4.1 DNA, genes and chromosomes + 4.2 DNA and protein synthesis Flashcards
What are the components of DNA
- Deoxyribose sugar
- Phosphate group
- Nitrogenous base: Adenine, Thymine, Cytosine, Guanine
What are the components of RNA
- Ribose sugar
- Phosphate group
- Nitrogenous base: Adenine, Uracil, Cytosine, Guanine
What is the main difference in structure between DNA and RNA
A DNA molecule is a double helix composed of two polynucleotides joined together by hydrogen bonds between complementary bases, whereas RNA is a relatively short polynucleotide chain.
What is a gene
A sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.
How many different amino acids can be made in the body
20
Describe the difference in structures between mRNA and tRNA
mRNA:
- Single, straight strand
- Longer (still shorter than DNA)
- Less stable than tRNA
- No paired bases or hydrogen bonds
- Has codons
- No amino acid binding site
tRNA:
- Clover shaped
- Shorter than mRNA
- More stable than mRNA
- Has some paired bases and hydrogen bonds
- Has anticodons
- Amino acid binding site
What is a (gene) locus
- The position on the DNA strand where a gene is located
- The same gene occupies the same locus on the DNA strand in all individuals of the same species
- E.g. In all humans, the gene for eye colour is found at the same locus for everyone
What are 3 things that describe the genetic code
- Non-overlapping
- Degenerate
- Universal
What does it mean that the genetic code is non-overlapping
- Each DNA base is part of one triplet code only
- Each triplet is only read once and triplets don’t share any bases
What does it mean that the genetic code is degenerate
Most amino acids are coded for by more than one triplet/codon
Why is it beneficial that the genetic code is degenerate
It reduces the number of mutations as if one of the bases in a codon is changed, it could still code for the same amino acid.
What does it mean that the genetic code is universal
The same codon/triplet always codes for the same amino acid
Describe the process of transcription
- DNA helicase breaks hydrogen bonds between DNA strands to separate them
- DNA-dependant RNA polymerase binds to a specific region (promoter) of DNA
- Free RNA nucleotides align by complementary base pairing
- RNA polymerase moves along DNA in the 5’ to 3’ direction of the coding strand, joining RNA nucleotides by condensation reactions between adjacent RNA nucleotides
- RNA polymerase allows hydrogen bonds to reform between the DNA strands
- Once RNA polymerase reaches a ‘stop’ codon, it detaches
- A single strand of pre-mRNA has been formed
- The pre-mRNA is then spliced
What is splicing
A process by which introns (non-coding DNA) is removed and functional exons are joined together.
Do prokaryotes splice mRNA
No as most prokaryotic cells don’t have introns, so transcription creates mRNA rather than pre-mRNA.
What happens to the mRNA once it has been spliced
The mRNA leaves the nucleus via the nuclear pore.
Describe the process of translation
- mRNA binds to a ribosome at the ‘start’ codon (AUG)
- mRNA has codons, these bind to specific complementary anticodons on tRNA molecules by hydrogen bonds
- Each tRNA molecule carries a specific amino acid which depends on its anticodon
- Peptide bonds formed by condensation reactions between amino acids (requires ATP) to form a polypeptide chain
- Ribosome moves to next codon
- The sequence of codons on mRNA determines the sequence of amino acids
- This continues until a ‘stop’ codon is reached. The ribosome, mRNA and polypeptide separate.
What are homologous chromosomes
- Set of one maternal and one paternal chromosome
- Pair up with each other inside a cell during meiosis
- Have the same genes in the same loci
What is an allele
An alternative form of a gene
