3 DNA and protein synthesis

define genome

an organisms complete set of DNA in a cell

define proteome

the full range of proteins that a cell is able to produce

define codon

three bases on mRNA that code for a specific amino acid

define start codon

three bases at the start of every gene that initiate translation

define stop codon

three bases at the end of every gene that cause ribosomes to detach and therefore stop translation

what are the different types of RNA?

• mRNA

• tRNA

where and when is messenger RNA (mRNA) made?

made during transcription in the nucleus, its copied from DNA and is therefore complementary to the DNA sequence

what’s the shape of mRNA?

short, single stranded molecule

where’s mRNA found?

the cytoplasm and nucleus

what are groups of adjacent bases in mRNA called?

triplets

what’s transfer DNA (tRNA) involved in?

translation

what does tRNA do?

carries specific amino acids to the mRNA on the ribosome

what’s the shape of tRNA?

single stranded polynucleotide that’s folded into a cloverleaf shape

what bonds are between the base pairs in tRNA?

hydrogen bonds which hold the shape

what does tRNA have at one end?

a specific sequence of three bases called an anticodon

what does tRNA have on the other end?

amino acid binding site

what’s protein synthesis?

the production of proteins (polypeptides). it involves two main stages

what stages does protein synthesis involve?

• transcription: where the DNA code is copied into a molecule called mRNA

• translation: where the mRNA joins with a ribosome and the code it carries is used to synthesise a protein

where does transcription occur in eukaryotes and prokaryotes?

nucleus for eukaryotes and cytoplasm for prokaryotes

what’s the process of transcription?

• hydrogen bonds between DNA bases break, causing the DNA double helix to separate and expose the template strand

• one of the strands is used as a template for the formation of RNA

• free RNA nucleotides align by complementary base pairs to the exposed bases on the DNA template strand

• in RNA, uracil base pairs with adenine on the DNA strand (uracil is used in place of thymine)

• the enzyme RNA polymerase catalyses the condensation reaction of joining adjacent RNA nucleotides together

• phosphodiester bonds form between adjacent RNA nucleotides, creating the sugar phosphate backbone of the RNA strand

once copied, the mRNA is modified and then leaves the nucleus through the nucleur envelope pores

what’s the initial RNA strand in eukaryotic cells called?

pre mRNA

what does the pre mRNA in eukaryotic cells undergo?

splicing, where introns are removed and exons are joined together to form mRNA

where does translation occur?

in the ribosomes in the cytoplasm

what happens during translation?

amino acids join together to make a polypeptide chain using both the mRNA base sequence and the tRNA

what does transcription in prokaryotes result in?

directly in the production of mRNA as prokaryotes do not contain any introns

how does translation occur?

• once the modified mRNA has left the nucleus, mRNA attaches to a ribosome in the cytoplasm

• the ribosome reads the sequence of codons (triplets of bases) on the mRNA one at a time

• a molecule of tRNA with an anticodon complementary to the codon on the mRNA binds to it by complementary base pairing (occurs first at the start codon)

• each tRNA molecule carries a specific amino acid, determined by its anticodon

• the ribosome moves along the mRNA to the next codon, the the process repeats, with new tRNA molecules binding, bringing more amino acids

• this continues until a stop codon is reached. no tRNA binds to the stop codon, and the polypeptide is released from the ribosome

• the polypeptide chain is now created and will enter the golgi body for folding and modification