DNA an the Genetic Code Flashcards
(16 cards)
What are the key features of the genetic code
- Code is non-overlapping- each base i the DNA sequence is only read once.
- Code is degenerate- most most amino acids have more than one possible code. With 64 sequences- allows for a degree of duplication. First 2 DNA bases- crucial in determining the amino acid produced.
- Code is universal- DNA code present in all living organisms
What is transcription
The process of forming complementary copies of mRNA from sequences of DNA that code for a polypeptide or protein
Why is messenger MRNA used instead of DNA
By remaining in the nucleus, DNA is always available for copying and is less likely to be damaged than in the more metabolically volatile cytoplasm
What are differences between mRNA and DNA
-mRNA is single strand- DNA is double stranded
-mRNA- ribose pentose sugar in short section backbone- DNA- deoxyribose pentose sugar
-mRNA- presence of uracil as nitrogenous base- DNA- thymine
-mRNA- shorter than DNA
Describe the process of transcription
- The enzyme DNA Helicase separates the two DNA strands of the relevant section of DNA to be separated. This happens by breaking the hydrogen bonds between the bonds, linking the two strands of the double helix, causing the DNA double helix to separate.
- The RNA polymerase moves along the template coding strand, linking the exposed nucleotides in that section, to free RNA ribo nucleotides, within the nucleotide pool in the nucleus. The building up of the mRNA strand alongside its complementary DMA template strand, follows the rules of complementary base pairing.
- The RNA polymerase joins adjacent mRNA nucleotides to each other by phosphodiester bonds as the mRNA nucleotide extends.
What happens when the final version of mRNA is formed
The functional mRNA moves out of the nucleus through a nucleur pore and into the cytoplasm where the formation of new proteins take place, known as translation
What helps attach amino acids in the cytoplasm to the appropriate tRNA molecule
Enzymes
What is the role of tRNA
- Transport amino acids within the cytoplasm to the tRNA- the site of protein synthesis
- Ensures each mRNA codon codes for a particular amino acid
Where are the two subunits of the ribosome assembled
Nucleolus in the nucleus and transported into the cytoplasm
What is the role of ribosomes
Hold the mRNA, tRNA and enzymes involved in protein synthesis
What is the process of translation
- A ribosome attaches to the start codon on the mRNA by its Acceptor site
- The tRNA molecule with the complementary anticodon moves to the ribosome and pairs up with the complementary codon on the mRNA molecule, following complementary base pairing rules. This tRNA carries the amino acid methionine to the ribosome. Peptide chains usually begin with methionine.
- As the ribosome moves along the mRNA to cover two codons, a tRNA molecule with a complementary anticodon pairs up with the second codon on the mRNA strand. Agin, this tRNA molecule carries its particular amino acid. By this stage, methionine will be in the Peptidyl site, leaving the Aminoacyl site free.
- A peptide bond links the two amino acids together to form a dipeptide
- As the ribosome notches along another codon, the tRNA for methionine is set free and the tRNA for the second amino acid is now in the peptidyl site. Again this leaves the aminoacyl site free and the third amino acid is brought into place by its tRNA molecule, following base pairing rules.
- The process continues and a polypeptide sequence of many amino acids is built up.
- Eventually, a stop codon is reached on the mRNA, and the newly synthesised polypeptide leaves the ribosome
Why are retroviruses useful
Retroviruses such as HIV, contain reverse transcriptase enzymes which can use a mRNA template to make complementary DNA
Explain one gene one protein
-appropriate when protein has one polypeptide
-less suitable when proteins are formed of more than one polypeptide, such as haemoglobin, which is formed from 4
-for proteins that are formed from more than one polypeptide, there is a different gene that codes for each polypeptide
Explain one gene one enzyme
-enzymes may be formed of more than one polypeptide
-not all proteins are enzymes
-eg haemoglobin and insulin aren’t enzymes
Explain one gene one polypeptide
A GENE (REPRESENTED BY A SEQUENCE OF BASES) CODES FOR THE SEQUENCE OF AMINO ACIDS (PRIMARY STRUCTURE OF A POLYPEPTIDE)
Define epigenetics
The study of heritable modifications to the genome that does not involve changes to the DNA base sequences
