Genetic information, variation and relationships between organisms Flashcards
Eukaryotic DNA
Long, linear associated with proteins called histones
Tightly coiled into chromosomes (DNA molecule and its associated proteins)
Prokaryotic DNA
Short, circular, not associated with proteins/histones
DNA in mitochondria and chloroplasts
Similar to prokaryotic DNA - short, circular, not associated with proteins/histones
Genes
Sequence of DNA bases that codes for the amino acid sequence of a polypeptide or a functional RNA molecule eg ribosomal RNAs and tRNAs
A gene occupies a fixed position, called a locus, on a particular DNA molecule
Features of the genetic code
Sequence of DNA triplets (or mRNA codons) codes for sequence of amino acids
Universal; the same DNA base triplets code for amino acids in all living organisms
Non-overlapping so it is discrete - each base can only be used once and in only one triplet
Degenerate, so the same amino acid can be coded for by more than one base triplet
DNA coding and non-coding in eukaryotes
Places between genes contain many non-coding sections of DNA (non-coding multiple repeats, same base sequence repeated multiple times) which do not code for any amino acids
Within genes, only exons code for amino acid sequences, which are separated by one or more non-coding sequences called introns
Genome
The complete set of genes in a cell, including those in mitochondria and/or chloroplasts
Proteome
The full range of proteins that a cell/genome is able to produce
Alleles
Different version (sequences of bases/triplets) of the same gene
Homologous pair of chromosomes
Same size chromosomes with the same genes, but different alleles
Codon
Sequence of three mRNA bases that codes for a specific amino acid
Anticodon
Sequence of three tRNA bases that are complementary to a codon
Triplet
Sequence of three DNA bases that codes for a specific amino acid
Protein synthesis: 2 stages
Transcription; production of mRNA from DNA within the nucleus
Translation; production of polypeptides from the sequence of codons carried by mRNA in the cytoplasm on ribosomes
Messenger RNA (mRNA)
Made by transcription in the nucleus and acts as a template for translation in the cytoplasm
It is a straight chain molecule. Sequence of bases on RNA determines sequence of amino acids in polypeptide chains
Sequence of bases on RNA determined by sequence of bases on DNA (triplets - codons)
It is chemically unstable so it breaks down after a few days
Transfer RNA (tRNA)
Carries an amino acid (binding site)
Single polynucleotide strand that is folded (3 hairpin loops) which is held together by hydrogen bonds
Has an anticodon (3 bases) which bases are complementary to mRNA codon
Each tRNA specific to one amino acid, in relation to its anticodon
Similarities/differences between the structure of mRNA and tRNA molecules
Similarities: both single polynucleotide strand
Differences: mRNA single helix/straight whereas tRNA is folded into a clover shape.
mRNA is longer, variable length whereas tRNA is shorter
mRNA contains no paired bases or hydrogen bonds, whereas tRNA has some paired bases and hydrogen bonds
Transcription
Occurs in the nucleus.
DNA double helix is unzipped by helicase and the hydrogen bonds are broken.
RNA nucleotides align next to their complementary bases on the template strand forming temporary hydrogen bonds (thymine is replaced by uracil in RNA)
RNA polymerase joins adjacent nucleotides in a condensation reaction forming phosphodiester bonds.
When RNA polymerase reaches stop codon, mRNA (prokaryotes) or pre-mRNA (eukaryotes) detaches from DNA
mRNA leaves nucleus via nuclear pore
Post transcriptional modification
Eukaryotic genes contain exons (coding regions) and introns (non-coding regions)
Whole gene is transcribed to pre-mRNA, which contains introns and exons.
Splicing is where introns are removed and exons are spliced together in different combos for different proteins
Prokaryotic DNA doesn’t contain introns and mRNA is directly produced from DNA, no splicing involved
Translation
Sequence of mRNA codons determines sequence of amino acids
tRNAs carry specific amino acids, in relation to their anticodon
At the ribosome tRNA anticodon binds to mRNA codon and hydrogen bonds are formed. The first codon is the start codon.
Two amino acids are joined by condensation, forming a peptide bond using energy from ATP.
tRNA detaches without its amino acid, ribosome moves along mRNA to next codon, which continues until stop codon (polypeptide is released)
Role of ATP in translation
Hydrolysis of ATP to ADP + Pi releases energy
For the bond between the amino acid and its corresponding tRNA molecule - the amino acid attaches at amino acid binding site.
For peptide bond formation between amino acids
Role of tRNA in translation
tRNA attaches to and transports a specific amino acid, in relation to its anticodon
tRNA anticodon has complementary base pairs to mRNA codon, forms hydrogen bonds
Two tRNAs bring amino acids together for the formation of a peptide bond
About 60 types of tRNAs to carry 20 different amino acids. Genetic code is degenerate.
Role of ribosomes in translation
Attaches to mRNA and houses tRNA, allowing codon-anticodon complementary base pairing
Allows peptide bonds to form between amino acids
Gene mutation
A change in the base sequence of DNA (on chromosomes)
Can arise spontaneously during DNA replication (interphase)
Involves base deletion/substitution
Mutation effects
Leads to the production of a non-functional protein/enzyme
Change in base/triplet sequence of DNA/gene
Changes sequence of codons on mRNA
Changes sequence of amino acids in the primary structure of the polypeptide
Changes position of hydrogen/ionic/disulfide bonds in tertiary structure of protein
Changes tertiary structure/shape of the protein (and active site if enzyme)
If enzyme, substrate can’t bind to active site and form an enzyme-substrate complex
Base deletion
One nucleotide/base removed from DNA sequence
Changes triplet/codon sequence from the point of mutation (frameshift)
Changes sequence of codons on mRNA after point of mutation
Changes sequence of amino acids in primary structure of polypeptide
Changes position of hydrogen/ionic/disulphide bonds in tertiary structure of protein
Changes tertiary structure/shape of protein i.e. non-functional or new and superior
Base substitution
Nucleotide/base in DNA replaced with another nucleotide/base
Change in one base; changes one triplet
1.) Changes one mRNA codon and one amino acid; sequence of amino acids in primary structure of polypeptide changes etc.
OR
2.) Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid so the sequence of amino acids in the primary structure of the polypeptide remains unchanged
Mutagenic agents
Increase the rate of gene mutation (above the rate of naturally occurring mutations) e.g. ultraviolet light or alpha particles
Pre meiosis
Before meiosis starts, DNA replicates so there are two copies of each chromosome, called sister chromatids, joined by a centromere.
Meiosis I (first division)
Separates homologous pairs
Chromosomes arrange into homologous pairs
Crossing over (prophase I) creates genetic variation in gametes.
Independent segregation (metaphase I) increases genetic variation in gametes (2n)
Meiosis II (second division)
Separates chromatids (n).
Creates 4 haploid cells (from a single diploid parent cell) that are genetically varied
How meiosis creates genetic variation
Crossing over between homologous chromosomes. Alleles exchanged between chromosomes. Creates new combinations of maternal and paternal alleles on chromosomes
Independent segregation of homologous chromosomes. Random alignment of homologous pairs at equator; random which chromosome from each pair goes to each daughter cell. Creates different combinations of maternal and paternal chromosomes and alleles in daughter cells
Random fertilisation when two gametes fuse to form a zygote
Importance of meiosis
Two divisions – creates haploid gametes (half number of chromosomes)
Diploid number restored at fertilisation
Maintains chromosome number from one generation to the next
Independent segregation and crossing over creates genetic variation
Mutations in the number of chromosomes – chromosome non-disjunction
Homologous chromosomes fail to separate during meiosis I OR sister chromatids fail to separate during meiosis II
One gamete has an extra copy of this chromosome and the other has none
Upon fertilisation, zygote has one fewer (dies) or one extra chromosome (survives)
Arises spontaneously
Causes genetic diseases e.g. down’s syndrome in humans – extra copy of chromosome 21
Differences between the outcomes of mitosis and meiosis
Mitosis produces diploid cells whereas meiosis produces haploid cells; two divisions in meiosis whereby homologous chromosomes separate then chromatids separate, whereas one division in mitosis whereby only sister chromatids separate
Daughter cells genetically identical to each other and parent cell in mitosis whereas in meiosis, daughter cells are genetically varied. Crossing over and independent segregation during meiosis I whereas no crossing over in mitosis
Mitosis produces 2 daughter cells whereas meiosis produces 4 daughter cells. Two divisions in meiosis whereas only one division in mitosis
