AS Booklet 9- DNA and Genetic Diversity Flashcards
Describe DNA, chromosomes, alleles, genes, genome, proteome.
DNA- genetic material in humans and most living organisms.
Chromosomes- structures that are friend from DNA and histones during cell division.
Alleles- different forms of the same gene that code for different types of same characteristic.
Genes- sequences of DNA based that code for specific amino acid sequence of polypeptide.
Genome- complete set of genes in a cell.
Proteome- full range of proteins that a cell can produce.
Explain the features of the genetic code
- Three nucleotide bases = triplets coding for specific amino acids.
- Four différent nucleotides: Guanine, Cytosine, Thymine and Adenine.
- Guanine and Cytosine pair, Thymine and Adenine pair.
- 64 (4 cubed) différent triplets or mRNA codons.
- 20 commonly occurring amino acids.
- Stop codes don’t code for amino acids but indicate end of code for specific polypeptide, stops translation for that sequence.
- Start codes don’t code for amino acids but indicate start of code for specific polypeptide, starts translation for that sequence.
- Code is non-overlapping.
- Code is universal: same triplet code for same amino acids in all organisms.
What is non-coding DNA and how does it get removed from the code during transcription?
Non-coding DNA (such as introns and multiple repeats) is removed during splicing after pre-mRNA is complete.
Introns are in a gene, multiple repeats are not in a gene.
Exons are base sequences within the gene that do code for amino acids.
What are the types of RNA and how are they different in function and structure? (use stuff from previous Booklet)
mRNA:
- linear structure containing codons.
- Involved in protein synthesis.
- formed in nucleus during transcription.
- moves into cytoplasm to ribosomes for translation.
tRNA:
- At least 20 different types of tRNA found in cytoplasm.
- clover shape due to presence of H bonds between complementary base pairs.
- at one end there’s an anticodon (3 unattached tRNA nucleotide bases).
- binding site for specific amino acid at other end.
rRNA:
- ribosomes are formed from rRNA and proteins.
Describe transcription
- takes place in nucleus.
- section of DNA transcribed into mRNA so it can translated by ribosome and proteins can be synthesised.
- section of DNA unzips/uncoils and strands separate as H bonds are broken by DNA helicase. One strand acts as template.
- free RNA nucleotides line up alongside DNA nucleotide bases on base template strand due to specific complementary base pairing.
- Uracil RNA nucleotide is complementary to Adenine (not Thymine in mRNA).
- RNA polymerase forms sugar-phosphate backbone along the back of free RNA nucleotides.
- pre-mRNA leaves through nuclear lore and goes through splicing (eukaryotes only) to remove introns.
- mRNA moves to ribosome.
Describe translation
- occurs at ribosome in the cytoplasm.
- tRNA molecule with complementary anticodon to first codon on mRNA strand moves to ribosome with specific amino acid attached at amino acid binding site.
- first amino acid is attached to amino acid on second tRNA molecule in ribosome by a peptide bind during condensation reaction that requires ATP + enzyme.
- first tRNA molecule then moves away, leaving amino acid behind and goes back to amino acid pool in cytoplasm.
- when stop codon is reached, specific polypeptide has been produced.
- polypeptide folds itself into secondary and tertiary structure, sometimes quaternary.
How is genetic diversity achieved?
- Gene mutations.
- Meiosis (independent segregation, crossing over).
- Random fusion of gametes during fertilisation.
- Chromosome mutations.
What are the types of mutations?
Deletion:
- removal of one or more bases.
- causes frame shift, alters base triplets and therefore base sequence from point of deletion.
- protein can be formed non-functional.
Substitution:
- the replacement of one or more bases.
- means new triplet code, can result in non-functional protein.
- one different amino acid can result in functional protein.
- one different amino acid could still result in same protein sequence due to degeneracy.
What causes mutations?
- spontaneous
* mutagenic agents such as radiation such as X-rays. UV light and benzene
What are the differences between meiosis and mitosis?
Meiosis: Two nuclear divisions Four cells produced Genetically different cells produced Haploid cells produced
Mitosis: One nuclear divisions Two cells produced Genetically identical cells produced Diploid cells
Describe meiosis
First meiotic division:
• Homologous pairs separate.
• Haploid number of chromosomes (but homologous pairs).
Second meiotic division:
• Sister chromatids separate.
• Haploid number of chromosomes.
Results:
Four genetically different daughter cells that are haploid and have one sister chromatid from each homologous pair.
Describe the different occurrences that result in genetic diversity
Independent segregation:
• Homologous pair, but then one sister chromatids of each pair goes into gametes.
• Means different combinations.
• 2n is used where ‘n’ is number of homologous pairs and is used to calculate possible combinations.
Crossing over:
• Chromatids of homologous chromosomes are next to each other.
• One chromatid from each homologous chromosome intertwines with the other forming a bivalent.
• Results in exchange of alleles if same gene to produce new combinations of alleles known as recombinants.
• Especially works with autosomally-linked alleles.
Non-disjunction:
• Happens when a homologous pair of chromosomes DON’T separate during first meiotic division.
• Both members of homologous air move to same pole of cell.
• Non-disjunction of one pair result in some gametes win an extra chromosome and one with one less than normal.
What are the principals of natural selection in evolution?
- variation already present in population due to random mutations, can result in new alleles of a gene.
- many mutations are harmful but some new alleles can be advantageous in certain environments and leads to increased reproductive success.
- change in environment results in organism with advantageous allele to be selected for as they’re better adapted.
- organisms with allele for selected phenotype survive and reproduce, pass on beneficial allele whole those without allele don’t- differential reproductive success.
- frequency of this allele increases over many generations as it is inherited by members of next generation.
Adaptations can be anatomical (change in what you have), physiological (change in how it works) or behavioural (hanged sound of call, for example)
What are the different types of natural selection?
Directional and stabilising.
Directional:
• change in environment.
• leads to selection for organisms with alleles for a phenotype towards one extreme of range (antibiotic resistance for example).
• over generations frequency of alleles for this phenotype are passed on until whole species has shifted to one extreme of range.
Stabilising:
• environment remains stable
• selection acts against the extreme phenotypes.
• selects for organisms with phenotype closest to mean (for example birth weight in humans).
• natural selection favours organisms with phenotypes close to middle of range, so organisms w phenotypes closer to mean are more likely to survive to breed and pass on these alleles to next generation
• frequency of allele increases with each generation.
What is degeneracy?
Some triplet codes are degenerate, meaning more than one triplet code codes for the same amino acids.
