRNA variants from the same gene
(i.e. splice different ways->diff mature mRNA->diff proteins)
-> allows cells to produce different proteins from a single gene.
-> helps organisms adapt by creating functional protein variations without requiring new genes.
what are the START and STOP codons in translation?
START: (AUG) *met
STOP: (UAA, UAG and UGA)
what is a group of ribosomes called?
a polysome
difference between mRNA and rRNA?
- mRNA formed during gene transcription has a short life
- rRNA which forms part of the ribosomes is very stable.
describe the structure of tRNA
- molecule consists of a single strand of 76 nucleotides coiled and paired with themselves
- at one end: anticodon.
- other end: region that attaches to one specific amino acid
*an enzyme, amino acyl tRNA synthetase, catalyses the linking of each amino acid to its specific tRNA carrier
describe the structure of genes **
coding region: contains the coded information for making a protein *segment of DNA double helix that includes the DNA template strand
flanking regions: on either side of coding region
upstream region: flanking region BEFORE START of coding region
downstream region: flanking region AFTER END of coding region
leader region: contain sections known as attenuators, involved in a process of gene regulation called attenuation.
-> involves the formation of hairpin loops and the stalling and detachment of the ribosome (thus also RNA polymerase when the structural genes do not need to be transcribed and translated)
operator region: found in prokaryotic genes.
-> between promoter and gene being transcribed.
-> binding site for repressor proteins. when a repressor (produced in another gene) binds to the operator, it PREVENTS RNA polymerase binding to the promoter, and thus transcription CANNOT be initiated.
promoter: section of DNA that transcription factors RNA polymerase binds to, activating transcription
terminator: section of DNA at the end of a gene that signals transcription to STOP
operator: a section of DNA that regulatory proteins bind to, located BETWEEN promoter and the DNA sequence being transcribed.
what is the TATA box?
- one part of the upstream region is rich in As and Ts and is often called the TATA box, because the sequence TATA AA (or similar) occurs there
*located around 25 to 35 base pairs from the transcription start site
*short base sequence consistently found in the upstream flanking region of the coding region of genes of many different species
what supports the idea that specific DNA sequences upstream of the coding region of a gene initiate transcription? *
- upstream sequences are invariably found in all organisms. (have been maintained during evolution)
- if upstream sequences are altered by mutation, the activity of the coding region of the gene may be reduced or even become inactive
- the absence of the correct upstream signal-> some inherited human diseases.
e.g. (a form of thalassaemia is due to a missing TATA group in the upstream region of the DNA of both copies of the specific gene) *? - upstream region includes segments of DNA to which hormones can attach. the fact that some hormones can bind to DNA provides one clue as to how hormones can influence the action of genes
