Prokaryotes & Eukaryotes

Question 1

Compare transcription in prokaryotes and eukaryotes. [4]

Answer 1

prokaryotes: eukaryotes

Binding of RNA
polymerase

RNA polymerase binds directly
to the promoter* via the sigma
factor;

RNA polymerase is recruited by
general transcription factors*
to bind to the promoter*
forming the transcription
initiation complex*;

Genes
transcribed

Transcription of several genes
in an operon controlled by one
promoter;

Transcription of one gene
controlled by one promoter;

mRNA formed Polycistronic* mRNA is

formed;

Monocistronic* mRNA is
formed;

Upregulation of
transcription

In lac operon, cAMP binds to
catabolic activator protein*
(CAP). Active CAP binds to
CAP binding site* in the
promoter region and increase
affinity of RNA polymerase*
binding to the promoter*, thus
increasing the frequency of
transcription;

Activator* binds to enhancer*
causing bending of spacer DNA
and promoting the assembly of
the transcription initiation
complex which will increase
frequency of transcription;

Downregulation
of transcription

Binding of repressor* to
operator* prevents the RNA
polymerase binding to the
promoter and hence preventing
transcription.
E.g. lac repressor bind to
operator in lac operon/ trp
repressor bind to operator in trp
operon;

Binding of repressor* to
silencer* will prevent the
assembly of the transcription
initiation complex which will
decrease frequency of
transcription;

Question 2

Compare how genetic variation arises in prokaryotes and eukaryotes. [10]

Answer 2

Prokaryote: eukaryote

1. Method of
   obtaining
   genetic
   variation

1A. Mainly by horizontal gene
transfer;

1B. Mainly by meiosis and
sexual reproduction;

2. Methods of
   genetic
   variation

2A1. Conjugation whereby the
F plasmids to transferred from
host to the recipient via a
mating bridge;

2A2. Specialised
transduction* whereby a
temperate phage takes up an
adjacent bacterial DNA of the
provirus when it undergoes the
lytic phase and phage infects
another bacteria. This is
transferred to another bacteria
via homologous recombination;
2A3. Transformation* whereby
fragments of naked DNA are
taken in by a competent
bacteria cell and introduced into
the bacterial chromosome by
crossing over of homologous
regions;

2B1. Meiosis: During
crossing over between
non-sister chromatids of
homologous
chromosomes* in prophase
1 results in new
combinations of alleles on
chromatids. (& eventually a
variety of offspring)
2B2. Independent
assortment of homologous
chromosomes at the
metaphase plate & their
subsequent separation
during metaphase I &
anaphase I respectively &
2B2. Random orientation of
non-identical sister
chromatids of each
chromosome at the
metaphase plate & their
subsequent separation
during metaphase II and
anaphase II respectively
2B3. This results in gametes
with different combinations
of maternal & paternal
chromosomes. (& eventually
in a variety of offspring)
3. Random fusion of
gamete occurs during sexual
reproduction/fertilisation
results in offspring with a
variety of genotypes &
possibly phenotypes (&
hence a variety of
offspring);

4. Mutations As there is only 1 chromosome, there will not be mutations that involve more than 1 chromosome, such as non-disjunction, polyploidy and translocation

As there are more than 1 chromosome, non-disjunction, translocation and polyploidy can occur;

Similarities:
5. Mutations are a source of genetic variation for both prokaryotes and eukaryotes;
6. Mutations such as point mutations and gene mutations such as inversion,
substitution, deletion and addition can occur in both;
7. Role of mutations in both can be significant as novel phenotypes can be selected for
during natural selection hence increasing the frequency of alleles coding for the
favourable phenotype in the subsequent generation;
8. QWC: Mention both similarities and differences.