DNA and protein synthesis

what does the order of bases on the DNA tell us?

• the order for combining amino acids to create particular proteins

what are the 2 processes in protein synthesis?

• transcription

• translation

what 2 molecules make up ribosomes?

• RNA

• proteins

mRNA?

• made during transcription

• it carries the genetic code from the DNA to the ribosomes, where it is used to make a protein during translation

• mRNA is a single polynucleotide strand

• 3 adjacent bases are called codons (or triplets/base triplets)

  • contains more nucleotides than tRNA

tRNA?

• involved in translation

• carries the amino acids that are used to make proteins to the ribosomes

• tRNA is a single polynucleotide strand that’s folded into clover shape.

• hydrogen bonds between specific base pairs that hold the molecule in this shape

• every tRNA molecule has a specific sequence of 3 bases at one end called an anticodon

• they also have an amino acid binding site at the other end

structural differences between mRNA and tRNA?

• mRNA does not have hydrogen bonds, but tRNA does

• mRNA is linear, tRNA is a cloverleaf

• mRNA does not have an amino acid binding site, tRNA does

• mRNA has more nucleotides/is longer

• different mRNAs have different lengths, all tRNAs are similar/same length

• mRNA has codons, tRNA has anticodons

accuracy of tRNA?

• it is important that the correct amino acid is brought to the ribosomes by the tRNA so that the protein is assembled correctly

• each t RNA molecule has an anticodon which is complementary to a specific codon on mRNA

• the amino acids that corresponds to a specific codon binds to a specific attachment site on the tRNA molecule

if the original DNA sequence has the codon GCC, the codon in each step is:

• mRNA complementary codon: CGG

• tRNA anticodon: GCC

• amino acids: arginine

what is transcription?

• the first step in protein syntehsis

steps in transcription (P1)

• RNA polymerase attaches to the DNA double helix at the beginning of a gene (locus of the gene)

• the hydrogen bonds that bind the 2 strands together break, causing the DNA strands to separate, and the bases of the target gene are exposed

  • this is different to DNA replication, where DNA helicase separates the strands

• RNA polymerase binds free-floating RNA nucleotides to the template strand

• the RNA nucleotides form a strand of mRNA that is complementary to the template strand

• the template strand in complementary to the gene, so this means mRNA is the copy of the gene

• free-floating nucleotides are joined together by RNA polymerase

  • phosphodiester bonds form between the nucleotides in a condensation reaction to form the completed strand of mRNA

• RNA polymerase eventually reaches the triplet of bases that signal stop

• the RNA polymerase stops separating the DNA and producing the mRNA

transcription steps: P2- removal of mRNA?

• the mRNA strand is separated from the template strand by RNA polymerase

• the hydrogen bonds between the 2 strands of DNA form again and the strands join together

• the completed mRNA strand leaves the nucleus and enters the cytoplasm

• mRNA Is used in translation, the next step is protein synthesis

easier transcription steps?

• hydrogen bonds between DNA bases break

• only one DNA strand acts as a template

• free RNA nucleotides align by complementary base pairing

• uracil base pairs with adenine on the DNA

• RNA polymerase joins the adjacent RNA nucleotides

• by phosphodiester bonds to form the sugar-phosphate backbone

• pre-MRNA is spliced to form mRNA/ introns are removed to form mRNA

easier translation steps?

• mRNA attaches/associates to the ribosomes

• ribosome moves to/finds the start codon

• tRNA anticodons are complementary to the mRNA codons

• tRNA brings a specific amino acid

  • ribosome fits around 2 codons/ribosome moves along to next codon

• process is repeated and amino acids are joined together by peptide bonds

• with the use of ATP

• tRNA is released after the amino acids is joined to the polypeptide

• the ribosome moves along the mRNA to form the polypeptide

what happens to the introns and exons in protein synthesis: eukaryotes?

• the introns and exons are both copied into mRNA during transcription

• mRNA strands containing introns and exons are called pre-mRNA

• a process called splicing then occurs, introns are removed and the exons are joined together, forming mRNA strands

• this takes place in the nucleus

  • the mRNA then leaves the nucleus for the next stage of protein synthesis (translation)

what happens to the introns and exons in protein synthesis: prokaryotes?

• mRNA is produced directly from the DNA- without splicing taking place

  • there’s no need for splicing, because there are no introns in prokaryotic DNA

what happens in translation?

• the produced mRNA binds to a ribosome in the cytoplasm

  • the ribosome is the site of protein synthesis

• 1 molecule of tRNA binds to the first codon in the ribosome (6 bases- 2 codons- can fit inside the ribosome at one tiem)

• the tRNA molecule has an anticodon that is complementary to a specific codon

• the anticodon allows the correct tRNA molecule to bind to the correct codon

• each tRNA molecule carries a specific amino acid into the ribosome

  • the amino acid is bound to tRNA using ATP

• A tRNA molecule binds to the second codon in the ribosome

  • when a tRNA molecule binds to mRNA, the corresponding amino acid is brought to the ribosome

• the 2 amino acids in the ribosome form a peptide bond

translation p2?

• when the 2 amino acids bind together, the ribosome moves along the mRNA strand so that a new codon enters the ribosome

• a complementary tRNA molecule binds to the new codon

• a new amino acid is brought into the ribosome

• a peptide bond forms between the new amino acid and the existing chain of amino acids

• when the ribosome reaches a STOP codon, there is no corresponding tRNA molecule

• the polypeptide chain is released from the ribosome

• the polypeptide chain has been formed and is ready to complete its function