module 4; genetic info, variation & relationships between organisms Flashcards
state the similarities between eukaryotic & prokaryotic DNA
made of DNA nucleotides containing deoxyribose, a phosphate group & nitrogenous base
nucleotides are joined together by phosphodiester bonds to make a polymer chain
state the differences between eukaryotic & prokaryotic DNA
eukaryotic DNA:
longer
linear
associated with histones
prokaryotic DNA:
shorter
circular
not associated with histones
state the similarities between mitochondria & chloroplast of eukaryotic cells & prokaryotic DNA
they are both short, circular & not associated with histones
define the term gene
a gene is a base sequence that codes for the amino acid sequence of a polypeptide & a functional RNA
what is a locus?
a locus is the particular position in which the gene occupies on the chromosome
what is the triplet code?
a sequence of 3 DNA bases, code for a specific amino acid
state the features of the genetic code
- degenerate
- universal
- non-overlapping
how many amino acids is the genetic code able to code for & why?
20 amino acids as there are 4 DNA bases (GCTA), & so 3 bases are needed to make enough combinations to code at least 20 amino acids
explain the proof in which the genetic code requires 3 bases to code for 20 amino acids
if 1 base is coded for 1 amino acid, it would allow for 4 aminos to be coded for - insufficient to code for 20 amino acids
if 2 bases code for 1 amino acid, it would allow for 16 aminos to be coded for (4x4 combinations) - also insufficient
if 3 bases code for 1 amino acid, it would allow for 64 aminos to be coded for (4x4x4 combinations) - more than needed to code for 20 aminos
explain the meaning of a degenerate genetic code
it’s where there are more combinations of amino acids to code for 20 aminos - it results in 1 amino being coded for by more than 1 triplet of bases (e.g. tyrosine is coded for by ATA & ATG)
how can we work out combinations of bases?
by using the genetic code wheel
what is an advantage of using the genetic code wheel?
it’s an advantage as if a point mutation occurs, even though the triplet bases will be different, it may still code for the same amino & so have no effect
what does universal mean in terms of coding for amino acids?
means the same triplet of bases codes for the same amino acid in all organisms
what does non-overlapping mean in terms of coding for amino acids?
means that each base in a gene is only part of 1 triplet of bases that codes for 1 amino acid - therefore, each codon, or triplet of bases, is read as a discrete unit
why is it an advantage that the genetic code doesn’t overlap?
it’s an advantage as if a point mutation occurs, it will only affect 1 codon, therefore 1 amino
define the term intron
introns are sections of DNA that do not code for polypeptides & are found in eukaryotic not prokaryotic DNA
define the term exon
exons are the sequences of DNA that code for amino acids
define the term codon
3 bases on mRNA that code for a specific amino acid
define the term start codon
3 bases at the start of every gene that initiate translation
define the term stop codon
3 bases at the end of every gene that causes ribosomes to detach & therefore stop translation
state the differences between a genome & proteome
the genome is an organism’s complete set of genes in a cell, whereas the proteome is a full range of proteins that a cell can produce
the genome should never change whereas the proteome of the cell is constantly changing depending on which proteins are currently needed
what is messenger RNA?
AKA - mRNA:
they are short, single stranded molecules found in the cytoplasm & nucleus
in mRNA, groups of 3 adjacent bases are called codons
how is mRNA made?
made during transcription - it’s copied from DNA & is therefore complementary to the DNA sequence
what is transfer RNA?
AKA tRNA:
in the cytoplasm, amino acids become attached to tRNA molecules - each tRNA is specific for 1 amino
what is an anticodon in terms of tRNA?
each tRNA molecule has a sequence of 3 bases called an anticodon - they are complementary to codons on the mRNA molecule
what is tRNA’s role in translation?
it carries the amino acids that are used to make proteins to the ribosomes
describe the structure & bonding of tRNA
it’s a single polynucleotide strand that is folded into a clover shape
H bonds between base pairs hold the shape
where are proteins created?
they are created on ribosomes
state & explain the stages of the production of proteins from the DNA code
- transcription:
where one gene on the DNA is copied into mRNA - translation:
where the mRNA joins with a ribosome & corresponding tRNA molecules bring the specific amino acid the codons code for
what is transcription?
this is where one gene on the DNA is copied on to the mRNA
why is mRNA able to carry the genetic code?
as mRNA is much shorter than DNA, it’s able to carry the genetic code to the ribosome in the cytoplasm to enable the protein to be made
describe the process of transcription
- the DNA helix unwinds to expose the bases to act as a template
- only one chain of the DNA acts as a template
- like with DNA replication, this unwinding & unzipping is catalysed by DNA helicase
- DNA helicase breaks the H bonds between bases
- free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases
- the enzyme RNA polymerase joins together the RNA nucleotides to create a new RNA polymer chain - one entire gene is copied
once copied, the mRNA of modified & then leaves the nucleus through the nuclear envelope pores
what is pre-mRNA & when is it formed in eukaryotes?
in eukaryotes, after transcription, pre-mRNA is made & it’s the mRNA that still contains the introns - the introns are spliced out by a protein called splicesome which leaves just the exons (the coding regions)
what does transcription create in prokaryotes?
it directly creates mRNA as prokaryotes don’t contain introns in their DNA
what is translation?
it’s the stage in which the polypeptide chain is created using both the mRNA base sequence & the tRNA
describe the process of translation
- once the modified mRNA has left the nucleus it’s attached to a ribosome in the cytoplasm
- the ribosome attaches at the start codon
- the tRNA molecule with the complementary anticodon to the start codon aligns opposite the mRNA, held
- the ribosome will move along 1 codon on the mRNA molecule to enable another complementary tRNA to attach to the next codon on the mRNA
- the 2 aminos that the tRNA has delivered are joined by a peptide bond - an enzyme catalyses this & requires ATP
- this continues until the ribosome reaches the stop codon at the end of the mRNA molecule - the stop codon doesn’t code for an amino & so the ribosome detaches & the translation ends
the polypeptide chain is now created & will enter the golgi body for folding & modification
define the term gene mutation
it’s a change in the base sequence of the DNA
when do gene mutations occur?
they randomly occur during DNA replication
what factors increase the likelihood of gene mutations occuring?
mutagenic agents can interfere with DNA replication such as; high energy radiation (UV light), ionisation radiation (gamma rays & x-rays) & chemicals (carcinogens)
how do gene mutations affect bases?
a gene mutation could result in a base being deleted or substituted for a different one
state the impact of a base substitution mutation
it may not have an impact because the new codon may still code for the same amino - this is because the genetic code is degenerate
state the impact of a base deletion mutation
base deletions result in a frameshift - removal of 1 base changes all of the subsequent codons, which is more harmful as multiple aminos being incorrectly coded for
define the term chromosome mutation
mutations in the no.of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis
define the term non-disjunction
non-disjunction is when the chromosomes or chromatids don’t split equally during anaphase
state the effects of chromosome non-disjunction
causes changes in the number or structure of whole chromosomes
state the forms in which chromosome non-disjunction during meiosis
- changes in whole sets of chromosomes (polyploidy)
- changes in the number of individual chromosomes (aneuploidy)
define the term polyploidy
it’s a process that changes in whole sets pf chromosomes occur when organisms have 3 or more sets of chromosomes rather than the usual 2 - mainly occurs in plants
describe the process of polyploidy
- there are 3 homologous chromosomes - each homologous pair is doubled due to DNA replication in interphase
- there is non-disjunction in meiosis I - all chromosomes fail to separate equally
- normal division in meiosis II - chromatids separate equally
- in meiosis II - no chromosomes in the 2 gametes (they won’t function)
- also in meiosis II - normal division so the chromatids will separate equally to the opposite poles in anaphase
- the resulting gametes have 2 copies of every chromosome so instead of a haploid gamete there is a diploid gamete
what occurs when a haploid gamete fuses with a diploid gamete?
a triploid gamete - 2 chromosomes from a diploid gamete & 1 chromosome from a haploid gamete
define the term aneuploidy
changes the number of individual chromosomes
state the cause of aneuploidy
causes when individual homologous pairs of chromosomes fail to separate during meiosis - this is non-disjunction & usually results in a gamete having 1 more or fewer chromosome
explain how down syndrome occurs
on fertilisation with a gamete that has the normal number of chromosomes, the resultant zygote will have more or fewer chromosomes than normal in all their body cells - down’s syndrome is 3 copies of chromosome 21
what is meiosis?
it involves 2 nuclear divisions & creates 4 haploid daughter cells from a single diploid parent cell
what does meiosis produce?
meiosis produces daughter cells. that are genetically different from each other
state the mechanisms involved in meiosis that introduce variation
- independent segregation of homologous chromosomes
- crossing over between homologous chromosomes
when do the mechanisms that introduce variation occur?
in the 1st round of division in meiosis (meiosis I)
describe the process of independent segregation
- homologous pairs of chromosomes line up opposite each other at the equator of the cell
- it’s random which side of the equator the paternal & maternal chromosomes from each homologous pair lie
- these pairs are separated, so one of each homologous pair ends up in the daughter cell
- this creates a large no.of possible combinations of chromosomes in the daughter cells produced - can be calculated with 2n
describe the process of crossing over
- when homologous pairs line up opposite each other at the equator in meiosis I, parts of chromatids can become twisted around each other
- this puts tension on the chromatids - causes parts of the chromatids to break
- the broken parts of the chromatid recombine with another chromatid
- results in new combinations of alleles
state the differences between meiosis & mitosis
meiosis:
2 nuclear divisions
results in a haploid cell (one set of chromosomes)
introduces genetic variation
whereas mitosis:
one nuclear division
results in diploid cells (two sets of chromosomes)
creates genetically identical cells
how can meiosis be identified in a life cycle?
meiosis involves a diploid (2n) parent cell dividing to become a haploid (n) cell
define the term genetic diversity
genetic diversity is the no.of different alleles of genes in a population
what does genetic diversity enable?
natural selection
define the term natural selection
it’s the process that leads to evolution in populations
what causes evolution?
the changes in allele frequency over many generations in a population
how does natural selection affect a species?
results in species becoming better adapted to their environment - the adaptation may be anatomical, physiological or behavioural
describe the process of natural selection
- random mutations create new alleles for a gene
- if the new alleles increase the chances of the individual to survive, then they are more likely to survive & reproduce
- this reproduction passes on the advantageous allele to the next generation
- results in: over many generations, the new allele increases in frequency in the population
state the types of selection
- directional selection
- stabilising selection
what is directional selection?
it’s when the advantageous allele codes for an extreme trait
when does directional selection occur?
occurs when there is a change in the environment - the modal trait changes
what is stabilising selection?
it’s when whenever the middling trait remains the selective advantage
when does stabilising selection occur?
occurs when there is no change in the environment - the modal trait remains the same
define the term species
a species is when two organisms belong to the same species if they can produce fertile offspring
how does species ensure their survival?
they reproduce to pass on their advantageous alleles - so courtship behaviour is essential for successful mating & species recognition
what is courtship behaviour?
a sequence of actions which is unique to each species - it’s how animals identify members of their species to reproduce with
state the importance of courtship
to ensure successful reproduction & survival of their offspring - enables them to recognise their species & opposite sex, synchronises mating behaviour
what can courtship sequences tell us about species?
can tell us how closely related different species are - if the courtship sequence is similar between different species they are more closely related
the more similar a species sequence is = the more similar its DNA sequence is
define the term phylogenetic classification
it arranges species into groups according to their evolutionary origins & relationships - tells us how closely related species are
what is a hierarchy?
smaller groups arranged within larger groups - no overlaps between groups
give an example of a hierarchy
domain - kingdom - phylum - class - order - family - genus - species
what is the binomial system?
it’s a universal way of identifying organisms using the binomial (2 names) - 1st name is genus & 2nd name is species
define the term species diversity
the no.of different species & individuals within each species in a community
define the term species richness
the no.of different species in a community
define the term genetic diversity
the variety of genes amongst all the individuals in a population of one species
define the term ecosystem diversity
the range of different habitats
give examples of farming techniques that reduce biodiversity
- destruction of hedgerows
- selective breeding
- monocultures
- over-grazing
- filling in ponds & draining wetlands
define the term index of diversity
it describes the relationship between the no.of species in a community (species richness) & the no.of individuals in each species (population)
how is the index of diversity calculated?
D = N(N - 1) / ∑n(n - 1)
N: the total no.of organisms of all species
n: total no.of organisms of a particular species
D: simpson’s diversity index
what do we compare to investigate biodiversity?
- the frequency of observable characteristics
- the base sequence of DNA
- the base sequence of mRNA
- the amino acid sequence of proteins
