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DNA coding for polypeptides (Transcription (A gene unwinds and unzips…
DNA coding for polypeptides
Transcription
Turns a gene into a length of mRNA
A gene unwinds and unzips
Hydrogen bonds between complementary nucleotide bases break
RNA polymerase
catalyses the formation of temporary hydrogen bonds between
RNA nucleotides
On one strand of unwound DNA (template strand)
Complementary unpaired bases
A length of RNA that is complementary to the template strand of the gene is produced
coding strand
The mRNA passes out the nucleus through the nuclear envelope
Attaches to a ribosome
Translation
tRNA molecules are made in the nucleolus
Single-stranded polynucleotides that can be twisted into a hairpin shape.
One end
Trio of nucleotide bases that recognises and attaches to a specific amino acid
Loop of the hairpin
Another triplet of bases
Anticodon
Complementary to a specific codon of bases on the mRNA
Ribosomes catalyse the synthesis of polypeptides
tRNA molecules bring the amino acids and find their place when the anticodon binds to the complementary codon on the mRNA molecule
via temporary hydrogen bonds
Amino acid sequence for the polypeptide is determined by the sequence of triplets of nucleotide bases on the length of DNA
The gene
As the ribosome moves along the length of mRNA, it reads the code
A peptide bond is formed between two adjacent amino acids
ATP is needed for polypeptide synthesis
The polypeptide has been assembled
mRNA breaks down
Its component molecules can be recycled into new lengths of mRNA
mRNA with different codon sequences
Newly synthesised polypeptide is folded into its 3D shape or tertiary structure
To carry out its function
Helped by chaperone proteins in the cell
Nature of the genetic code
Universal
Same triplet of DNA bases codes for the same amino acid
Degenerate
Nearly all amino acids have more than one base triplet
This may reduce the effect of point mutations
A change in one base of the triplet could still produce another base triplet that still codes for the same amino acid
Non-overlapping
Genetic code is read starting from a fixed point in groups of three bases
If a base is added or deleted
Frame shift
Every base triplet after that and every amino acid coded for is changed
RNA
Structurally different from DNA
Sugar molecule in each nucleotide is ribose
Uracil pyrimidine replaces thymine pyrimidine
Polynucleotide chain is usually single-stranded
Three forms of RNA
Messenger RNA (mRNA)
Transfer RNA (tRNA)
ribosomal RNA (rRNA)