Genetic Variation Flashcards
What is a gene
A section of DNA that contains the coded information for making polypeptides and functional RNA
What do genes determine
The proteins of an organism
What is a locus
The location of gene located on a dna molecule
A gene is a base sequence of DNA that codes for :
The amino acid sequence of a polyeptide
A functional RNA , including ribosomal rna and transfer RNAs
What were scienctists reasons of suggesting that there must be a minimum of 3 bases that coded for each amino acid
Only 20 different amino acids regularly occur in proteins
Each amino acid must have its own code of bases on the DNA
Each 4 bases (ATCG) are present in DNA
If each base coded for a different amino acid , only 4 different amino acids could be coded for
So 3 bases produce 64 different codes , more than enough to satisfy the requirements of 20 amino acids
3 features of the genetic code
Degenerate
Non over lapping
Universal
Why is genetic code known as degenerate
Most amino acid sequences are coded for by more than one triplet
What are the 3 triplet so-called that dont code for any amino acid
Stop codons + mark the end of the polypeptide chain
Why is the genetic code non-overlapping
Each base in the sequence is only read once
Why is it universal
Each triplet codes for the same amino acid in all organisms
Indirect evidence for evolution
What are coding sequences called
Exons
Non coding sequences
Introns
DNA in eukaryotic cells
DNA molecules are longer and linear
Associated with histones to form chromosomes
Mitochondria + chloroplasts also contain DNA (shorter and circular + not associated with proteins )
DNA in prokaryotic cells
DNA molecules are shorter and circular
DNA is not associated with protein molecules
Prokaryotic cells therefore do not have chromosomes
What do chromosomes appear as when they first become visible, why
At the start of cell division chromosomes appear as 2 threads lines joined at the centromere
Each thread is a chromatid because DNA has already replicated to give 2 identical DNA molecules
What is DNA in chromosomes held by
Histones
What are homologous chromosomes
CARRY THE SAME GENES
Pair of chromosomes in a diploid organism that
Have the same genes at the same loci
Are the same size and shaoe
Have the same centromere position
One is paternal one is maternal
What is an allele
One of a number of alternative forms of a gene
What happens when 2 alleles are different
Each allele has a different base sequence , therefore a different amino acid sequence , so produces a different polypeptide
What is a mutation
Any changes in the base sequence of DNA of a gene
What does a mutation in the base sequence of a gene produce
A new allele of that gene
Results in a different sequence of amino acids being coded for
Leading to the production of a different polypeptide and hence a different protein and h
What happens if the new protein produced is an enzyme
It may have a different shape the new shape may not fir teh substrate
Enzyme won’t be able to function having serious consequences for the organism
How is coded information on the DNA molecule in the nucleus transferred to the cytoplasm where it is translated into proteins
The sections of the DNA code are transcribes onto a single stranded molecule - RNA
What is mRNA
The rna that transfers the DNA code from the nucleus to the cytoplasm where the coded information that it contains is used to determine the sequence of amino acids which are synthesised there
As it is small enough to leave via nuclear pores
What does codon refer to
The sequence of 3 bases on mRNA that code for a single amino acid
Define genome
Complete set of genes in a cell , including those in mitochondria and/or chloroplasts
Define proteome
The full range of proteins produced by the genome (complete proteome, in which case proteome refers to the proteins procured by a given type of cell under a certain set of conditions )
Full amount of protein that can be produced by the cell
Describe the structure of mRNA
Long strand arranged into a single helix
Consisting of thousands of mononucleotides
Linear
What is the base sequence of mRNA determined by
The sequence of bases on a length of DNA in transcription
Where does mRNA act as a template for protein synsthesis
Once leaving the nucleus via nuclear pores in the nuclear envelope.
It enters the cytoplasm and associated with the ribosomes
Why is mRNAs structure suited to its function
It possesses information in the form of codons
The sequence of codons determines the amino acid sequence of a specific polypeptise that will be made
Structure of tRNA
A relatively small molecule (80 nucleotides)
Single stranded chain folded into a clover leaf shape
With one end of the chain extending beyond the other - where an amino acid can easily attach
What is on the opposite end of a tRNA molecule
Anticodon
What is each tRNA molecule specific to
One amino acid and has an anticodon specific to that amino acid
What is thymine replaced by in RNA
Uracil
What is mRNAs role
Lining up amino acids on the mRNA template during protein synthesis
Order to make a protein - basic
DNA provides the instructions in the form of a long sequence of bases
A complementary section of part of this sequence is made in the form of a premRNA = transcription
Pre mRNA is spliced to from mRNA
This is used as a template to which complementary t RNA molecules attached a and the amino acids are linked to form a polypeptide
= translation
What is transcription the process of
Making pre-mRNA using part of the DNA as a template
Process of transcription
DNA helicase breaks the hydrogen bonds between complementary base pairs , sepeatng the strand to expose the nucleotide bases in that region
Nucleotide bases on template strand pair with complementary nucleotides from the pool which is. Present in the nucleus
RNA polymerase moves along the strand and joins complementary nucleotides together to form a pre mRNA molecule
When rna polymerase reaches a particular sequence of bases of DNA - stop codon
It detaches and the production of pre mRNA is complete
What is splicing
The process where introns are removed from the pre-mRNA and exons are joined together to form mrna
Only happens in eukaryotes prokaryotes no not have introns
Process of translation in detail
A ribosome becomes attached to the start codon at one end of the mRNA molecule
tRNA molecule with the complementary anticodon sequence moves to the ribosome and pairs up with the codon on the mRNA
This tRNA carries a specific amino acid
A tRNA molecule with a complementary anticodon pairs with the next codon on the mRNA - this tRNA molecule carries another amino acid
The ribosome moves along the mRNA , bringing together 2 tRNA molecules at 1 time , each pairing with the corresponding 2 codons on the mRNA
2 amino acids on tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed to provide required energy
Ribosome moves onto the 3rd codon - links amino acids on 2nd and 3rd tRNA molecules
As this happens the first tRNA molecule is released from its amino acid and is free to collect another amino acid
Process continues until a polypeptide chan is built up
Synthesis stops when ribosome reaches a stop codon
Where ribosome , mRNA and last tRNA molecule all separate and the polypeptide chain is complete
What is the start codon
AUG
What do the sequence of codons on the mRNA determine
Order in which tRNA molecules line up
-sequence of amino acids in the polypeptide
What is a functional protein
A protein that has folded into its correct 3D shape and can carry out its specific biological role in the cell
What happens to the polypeptide after translation
It’s coiled / folded producing its secondary structure
Secondary structure is folded producing ytertiary structure
Different polypeptide chains are linked to form quaternary structure
4 structural differences between a dna and an mRNA molecule
DNA is long mRNA is short
DNA is a double helix whereas rna is single stranded
DNA is bound to histone proteins mRNA is not
DNA contains thymine mRNA has uracil
2 types of molecule from which a ribosome is made
Functional rna and proteins
3 ways in which the DNA in a chloroplast os different from DNA in a nucleus
DNA in chloroplast is a circular shape wheras is a nucleus it is linear
DNA in a chloroplast contains only exons whereas dna in a nucleus contains exons and introns
DNA in a nucleus is longer than dna in a chloroplast
Why don’t all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide
Only introns mutate as the code is degenerate
All introns are spliced therefore cant cause a change in the sequence of amino acids determining the structure
Starting with mRNA in the cytoplasm describe how translation leads to the production of a polypeptide
tRNA molecules bring a specific amino acid to the ribosome at yhe start codon on mRNA
tRNA molecules contain an anticodon which is complementary to the codon on mRNA
Amino acids join together by a peptide bond
2 biological molecule that can be coded for by a gene
tRNA + mRNA
Role of ATP in translation
Provides required energy to form peptide bond between amino acids to form the polypeptide chain
How is mRNA formed by translation in eukaryotes
Hydrogen bonds break
Only one dna strand acts as a template
RNA nucleotides align by complementary pairing
In RNA uracil replaces thymine
RNA polymerase joins adjacent nucleotides by phosphodiester bonds
Describe how a polypeptide is formed by translation of mRNA
mRNA attatches to ribosomes
Anticodons bind to complementary codons
Amino acids join together by peptide bonds use of ATP
And tRNA s released
Ribosome moves along the mrna to form the polypeptide
What’s a difference between the structure of mRNA and pre-mRNA
PremRNA has exons and introns as splicing hasn’t occured
Which is why mRNA has no introns
Suggest one way the structure of the chromosome could differ along its length to result in the strain binding more in some areas
Difference in base sequences
Difference in histone interaction
Not all mutations in the nucleotide sequence of a gene cause a change in the structure of the polypeptide , 2 reasons why
Occurs in introns
Triplets code for the same amino acid - degenerate
Define non-coding base sequences and where they are positioned in the genome
DNA that does not code for a protein
Positioned between genes
