genetics, biodiversity and classification Flashcards
what are DNA molecules like in prokaryotic cells and in mitochondria and chloroplasts?
short, circular and not
associated with proteins.
what are DNA molecules like in eukaryotic cells? (2)
1) found in the nucleus
2) long, linear and associated with proteins called histones to form chromosomes
when are chromosomes first visible and what are they made of? (4)
1) at the start of cell division
2) is the result of the DNA being tightly coiled around the histones
3) each thread is called a chromatid
4) DNA is highly coiled and folded to form the chromosome
what is a gene? (3)
1) a base sequence of DNA that codes for:
2) the amino acid sequence of a polypeptide
3) a functional RNA (including ribosomal RNA and tRNAs).
where are genes found? (2)
1) genes occupy a fixed position, called a locus
2) on a particular DNA molecule
what is a triplet?
A sequence of three DNA bases
what does a triplet code for?
a specific amino acid
how can you describe the genetic code? (4)
1) universal- each triplet codes for the same amino acid in all organisms. This is indirect evidence for evolution (with a few minor exceptions)
2) non-overlapping- each base in the sequence is only read once as triplets.
3) degenerate- most amino acids are coded for by more than one triplet
4) The genetic codes contains start and stop codons.
what are exons? (2)
1) A sequence of DNA that codes for an amino acid sequence. within a gene there are only some exons.
2) Within the gene, these exons are separated by one or more non-coding sequences, called introns.
In eukaryotes, much of the nuclear DNA does not code for (1)
There are, for example (2) of base sequences between (3)
1) polypeptides
2) non-coding multiple repeats
3) genes
what is the genome?
the complete set of genes in a cell
what is the proteome?
the full range of proteins that a cell is able to produce.
what is a codon?
a sequence of three bases on mRNA that codes for a specific amino acid.
what is an anti codon
a sequence of three nucleotide bases at one end of a tRNA molecule that is specific to an mRNA codon.
what is a homologous pair? (2)
1) two chromosomes that carry the same genes on the same location
2) they are not identical as they can carry different alleles of the same gene.
why is each amino acid represented by more than one triplet? (3)
1) there are four nucleotide bases which code for 20 different amino acids
2) Scientists were able to conclude that each amnio acid is coded for by one or more combination of triplets. 3) There are therefore 64 possible triplets (4^3) meaning that each amino acid is represented by more than one triplet.
what is an allele?
an alternative form of the same gene
what is the structure of mRNA? (5)
1) a long single polynucleotide chain strand arranged in a single helix, created during transcription in which the base sequence is complementary to DNA.
2) Each set of three bases, called the codon, match a triplet on the DNA as well as the tRNA.
3) pentose sugar is ribose
4) molecule is smaller than DNA but larger than tRNA
5) organic bases are adenine, uracil, guanine and cytosine
what is the structure of tRNA? (6)
1) a small molecule made up of around 80 nucleotides with bases U, A C and G
2) a single polynucleotide chain that is folded into a clover leaf shape in which one end extends from the others where an amino acids attaches
3) At the opposite end is an anti- codon which is specific to the amino acid that the tRNA attaches to
4) anticodon can recognize and decode an mRNA codon
5) pentose sugar is ribose
6) many types of tRNA each of which binds to a specific amino acid
what is transcription in simple terms?
the production of mRNA from DNA
outline the stages of transcription (5)
1) an enzyme acts on a specific region of the DNA causing the two strands to separate as hydrogen bonds between the complementary bases break and expose the nucleotide bases in that region
2) one of the DNA strands is used as a template strand to make the mRNA molecule and pairs with their complementary nucleotides from the pool which is present in the nucleus.
3) The enzyme RNA polymerase then moves along the strand and joins the nucleotides together by catalysing the formation of phosphodiester bonds to form a pre mRNA molecule
4) As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin behind it. As a result, only about 12 base pairs on the DNA are exposed at any one time.
5) When the RNA polymerase reaches a stop triplet code on the DNA, it detaches and the production of pre-mRNA is then complete
4) In eukaryotic cells the pre-mRNA is then spliced to remove the introns leaving just a strand of exons. The mRNA then moves out of the nucleus through a pore and attaches to a ribosome in the cytoplasm which is the site of next stage of protein synthesis called translation.
why do different species produce some unique proteins?
each genome is specific to different species
what does transcription result in, in prokaryotes?
the production of mRNA from DNA.
what does transcription result in, in eukaryotes?
the production of pre- mRNA; this is then spliced to form mRNA
what is translation in simple terms?
the production of polypeptides from the sequence of codons carried by mRNA
outline the process of pre-mRNA splicing (6)
1) pre-mRNA contains introns (non coding base sequences) which need to be removed
2) the introns would prevent the synthesis of the polypeptide
3) joining together the remaining exon sequence is called splicing
4) introns are removed by enzymes within a splicosome complex and exons are joined/spliced together\
5) the mRNA molecule leaves the nucleus through nuclear pores and is attracted to ribosomes
6) most prokaryotic DNA does not have introns
outline the process of translation (6)
1) a ribosome becomes attached to the starting codon at one end of the mRNA molecule
2) a tRNA molecule with the complementary anticodon sequence moves to the ribosome and pairs up with the codon on the mRNA. The tRNA carries a specific amino acid
3) the ribosome moves along the mRNA, bringing together two tRNA molecules. The amino acids attached to two tRNA molecules are joined by a peptide bond using an enzyme and ATP which is hydrolysed to provide the required energy.
4) as the ribosome moves along the codons, tRNA molecules detach themselves from the amino acids, leaving them behind and are free to collect another amino acid from the cytoplasm
5) the process continues with up to 15 amino acids being added each second, until a polypeptide chain is built up.
6) the synthesis of a polypeptide continues until a ribosome reaches a stop codon. At this point, the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide chain is complete
how do genes control the activities of cells? (5)
1) DNA sequence of triplets that make up a gene determine the sequence of codons on mRNA
2) sequence of codons on the mRNA determine the order in which tRNA molecules line up
3) this determines the sequence of amino acids in the polypeptide
4) genes determine which proteins a cell manufactures
5) many of these proteins are, enzymes
how can several proteins can be assembled simultaneously during translation? (3)
1) a ribosome can join up to 15 amino acids per second until the stop codon is reached
2) up to 50 ribosomes can move along the same strand of mRNA behind one another
3) many identical polypeptides can be assembled simultaneously
give 2 molecules from which a ribosome is made (2)
1) rRNA
2) protein
give 4 structural differences between a DNA molecule and a mRNA molecule
1) DNA- double helix/stranded mRNA- single helix/ strand
2) DNA- thymine mRNA- uracil
3) DNA- H bonds mRNA- no H bonds
4) DNA- long mRNA- short
give 5 ways in which the DNA in a chloroplast is different from DNA in the nucleus
1) DNA is shorter/ accept smaller
2) fewer genes
3) DNA is circular not linear
4) DNA not associated with proteins
5) has no introns
why do some mutations in the nucleotide sequence of a gene not cause a change in the structure of a polypeptide? (2)
1) more than one triplet can code for the same amino acid/ genetic code is degenerate
2) mutations may occur in introns
describe 4 differences between the structure of a tRNA molecule and the structure of a mRNA molecule
1) mRNA is linear whereas tRNA is clover leaf shaped
2) tRNA has hydrogen bonds but mRNA doesn’t
3) tRNA has an amino acid binding site mRNA doesn’t
4) tRNA has an anticodon, mRNA has a codon
describe and explain the difference in the structure of pre-mRNA and mRNA (2)
1) pre mRNA is longer as it has both exons and introns/ more nucleotides
2) mRNA only has exons/ less nucleotides as a result of splicing
describe how one amino acid is added to a polypeptide that is being formed at a ribosome during translation (3)
1) tRNA brings specific amino acid to ribosome
2) anticodon on tRNA binds to codon on mRNA
3) amino acids join by condensation reaction to form a peptide bond using ATP
why does a change in what bonds form affect the properties of a protein?
a change in which bonds form changes the tertiary structure of a protein
describe how mRNA is produced from an exposed template strand of DNA (3)
1) free nucleotides form complementary base pairs
2) phosphodiester bonds form between adjacent nucleotides
3) by action of RNA polymerase
what would the effect be of an addition of one nucleotide within a sequence in terms of mutation? (2)
1) frameshift occurs as all triplets are moved
2) so affects more than one amino acid
what is a mutation?
1) any change in the base sequence or quantity of DNA
2) gene mutations involve a change in the base sequence of chromosomes
when can gene mutations arise and what do they include (2)
1) can arise spontaneously during DNA replication
2) includes base deletion and base substitution
what can increase the rate of gene mutation?
Mutagenic agents
describe how base substitution can result in a different amino acid sequence in polypeptides and its effects? (5)
1) a nucleotide in the DNA sequence is replaced by another nucleotide that has a different base
2) original DNA triplet is replaced and so codes for a different amino acid
3) polypeptide produced will differ in a single amino acid
4) the effect of this change depends on the role of the original amino acid in the overall shape and function of the protein
5) a substitution may not always be harmful as the substituted nucleotide may code in that triplet for the same amino acid.
how can base substitution lead to a non functioning protein? (6)
1) a nucleotide in a DNA a molecule is replaced by another nucleotide that has a different base
2) original DNA triplet is replaced and so codes for a different amino acid
3) polypeptide produced will differ in a single amino acid
4) amino acid may be important in forming bonds that determine the tertiary structure of the final protein
5) replacement amino acid may not form the same bonds
6) protein may be a different shape and therefore not function properly.
what is the effect of base substitution if the new triplet of bases still codes for the same amino acid? (2)
1) genetic code is degenerate so most amino acids have more than one codon
2) If both DNA triplets code for the same amino acid there will be no change in the polypeptide produced and so the mutation will have no effect
how can base deletion result in different amino acid sequences in polypeptides? (4)
1) a deletion event is when a nucleotide in the DNA sequence is lost
2) the loss of a single nucleotide can have a significant impact as it leads to a frame shift as each subsequent triplet has been shifted to the left by one base, resulting in completely different amino acids being coded for
3) usually the amino acid sequence of the polypeptide is entirely different and so the polypeptide is unlikely to function correctly
4) this is because the sequence of bases in DNA is read in units of 3 bases/ triplets
what is a chromosome mutation?
changes in the structure or number of whole chromosomes
outline chromosome non-disjunction (4)
1) non-disjunction occurs when chromosomes/individual homologous pairs of chromosomes fail to separate correctly in meiosis
2) as a result the gametes and any zygotes formed will have one more or one less chromosome than they should
3) on fertilisation with a gamete that has the normal complement of chromosomes, the resultant offspring have more of fewer chromosomes than normal in all their body cells
4) Down’s syndrome is the result of non-disjunction where individuals have an extra chromosome 21
outline chromosome polyploidy (2)
1) changes can occur in the whole set of chromosomes so that an individual has three of more sets of chromosomes instead of two.
2) This is common in plants, with many modern wheats arising to be polyploidy.
how does mutation in the number of chromosomes arise?
can arise spontaneously by chromosome non-disjunction during meiosis
explain why a deletion gene mutation is more likely to result in a change to an organism than a substitution gene mutation (3)
1) in deletion all subsequent codons after the deletion are affected due to frameshift occurring
2) as a result most amino acids coded for by these codons will be different and the polypeptide produced will be affected more severely
3) in substitution only a single codon may change and may still code for the same amino acid due to the degenerate nature of the genetic code. Therefore a single amino acid will be affected and the effect on the polypeptide less severe
errors in transcription occur about 100,000 times more often than errors in DNA replication. Explain why errors in DNA replication can be far more damaging than errors in transcription (2)
1) DNA errors can be inherited and may have a permanent effect on the whole organism
2) errors in transcription are temporary in cells and not inherited
summarise meiosis (3)
1) two nuclear divisions result usually in the formation of four haploid daughter cells from a single diploid parent cell
2) genetically different daughter cells result from the independent segregation of homologous chromosomes
3) crossing over between homologous chromosomes results in further genetic variation among daughter cells.
what is the main role of meiosis? (2)
1) the production of haploid gametes
2) which is necessary to maintain a stable number of chromosomes
how is genetic variation achieved through meiosis? (3)
1) Independent segregation – there are various combinations of chromosome arrangement. During meiosis 1 homologous chromosomes line up in pairs, the arrangement of these pairs is random, meaning that the division into the daughter cells is also random
2) Crossing over of chromatids/ non-sister chromatids - When pairs of chromosomes line up they can exchange some of their genetic material. Crossing over occurs when one chromosome may swap places with the same part of its homologous pair leading to a different combination of alleles on the gene
3) both happen in meiosis 1/ first round of division
summarise the first nuclear division in meiosis/ meiosis 1 (2)
1) homologous chromosomes line up in pairs at the equator and are separated into 2 new cells - crossing over and independent segregation happens
2) The cell divides so each daughter cell contains one chromosome from each homologous pair
summarise the second nuclear division in meiosis/ meiosis 2
1) chromosomes line up at the equator and spindle fibres attach to centromeres, splitting them apart so sister chromatids are separated to either pole.
2) the chromatids of each chromosome are separated producing 4 genetically different haploid daughter cells
how can you identify where meiosis occurs when given information about an unfamiliar life cycle
look on the diagram where it goes from being a diploid parent cell to a haploid daughter cell/ or as 2n becoming n- this is where meiosis is happening
how can you calculate how many different possible gametes which can be made based on the no. of chromosomes a particular species has?
1)2^n
2) for humans this would be 2^23 bc we have 23 pairs of chromosomes
describe how crossing over introduces variation (4)
1) occurs in meiosis 1
2) homologous chromosomes align at the equator
3) non sister chromatids twist putting tension on chromatid so it breaks and rejoins on another chromatid
4) creates new combinations of alleles in gametes
describe how independent segregation introduces variation (5)
1) homologous chromosomes align opposite at the equator
2) it is random which side of the equator paternal and maternal chromosomes end up on
3) these separate into daughter cells
4) large possible combinations of chromosomes in daughter cells
5) use 2^n to work out the different combinations
after meiosis 1, are cells haploid or diploid?
the cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present.
why does meiosis 2 not change chromosome number?
1) each chromosome then splits into its two chromatids, giving one to each daughter cell.
2) Thus, the four cells remain haploid, but now with single-stranded chromosomes
explain the different outcome of mitosis and meiosis
1) Meiosis produces four cells
Mitosis produces two cells
2) -Meiosis produces haploid cells. The number of chromosomes in the daughter cells is half the number in the parent cells.
E.g. Meiosis in humans produces cells with 23 chromosomes.
-Mitosis produces diploid cells. The number of chromosomes in the daughter cells is the same as the parent cells.
E.g. Mitosis in humans produces cells with 46 chromosomes
3) Meiosis produces genetically different daughter cells.
Mitosis produces genetically identical daughter cells.
4) Meiosis only produces gametes (sex cells).
Mitosis allows the replication of every body cell.
explain how random fertilisation of haploid gametes further
increases genetic variation within a species (2)
1) Any sperm can fertilise any egg
2) Random fertilisation produces zygotes with different combinations of chromosomes to both parents
what is genetic diversity
the number of different alleles of genes in a population.
what enables natural selection?
Genetic diversity is a factor enabling natural selection to occur
how do the principles of natural selection link to the evolution of populations? (8)
1) Random mutation can result in new alleles of a gene
2) Many mutations are harmful but, in certain environments, the
new allele of a gene might give its possessor an advantage over other individuals in the population
3) these individuals will be better adapted to survive their environment and will outcompete others in the population
4) these individuals are more likely to obtain the available resources and so will grow more rapidly and live longer.
5) As a result, they will have a better chance of breeding successfully and producing more offspring
6) The advantageous allele is inherited by offspring
7) offspring with this advantageous are in turn more likely to survive and so reproduce successfully
8) As a result, over many generations, the new allele increases
in frequency in the population.
why aren’t all alleles of a population equally likely to be passed on to the next generation?
only certain individuals are reproductively successful and so pass on their alleles
what affects allele frequency in populations?
differences between the reproductive success of individuals
the larger the number of alleles of genes in the population…
the greater the genetic diversity for that population (larger variation)
what does natural selection result in? (3)
1) results in species that are better adapted to their environment.
2) these adaptations may be anatomical, physiological or behavioural
3) this is because alleles (genes) code for proteins which form living tissue
why is a larger genetic diversity in a population good? (2)
1) a larger genetic diversity (wider range of alleles in a population) results in a wider range of characteristics (more variation)
2) this means that it is more likely that at least some individuals within the population are better suited/ possess the characteristics to survive an environmental change
