Topic 4A - DNA, genes and chromosomes Flashcards
Eukaryotic DNA
Found in the nucleus
Linear DNA molecules that exist as chromosomes
DNA is wrapped around proteins called histones
Longer than prokaryotic
Histone proteins help support DNA
Mitochondria and chloroplasts have their own DNA which is circular, shorter and not associated with proteins
Has introns and exons
Prokaryotic DNA
DNA is shorter and circular
DNA carried as chromosomes
DNA isn’t wrapped around histones
It condenses to fit in the cell by supercoiling
Does not have introns
What is a gene?
A gene is a sequence of DNA bases that code for a polypeptide of functional RNA
It’s the order of bases in a gene that determines the order of amino acids in a polypeptide
Each amino acid is coded by a triplet
How do you make a polypeptide?
DNA is copied into messenger RNA (mRNA). This is the first stage of protein synthesis
What do genes that don’t code for a polypeptide code for instead?
Functional RNA - RNA molecules other than mRNA e.g. tRNA and rRNA
What is an intron?
A non-coding nucleotide sequence within a gene
The parts of the gene that do code for amino acids are called exons
Introns are removed during protein synthesis - they don’t effect amino acid order
What are non-coding repeats?
DNA sequences that repeat over and over
Don’t code for amino acids
Alleles
Different versions of the same gene
The order of bases in the sequence differs slightly so code for slightly different versions of the same polypeptide
All about messenger RNA
Made during transcription
Carries the genetic code from the DNA to the ribosomes where it’s used to make a protein during translation
A single polynucleotide strand
In mRNA, groups of 3 adjacent bases are called codons (sometimes triplets or base triplets)
All about transfer RNA
tRNA is involved in translation
Carries the amino acids used to make proteins to the ribosomes
A single polynucleotide strand that’s folded into a clover shape
Hydrogen bonds between specific base pairing holds the molecules in this shape
Each tRNA molecules has a specific sequence of 3 bases at one end called an anticodon
They have an amino acid binding site at the other end
Stage 1 of protein synthesis - transcription
An mRNA copy of a gene is made from DNA
Eukaryotes - nucleus
Prokaryotes - cytoplasm
1) DNA helicase breaks hydrogen bonds between DNA strands and exposes the bases
2) One of the strands is used as a template to make an mRNA copy
3) RNA polymerase lines up free floating RNA nucleotides alongside the exposed bases
4) Complementary base pairing
Thymine replaced by Uracil
5) Hydrogen bonds reform between original DNA strand
8) RNA polymerase reaches a stop signal
9) In eukaryotes, mRNA moves out of the nucleus through nuclear pores and attaches to a ribosome in the cytoplasm
Stage 2 of protein synthesis - translation
Amino acids are joined together to make a polypeptide chain (protein)
1) mRNA attaches to ribosome and tRNA carry amino acids to it
2) A tRNA molecule with an anticodon that’s complementary to the first codon on the mRNA attaches to the mRNA by complementary base pairing
3) A second tRNA molecule attaches to the next codon in the same way
4) The two amino acids attached to the tRNA now join by a peptide bond
5) The first tRNA molecule moves away leaving its amino acid behind and a third binds to the next codon on the mRNA
6) This process continues producing a polypeptide chain until a stop signal is reached on the mRNA
7) The polypeptide moves away from the ribosome and translation is complete
Transcription products in eukaryotes and prokaryotes
In eukaryotes -
Introns and exons are copied into mRNA. mRNA strands containing introns and exons are called pre-mRNA. A process called splicing then occurs - where introns are removed, leaving exons and forming mRNA strands. This takes place in the nucleus
In prokaryotes -
mRNA is produced directly from DNA with no splicing as there are no introns in prokaryotic DNA
Features of the genetic code
Non-overlapping
Degenerate - more possible combinations of triplets than there are amino acids
Universal
