Genetics, Biodiversity and Classification Flashcards
what is a gene?
a section of DNA that contains a code for making a polypeptide and functional RNA - the code is a specific sequence of bases
what is a locus?
location of a particular gene on a chromosome
what is an allele?
one of a number of alternative forms of a gene
what are homologous pairs?
chromosomes that have exactly the same genes, but might have different alleles and are exactly the same size
how is DNA stored in a eukaryote?
- stored as chromosomes in nucleus
- chromosomes in eukaryotic cells are linear in shape
- to tightly coil DNA to fit in nucleus as chromosomes, DNA tightly wound around histones (a protein) to form nucleosomes
what is a nucleosome?
complex of DNA wrapped around a histone
how is DNA stored in prokaryotic cells?
- prokaryotes also carry DNA in chromosomes but the DNA molecules are shorter + circular
- DNA is NOT wound around histones and instead supercoils to fit in the cell
describe the DNA found in chloroplasts and mitochondria
- short and circular
- not protein bound
what are the three key features of the genetic code?
- degenerate
- universal
- non-overlapping
what is a start codon? what does it do?
- first codon on the DNA and mRNA
- initiates translation
what is a stop codon? what does it do?
- final codon
- does not code for an amino acid + therefore there is no complementray anticodon with a particular amino acid
- so in translation causes the ribosome to detach, therefore stopping translation
why is the genetic code described as being degenerate? why is this an advantage?
- most amino acids can be coded for by more than one codon
- if a point mutation occurs, even though the codon will be different, it may still code for the same amino acid and therefore have no effect
why is the genetic code described as being universal? why is this an advantage?
- the same triplet of bases codes for the same amino acid in all organisms
- means genetic engineering is possible
why is the genetic code described as being non-overlapping? why is this an advantage?
- each base in a gene is only part of one codon that codes for one amino acid
- therefore each codon is read as a discrete unit
- if point mutation occurs, will only affect 1 codon and therefore only 1 amino acid
what are introns? where are they found?
- sections of DNA that do NOT code for amino acids and therefore polypeptide chains
- found in eukaryotic DNA but not in prokaryotic DNA
- get spliced out of mRNA molecules
what are exons?
sections of DNA that code for a sequence of amino acids
what is a genome?
complete set of DNA in one cell
what is a proteome?
full range of proteins in one cell
compare a genome to a proteome
- the genome is the complete set of DNA in one cell whereas the proteome is the full rangeof proteins one cell
- genome should never change whereas the proteome of the cell is constantly changing depending on which proteins are currently needed
where does transcription take place?
nucleus
outline the process of transcription
- DNA helicase breaks hydrogen bonds between bases, helix unwinds + bases exposed and act as a template
- only one chain of the DNA acts as a template
- free mRNA nucleotides in nucleus align opposite exposed complementary DNA bases
- RNA polymerase bonds together the RNA nucleotides to form pre-mRNA
what happens to pre-mRNA after transcription?
- has to be modified to become mRNA ready to leave the nucleus and take part in translation
- introns are spliced out by splicesome proteins leaving only exons behind
outline the process of translation
- once modified mRNA left nucleus, attaches to ribosome in cytoplasm
- ribosome attaches to start codon
- tRNA molecule with complementary anticodon to start codon aligns opposite mRNA + is held in place by ribosome
- ribosome move along mRNA molecule to enable another complementary tRNA to attach next to codon on mRNA
- the 2 amino acids that were delivered by tRNA molecule are joined by a peptide bond - catalysed by an enzyme + requires ATP
- continues until ribosome reaches stop codon at end of mRNA molecule. stop codon doesnt code for amino acid + so ribosome detaches + translation ends
- polypeptide is now created and will enter the golgi body for folding + modification
which 2 mechanisms introduce variation in organisms?
- independent segregation (of homologous chromosomes)
- crossing over (between homologous chromosomes)
what is independent segregation? how can the number of combinations this produces be calculated?
- in meiosis 1, homologous pairs of chromosomes line up opposite each other at the equator of the cell
- it is random which side of the equator the parernal and maternal chromosomes from each pair lie
- these pairs are separated so one of each homologous pair ends up in the daughter cell, which creates a large number of possible combinations of chromosomes in the daughter cells produced
- calculate using 2n, where n = number of homologous pairs (e.g. in humans 223)
what is crossing over?
- when homologous pairs line up opposite each other at the equator in meiosis 1, parts of the chromatids become twisted around each other
- this puts tension on the chromatids causing parts of the chromatids to break
- broken parts of the chromatid recombine with another chromatid, resulting in new combos of alleles
compare meiosis and mitosis
meiosis: 2 nuclear divisions, haploid cell (one set of chromosomes), introduces genetic variation
mitosis: 1 nuclear division, diploid cells (two sets of chromosomes), creates genetically identical cells