Lecture 5: Cellular Reproduction

Cell division causes no change in

Allele frequency

What does cell division occur for

Growth

Cell replacement

Healing

Reproduction

What are the 3 main steps of cell reproduction

1. Copy genetic information

2. Separate copies

3. Cell division

Define DNA replication

The process by which a single parental molecule of DNA produces two daughter molecules

What process do prokaryotic cells divide by

Binary fission

What is the circular genome of bacteria attached to and what by

The cell membrane, by proteins

What is the first step of binary fission - where is this initiated

DNA replication - initiated at the origin of replication

How does replication occur, resulting in what

In opposite directions around circle, results in 2 molecules of DNA attached to the cell membrane at a different site

What happens when the cell then elongates

The two attachment sites start close and move further apart

When the DNA molecules are well separated, what forms

A constriction forms at the midpoint

What is the result of this

Two genetically identical daughter cells, identical also to the parent

Where are the highly expressed genes in a bacterial genome located

Near the origin of replication (Ori)

What are the main differences in genomes and DNA locations between eukaryotes and prokaryotes

Eukaryotes - Large genome, linear - DNA in nucleus

Prokaryotes - Genome small, circular - DNA in cytoplasm

What is the general structure of a chromosome

Telomere at top and bottom

Centromere in middle (connects sister chromatids)

1 chromosome can either consist of 1 chromatid or 2 sister chromatids - both are still ONE chromosome

What is the centromere

Constricted region of the chromosome where the kinetochores form and the spindle microtubules attach

Draw the cell cycle

What 2 stages does the M phase consist of

Mitosis (chromosome separation) and cytokinesis (cell division)

What is interphase

The time between 2 successive m phases

What 3 phases is interphase divided into

S phase (synthesis)

G1 phase

G2 phase

What happens to the centrosome in the S phase, what forms around it

Duplicates and migrate to opposite poles, tubulin dimers assemble around them, forming microtubules radiating from each centrosome

What important molecules are synthesised in G1

Regulatory proteins like kinases, which activate enzymes to synthesise DNA

What are the man functions of G1 and G2

G1 - preps the cell for DNA synthesis

G2 - preps the cell for mitosis and cytokinesis

How is G0 distinguished from G1

The absence of preparations for DNA synthesis

Why do cells enter G0

If they are not actively dividing - means they exit the cell cycle and division is dormant (i.e. liver cells)

What are the stages of mitosis

Prophase

(prometaphase)

Metaphase

Anaphase

Telophase

What are homologous chromosomes

Pairs of chromosomes, matching in size and appearance and carrying the same set of genes

One inherited from mother, one from father

Main process in prophase

Chromosomes condense and become visible

What is the change in structure that chromosomes undergo to become visible

From long, thin to short and dense

What happens in the cytoskeleton during prophase

Reorganised - microtubules assemble the mitotic spindle which radiate from the centrosome (microtubules-organising plant for animal cells)

What happens in prometaphase

Chromosomes attach to the microtubules of the mitotic spindle at their centromeres after the nuclear envelope breaks down

What are kinetochores

2 protein complexes on either side of the centromere of each chromosome, they form the site of attachment for a spindle microtubule which eventually pull each sister chromatid to the opposite cel

What is the fundamental of metaphase

Chromosomes align at the equator as a result of dynamic changes in the mitotic spindle

How does this occur

Once each chromosome is attached to the mitotic spindles, the microtubules of the spindle lengthen/shorten to move the chromosomes to the centre of the cell (the equator)

What happens in anaphase

Sister chromatids fully separate

What happens to the centromere

It splits, as the spindle microtubules shorten and pull the chromosomes to opposite poles of the cells

What happens in telophase

Nuclear envelopes re-form around newly segregated chromosomes

What happens to the microtubules in this stage

They break down and disappear

What is formed during telophase, what happens to the chromosomes after

New nuclear envelope forms around each set of chromosomes, creating 2 new nuclei - chromosomes then decondense and become less visible.

What is the process of cell splitting called

Cytokinesis

What forms in order for the cells to split, how does it split the cells

A contractile ring forms against the inner face of the cell membrane

The ring contracts to divide the cytoplasm

What drives the constriction of the contractile ring

Motor proteins that slide bundles of actin filaments in opposite directions

Since plant cells have a cell wall, how does telophase differ

Instead of a contractile ring, a phragmoplast forms (overlapping microtubules which guide vesicles of cell wall components to the centre of the cell)

What are the products of mitosis

2 diploid, genetically identical daughter cells

How are gametes (egg/sperm cells) formed

Meiotic cell division

What is fertilisation

The fusion of 2 haploid gametes to form a diploid zygote

How many rounds of DNA replication are there in meiosis, how many cell divisions

1, 2

What is separated in meiosis 1, what effect does this have on the total number

Homologous chromosomes, reduces the total number of chromosomes by half

What is separated in meiosis 2

Sister-chromatids (like in mitosis)

What happens in prophase 1

Same as in Prophase of mitosis, with the addition of bivalents forming between non-sister chromatids

The chromosomes then undergo synapsis, what is this?

The precise pairing of homologous chromosomes, where they lie side-by-side

What does synapsis allow to occur

Crossing over and the formation of a bivalent (the structure formed when 2 pairs of homologous chromosomes cross over)

What do non-sister chromatids result from

The replication of homologous chromosomes, so have same set of genes in the same order, but aren’t genetically identical

What do sister chromatids result from

Replication of a single chromosome, so are genetically identical

What is the site of crossing over called, why can this result in genetic variation

Chiasma, the position of chiasmata on a chromosome is random

Why does crossing over increase genetic variation

Recombinant chromatids carry different combinations of partially paternal/maternal genes

What happens in prometaphase I

Spindles attach to kinetochores on chromosomes after nuclear envelope breaks down

What happens to the bivalents in metaphase 1, why does this cause genetic variation

They line up so the two centromeres lie on opposite sides of the plane, but it is random which pole the maternal homolog/paternal homolog is taken to

What happens to these bivalents in anaphase 1, why does it differ to mitotic anaphase

The 2 homologous chromosomes of each bivalent separate and are pulled to opposite poles

The main difference is that the centromeres don’t split so the sister chromatids remain attached

What is the number of chromosomes at each pole in anaphase 1

23 (haploid number)

Why is meiosis 1 called reduction all division

Number of chromosomes in the daughter cells reduces as each chromosome consists of 2 chromatids attached to a single centromere

(i.e. there are 2 bivalents in prophase 1, each bivalent gets carried to either pole, so there are only 2 chromosomes in each daughter cell)

What is the main difference between mitotic telophase and meiotic telophase

In meiotic telophase, the nuclear envelope only briefly reappears and the chromosomes do not completely decondense

Meiosis 2 is the same as

Mitosis

What is meiosis 2 often called, why

Equational division - as the cells have the same number of chromosomes at the beginning and the end

What is produced in meiosis

4 genetically different, haploid daughter cells

Draw mitosis/meiosis

What are the 3 sources of genetic variation in meiosis

Crossing over, Independent segregation, Random gamete fusion in fertilisation

What does it mean that meiosis is fair

½ the haploid gametes from each diploid allele

No change in allele frequency

How do cyclin-CDK complexes regulate the cell cycle

The level of cyclins (regulatory proteins) rises and falls during the cell cycle, activating kinases which target and phosphorylate proteins promoting cell division

What are these kinases called, when are they active

Cyclin-dependent kinases, activate when bound to appropriate cyclin

What are the 4 types of cyclin, when do they activate CDKs and what do they do

Cyclin A - Levels rise during S phase, activating CDKs which help to initiate DNA synthesis

Cyclin B- Levels rise in G2, activating CDKs which help to prepare the cell for mitosis (i.e. targeting proteins which trigger the breakdown of the nuclear envelope and formation of mitotic spindle)

Cyclin D and E - levels rise in G1 and prepare the cell for DNA replication by activating transcription factors and DNA polymerase

What else is used for the regulation of the cell cycle

Checkpoints

What are these checkpoints/ where

DNA damage checkpoint - between G1 and S

DNA replication checkpoint - between G2 and M

Spindle assembly checkpoint - before anaphase in M

How do these checkpoints work

If damage/incomplete replication is detected, cyclin-CDK activity is blocked and the cell cycle is halted until issue is fixed

When DNA is damaged by radiation, what protein is activated and how

p53, an activated protein kinase phosphorylated p53

What does p53 do

Binds to DNA and turns on expression of several genes one of which blocks cyclin-CDK complexes between G1 and S, arresting the cell here until fixed