subject guide

D1.2.1—Transcription as the synthesis of RNA using a DNA template

in eukaryotic cells, transcription takes place in the nucleus (as that’s where DNA is)

in prokaryotic cells, transcription takes in place in cytoplasm (where DNA is)

RNA polymerase is involved in transcription

D1.2.2—Role of hydrogen bonding and complementary base pairing in transcription

during initiation phase, RNA polymerase breaks hydrogen bonds holding together the nitrogenous bases between the 2 strands

  • separates the 2 strands

RNA polymerase adds free RNA nucleotides based on complementary base pairing

  • DNA contains thymine, but it’s replaced with uracil in RNA

  • so when RNA polymerase encounters T on template strand, it adds U to mRNA strand

  • temporary hydrogen bonds form between the nitrogenous bases on the template & mRNA strand

D1.2.3—Stability of DNA templates

its important that DNA is stable, as it is transcribed multiple times and needs to maintain unchanged so that no genetic mutations occur & the protein can be produced

D1.2.4—Transcription as a process required for the expression of genes

only genes that’re transcribed. are active within a cell

enables cells to turn on/express genes whose products are needed, & switch off other genes

regulating transcription = regulate other cellular activities

transcriptional regulation also results in cell differentiation

D1.2.5—Translation as the synthesis of polypeptides from mRNA

ribosomes read the genetic info in mRNA, in order to synthesize proteins

  • AKA: base sequence of mRNA is translated into the amino acid sequence of a polypeptide

D1.2.6—Roles of mRNA, ribosomes and tRNA in translation

mRNA contains the genetic info needed to synthesize proteins

ribosomes are where proteins are made

tRNA have their own amino acids & are arranged based on mRNA sequence (the codons?)

ribosomes hv small & large subunit, with 3 binding sites for 3 tRNA molecules

  • mRNA binds to small subunit

  • 2 tRNAs bind simultaneously to larger subunit

D1.2.7—Complementary base pairing between tRNA and mRNA

as mRNA binds to the small subunit of ribosome, its code is read in groups of 3 (codons)

tRNA molecules hv their own 3-base code that’s complementary to the codon on the mRNA (anticodon)

since codon & anticodon are complementary, this ensures the correct amino acid is placed in the sequence

D1.2.8—Features of the genetic code

genetic code is broken down into codons found on mRNA

each codon represents specific amino acid

but some amino acids are coded for by more than one codon

  • account for the degeneracy of the genetic code

genetic code is also universal, as it’s found in most organisms

  • this provides proof that all life on Earth likely originated from a common ancestor

D1.2.9—Using the genetic code expressed as a table of mRNA codons

need to know how to read the genetic code table

D1.2.10—Stepwise movement of the ribosome along mRNA and linkage of amino acids by peptide bonding to the growing polypeptide chain

during elongation phase, mRNA moves through ribosome, one codon at a time

as each new codon is positioned, a new tRNA attaches to large subunit

  • move previous tRNA to next position

as new amino acids are delivered, condensation reactions occur, forming a polypeptide bond between the amino acids

D1.2.11—Mutations that change protein structure

mutations are important as they introduce genetic variation

point mutation results in frameshift mutation

  • all the codons following that mutation are now altered, cuz there’s been a change in the base sequence

  • example - sickle cell anaemia

    • result of a single point mutation in gene responsible for producing one of the polypeptides in haemoglobin

can lead to a different amino acid being added in the polypeptide = different protein produced

or sometimes silent mutation could occur = no change in produced protein

D1.2.12—Directionality of transcription and translation

transcription occurs in 5’-3’ direction

  • RNA polymerase can only catalyse formation of phosphodiester bonds between 3’ end of one RNA nucleotide & 5’ end of other nucleotide

translation occurs in a 5’-3’ direction

  • meaning the mRNA moves through the ribosome in a 5’-3’ direction

    • mRNA fits the small subunit of ribosome only if its positioned correctly

    • if mRNA could fit the subunit when it’s backwards, then the wrong polypeptide would be synthesized

D1.2.13—Initiation of transcription at the promoter

at promoter, transcription factors will bind & these initiate transcription

  • if transcription factors bind in the correct orientation, then RNA polymerase will be able to bind & begin transcription

D1.2.14—Non-coding sequences in DNA do not code for polypeptides

regions of DNA that dont code for proteins include:

  • regulators of gene expression (DNA sequences that regulate gene expression in other ways)

    • ex: promoter - function as binding region for RNA polymerase

  • introns

  • telomeres

  • genes for tRNAs & rRNAs

D1.2.15—Post-transcriptional modification in eukaryotic cells

in eukaryotes, mRNA undergoes post-transcriptional modification

before undergoing post-transcriptional modification, mRNA is referred to as pre-mRNA

  • poly-A tail & 5’ cap are added to mRNA

    • helps protect the molecule from degradation

  • splicing also occurs

    • introns are removed from pre-mRNA & exons are ligated to produce mature mRNA

D1.2.16—Alternative splicing of exons to produce variants of a protein from a single gene

alternative splicing produces different versions of a protein that function differently

this allows one gene to code for different polypeptides

D1.2.17—Initiation of translation

begins when small ribosomal subunit attaches to 5’ terminal of mature mRNA

tRNA linked to start codon always carries amino acid methionine

  • all proteins begin with this amino acid

ribosomal subunit attaches

A site of ribosome is where incoming tRNA binds

P site of ribosome is where tRNA from A site moves, after it’s amino acid has been added to the polypeptide chain

E site is where tRNA moves & prepares to exit ribosome

D1.2.18—Modification of polypeptides into their functional state

many polypeptides hv to be modified before they can function

post-translational modification occurs in Golgi apparatus

  • chemical groups can be added

ex: modifying pre-proinsulin into insulin

  • pre-proinsulin has 4 main sections

    • signal peptide

    • A chain

    • B chain

    • C peptide

  • when pre-proinsulin enters ER, signal peptide is removed

  • remaining protein - proinsulin

  • disulfide bridges form between A chain & B chain

  • proinsulin packaged into vesicles that move to Golgi apparatus

  • at Golgi apparatus, C-peptide removed & mature insulin remains

D1.2.19—Recycling of amino acids by proteasomes

body cell’s constantly producing proteins to maintain proteasomes

sustaining a functional proteasome requires constant protein breakdown & synthesis