Transmission genetics 1: cell and organismic reproduction
Important message:
In both prokaryotes and eukaryotes cell division has no
effects on allele frequencies. In eukaryotes meiosis
(making haploid cells) is also a “fair” process: half the
gametes of an Aa heterozygote are A and half are a.
These 2 insights are key to transmission & population
genetics.
1.1 prokaryotic and eukaryotic cells differ in a number of
genetic characteristics
Prokaryotes:
Unicellular (mostly), with no compartmentalised cell
structure
Prokaryotic DNA does not exist in the highly ordered and
packaged arrangement (but can have some structure e.g.
supercoiling)
Made up of eubacteria and archaea
Eukaryotes:
Both unicellular and multicellular with compartmentalised
cell structure
Its genetic material is surrounded in a nuclear envelope to
form a nucleus
DNA is closely associated with histones to form tightly
packed chromosomes (couldn’t get all that DNA in our
cells without packing. Sperm use protamines rather than
histones to ensure even tighter packing)
Eubacteria cell replication:
Simple division: separation of replicated circular
chromosome
Single Origin of replication (not true in archea) – in many
species genes commonly co-orient with the direction of
replication to prevent DNA polymerase (for replication)
and RNA polymerase (for transcription) clashing head on
, High rate of replication – in some species, genes near the
origin of replication are the most highly expressed as they
are diploid more often
Genes with related functions tend to cluster (both in and
out of operons) and be co-expressed
Eukaryotic cell replication: chromosomes:
In a diploid cell, chromosomes form a homologous pair
(humans have 23 pairs of chromosomes)
The chromosomes have specialist anatomy to enable cell
division:
- Centromere: attachment point for spindle microtubules.
Tend often to be gene poor domains.
- Telomeres: tips of a linear chromosome – need to be
stabilised to stop them falling apart
- Origins of replication: where DNA synthesis begins. There
are multiple on every chromosome.
The cell cycle is tightly coordinated with key check points
1.2 they face the same problem: cell reproduction requires
the copying of the genetic material, separation of the
copies, and cell division
The Cell Cycle:
Interphase: an extended period between cell divisions,
DNA synthesis, and chromosome replication phase
M phase (mitotic phase): two different parts:
- Mitosis: separation of sister chromatids
- Cytokinesis: separation of cytoplasm
Phase check points: key transition/ quality control
points
Important message:
In both prokaryotes and eukaryotes cell division has no
effects on allele frequencies. In eukaryotes meiosis
(making haploid cells) is also a “fair” process: half the
gametes of an Aa heterozygote are A and half are a.
These 2 insights are key to transmission & population
genetics.
1.1 prokaryotic and eukaryotic cells differ in a number of
genetic characteristics
Prokaryotes:
Unicellular (mostly), with no compartmentalised cell
structure
Prokaryotic DNA does not exist in the highly ordered and
packaged arrangement (but can have some structure e.g.
supercoiling)
Made up of eubacteria and archaea
Eukaryotes:
Both unicellular and multicellular with compartmentalised
cell structure
Its genetic material is surrounded in a nuclear envelope to
form a nucleus
DNA is closely associated with histones to form tightly
packed chromosomes (couldn’t get all that DNA in our
cells without packing. Sperm use protamines rather than
histones to ensure even tighter packing)
Eubacteria cell replication:
Simple division: separation of replicated circular
chromosome
Single Origin of replication (not true in archea) – in many
species genes commonly co-orient with the direction of
replication to prevent DNA polymerase (for replication)
and RNA polymerase (for transcription) clashing head on
, High rate of replication – in some species, genes near the
origin of replication are the most highly expressed as they
are diploid more often
Genes with related functions tend to cluster (both in and
out of operons) and be co-expressed
Eukaryotic cell replication: chromosomes:
In a diploid cell, chromosomes form a homologous pair
(humans have 23 pairs of chromosomes)
The chromosomes have specialist anatomy to enable cell
division:
- Centromere: attachment point for spindle microtubules.
Tend often to be gene poor domains.
- Telomeres: tips of a linear chromosome – need to be
stabilised to stop them falling apart
- Origins of replication: where DNA synthesis begins. There
are multiple on every chromosome.
The cell cycle is tightly coordinated with key check points
1.2 they face the same problem: cell reproduction requires
the copying of the genetic material, separation of the
copies, and cell division
The Cell Cycle:
Interphase: an extended period between cell divisions,
DNA synthesis, and chromosome replication phase
M phase (mitotic phase): two different parts:
- Mitosis: separation of sister chromatids
- Cytokinesis: separation of cytoplasm
Phase check points: key transition/ quality control
points
