chapter 2 (nucleic acids and proteins) Flashcards
Protein functional diversity
- proteins form many different functions
e.g structural support, transport, enzymes, defence and hormones
Monomers of proteins and nucleic acids
proteins = amino acids
nucleic acids = nucleotides
amino acid structure
- amino group
- central carbon
- carboxyl group
- r-group
(r-group is specific for each of the 20 amino acids)
Condensation reactions
- joins the amino acids together
- forms phosphodiester bonds
- releases by-products (water)
Protein structure levels - primary
- sequence of amino acids in a polypeptide chain
- written as three letter codes
Protein structure levels - secondary
- arrangement into alpha helices, beta pleated sheets and random coils
- polypeptide chain folds and coils
- structure is held with hydrogen bonds
Protein structure levels - tertiary
- functional 3D shape of a protein
- secondary structures may fold and join together
- disulphide bonds further stabilise 3D structure
- protein must at minimum have a tertiary structure to function
Protein structure levels - quaternary
- bonding of multiple polypeptide chains with a tertiary structure together
- held together by hydrogen bonds
haemoglobin
- quaternary structure
- composed of 4 polypeptide chains bonded together
- carries oxygen in red blood cells
Structure of DNA
- two polynucleotides chains run antiparallel (3’ to 5’) and (5’ to 3’)
- joined together via complementary base pairing (A & T) (C & G)
(A & T = 2 hydrogen bonds)
(C & G = 3 hydrogen bonds) - phosphate added at 5’ end
- OH added at 3’ end
rnucleic acid structure & primers
- phosphate (circle)
- five-carbon sugar (pentagon)
- nitrogenous base (rectangle)
- 1’ attaches to nitrogenous base
- 5’ attaches to sugar phosphate group
- 3’ attaches to phosphate of following nucleotide)
Purines and pyrimidines
purines = adenine & guanine
(2 nitrogen carbon rings)
pyrimidines = thymine & cytosine
(1 nitrogen carbon ring)
different types of RNA
mRNA - messenger RNA: carries genetic information from nucleus to ribosomes for protein synthesis
tRNA - transfer RNA: delivers specific amnio acids to the ribosome after recognising specific nucleotide sequence
rRNA - ribosomal RNA: serves as main structural component of ribosomes
DNA v RNA similarities and differences
similarities:
- same basic structure (phosphate, sugar and nitrogen base)
- contain adenine, guanine & cytosine
- contain sugar-phosphate backbone
differences:
DNA - double stranded
- deoxyribose sugar
- thymine
- inherited and long-term storage
RNA - single stranded
- ribose sugar
- uracil
- temporary and short-lived molecule
Translation table
- first base, second base, third base
- START CODON: AUG (met) is always the start codon
- STOP CONDONS: UAA, UAG, UGA
(do not code for a amino acid)
exons, introns & operator
exons - regions of coding DNA (in both prokaryotes & eukaryotes)
introns - regions of non-coding DNA (in eukaryotes)
operator - serves as binding site for repressor (in prokaryotes)
key genetic code
unambiguous: codon is only capable for coding one specific amino acid
degenerate: each amino acid may be coded for by multiple different codons
non-overlapping: each codon is read independently without overlapping
universal: nearly every organism uses the same codon to code for a specific amino acid
types of gene
constitutive - genes that are always switched on (what we need)(e.g blood and growth)
regulatory - control the expression of other genes, does not code for a protein, turn structural genes off or on
structural gene - codes for RNA or protein, achieves an effect within the organism, producing useful proteins
(e.g structural protein, enzyme, RNA molecule)
3 stages of transcription
- copying of DNA code
- first stage of expression and creation of pre-mRNA and occurs in the nucleus
INITIATION: - RNA polymerase binds to the promoter region
- DNA unwinds
ELONGATION: - RNA polymerase reads the DNA template strand and uses complementary RNA nucleotides to catalyse the formation of pre-mRNA
- strand of DNA that is not read is called the coding strand
TERMINATION: - transcription is terminated when the termination sequence is recognised
- pre-mRNA is then processed to become mRNA
RNA proccesing
- ‘editing’ and ‘proofreading’
- post-transcriptional modifications
- occurs in the nucleus
5’METHYL-GCAP & 3’ POLY-ATAIL: - serves to stabilise the mRNA molecule
- prevents degrading
- allows it to bind to ribosomes
SPLICING: - introns non-coding regions of DNA (are removed)
- exons coding regions of DNA
(must be joined together) - splicing occurs via spliceosomes
ALTERNATIVE SPLICING: - sometimes exons can also be removed
3 stages of translation
- reading and converting mRNA into a polypeptide chain
- occurs in the ribosome
INITATION: - mRNA molecule binds to the ribosomes
- tRNA complementary to mRNA deliver corresponding amino acids to the ribosome
ELONGATION: - adjacent amino acids are joined with peptide bonds via a condensation reaction to form a polypeptide
TERMINATION: - translation ends when a STOP codon is recognised
promoter & operator
promoter: binding site for RNA polymerase (recognition site)
operator: binding site of repressor protein
repression
LOW LEVEL OF TRYPTOPHAN:
- insufficient tryptophan to bind to repressor protein
- causes repressor protein to detach from operator region
- allowing RNA polymerase to transcribe the trp gene and increase tryptophan levels
HIGH LEVEL OF TRYPTOPHAN:
-sufficent tryptophan to bind to the repressor protein
- causing conformational change and repressor protien to stay attached
- RNA polymerase will not transcribe the trp gene and no tryptophan will be produced
attenuation
- transcription and translation occur simultaneously
LOW LEVEL OF TRYPTOPHAN: - due to no tRNA bound tryptophan in cell ribosomes pause
- causes mRNA molecule to fold and form anti-terminator hairpin loop
- RNA polymerase continues and transcribes genes to synthesise tryptophan
HIGH LEVEL OF TRYPTOPHAN: - tRNA bound tryptophan travels to ribosomes and is added to proteins
- causes mRNA to fold and form terminator hairpin loop
- causes mRNA to seperate and RNA polymerase to detach stop transcription and no new tryptophan to be synthesised
exocytosis
- secretory products transported to plasma membrane via vesicles
- membrane of vesicles fuse with the plasma membrane
- secretory products are released from the cell
protein secretory pathway
- ribosomes, rough endoplasmic reticulum, transport vesicle, golgi apparatus, secretory vesicles
