Biology - D2.1 Cell and nuclear division

Purpose of cell division

To grow, replace and asexually reproduce cells

Definition

Splitting of cytoplasm in a parent cell between daughter cells

How does cytokinesis occur in animal cells?

1) Ring of conctractile proteins (myosin and actin) form just inside the plasma membrane

2) Pulls the plasma membrane inwards at the equator by contracting and pinching inwards

3) Cleavage furrow is created

4) Cleavage furrow reaches the center and splits, so the cells are pulled apart

Cytokinesis in plant cells

• Plants have cells walls so its more difficult

1) Vesiscles move towards the equator and fuse, forming 2 new plasma membranes = cell plate

2) other vesicle fuse releasing pectin through exocytosis which then makes up the middle lamella

3) 3rd set of vesicles releases cellulase making a cell wall

Why and when is cytokinesis equal?

• To survive and grow, both daughter cells need a nucleus and at least 1 complex organelle not made by components of the cell (e.g. chloroplasts or mitochondria)

• Equal cytokinesis is during most cell divisions

Examples of unequal cytokinesis

• Budding in yeast

• Oogenesis in humans (production of eggs), bc when one is too small (the polar body) to survive it dies, leaving only one egg, which is what you want

How does unequal cytokinesis occur?

• A smaller cell buds off from the other cell, hence unequal

• ½ DNA into one cell and ½ DNA into the other

• Sometimes smaller one dies

What is the middle lamella

It is the area that forms when the second set of vesicle fuses at the cell plate releasing proteins in a plant cell during cytokinesis

What is the cell plate

It is the two new plasma membranes that form (temporarily) because the first set of vesicles has fused at the equator

Role of mitosis and meiosis in eukaryotes

They are methods of nuclear division

It is necessary to divide nucleus before cell division to avoid anucleate cells

Differences between mitosis and meiosis

Mitosis: maintains chromosome number and genome of cells, for growth, repair and asexual reproduction, 2 daughter cells

Meiosis: halves the chromosome number and generates genetic diversity, production of gametes

How many chromosomes normally

23 types of chromosomes, and 2 of each type for diploid cells

(y&x is male, x&x is female)

Definition homologous chromosomes

Same type of chromosomes with the same length and code for the same genes

Importance of DNA replication

DNA replication is a prerequisite for both mitosis and meiosis

• Double the DNA, but still 2 homologous chromosomes bc still just 2 types

• Copy and original are identical

• Sister chromatids held together until anaphase

Condensation of chromosomes

Chromosomes condense to become visible, avoid breakages and tangling up

• Includes histones to supercoil

• Occurs before both mitosis and meiosis

Cohesin role with sister chromatids

Sister chromatids are held together by loops of a protein complex called cohesin (cohesin loops)

Occurs in both mitosis and meiosis

How do the chromosomes move during anaphase?

• Chromatids are separated and move towards poles

• Microtubules are hollow cylinders made with proteins called tubulin

• Microtubules are attached to both the kinetochore and the poles of the cell at the microtubule organizing center

• Tubulin is removed from microtubules through depolymerisation, shrinking the microtubule

• The microtubule stays attached to both sides but gets shorts so pulls chromatid towards poles

Definition kinetochore

The middle of the chromosome, where the microtubules attach

Explanation microtubules

Microtubules are made of many tubulins and break down (shrink, depolymerisation) or grow (polymerisation) and attach to kinetochore

What are the phases of mitosis?

Prophase, metaphase, anaphase and telophase

With interphase beforehand (although it is not a part of mitosis nuclear division)

What happens during prophase in mitosis?

• Supercoiling, condensation of chromosomes

• Microtubules start to grow from microtubule organisation centres at poles of cell

• Nucleolus breaks down

• Nuclear membrane breaks down

What happens during interphase?

• DNA replication to form sister chromatids

• Chromosomes are still long and thin

What happens during metaphase in mitosis?

• Microtubules from the poles attach to the kinetochore

• Chromosomes, consisting of sister chromatids are aligned at the equator

Anaphase mitosis

• Cohesin loops holding the sister chromatids together are cut

• Sister chromatids seperate

• Kinetochore shortens the microtubules by removing tubulin, pulling the sister chromatids/chromosomes to opposite poles

Telophase mitosis

• Chromosomes reach poles in a tight group around MTOCs

• Nuclear envelope forms

• Chromosomes decondense

• Nucleolus reforms

• Cytokinesis usually happens at the end of this phase

Why is meiosis necessary?

Enables variation as a result of sexual reproduction, conserves number of chromosomes particular to that species by making haploid gametes

What is reduction division?

Number of chromosomes are halved when making new cells, e.g. meiosis

Haploid and diploid definitions

Haploid: one of each type of chromosomes (so only the 23, and one of each)

Diploid: two of each type of chromosomes (so 46 chromosomes, 23 pairs)

What is the process of meiosis?

Prophase 1, metaphase 1, anaphase 1, telophase 1

Everything again in meiosis II which is similar to mitosis (prophase II, metaphase II, anaphase II and telophase II)

Prophase I, meiosis

• Homologous chromosomes pair up in synapsis = bivalent or tetrad formation

• Crossing over occurs between homologous pairs

• Spindles start to form

• Nucleolus and nuclear envelope break down

Metaphase I, meiosis

• Bivalents line up a the equator and attach to the spindle microtubules

• Can align in 2^(number of pairs) ways by switching sides

Anaphase I, meiosis

• Kinetochores shorten the microtubules by removing tubulin, pulling the pairs towards poles

Telophase 1, meiosis

• Homologous chromosomes reach opposite poles

• Nuclear envelope and nucleolus reform

• Chromosomes decondense

• Cytokinesis (after)

• Cells are now haploid with one of each type per cell

What happens in meiosis II?

• Same stages and steps as mitosis

• Sister chromatids are a when cohesin loops are cut

• Cells remain haploid

How many cells are there after one meiosis division cycle?

From 1 parent cell there are 4 daughter cells

What is the bivalent or tetrad formation?

After interphase or DNA replication, the homologous pairs of chromosomes are bonded together for meiosis 1

What are the ways in which meiosis causes genetic variation, and why?

Crossing over during prophase 1 and random orientation during metaphase 1

Genetic variation is crucial for natural selection and evolution

How does crossing over occur?

When the homologous chromosomes have come together through synapsis to form a tetrad or a bivalent, crossing over can take place

• Parts of a chromatid move and wind around the equivalent parts of non-sister chromatid

• DNA at these locations break off and reattach to the different chromatids

• The locations are called chiasmata

• The new combinations, if they are different to either parent are called recombinants

What are chiasmata?

The points at which the bivalents are held together and the point at which the genetic material is exchanged

What are recombinants?

When crossing over has occurred and new combinations, combinations that are different than that of the parents, are created

Random orientation for genetic variation during meiosis

• During metaphase 1 the homologous pairs line up, but can face either direction and the chance is random

• Assorted independently as the orientation of one homologous pair doesn’t affect that of the other one

• Is 2^(number of pairs) possibilities, 2²³ = 8 million

Definition non-disjunction

If the chromosomes don’t separate correctly during anaphase I or anaphase II, and gametes receive one extra or one too few chromosomes

Consequences of non-disjunction

Leads to chromosomal abnormalities, most common being down syndrome with an extra 21st chromosome

There are other conditions, but most lead to the death of the haploid cell

Definition cell proliferation

Rapid increase in the number of cells, with mitosis

Where the number of new cells is greater than the number of cells dying

Uses of cell proliferation

For growth, cell replacement or tissue repair

Growth and cell proliferation examples

• Embryo has a lot of cell proliferation to grow

• Juvenile animals/children have growth at bone growth plates

• In plants only the meristem regions grow

  • Shoot apical meristem which is at the leaves and buds

  • Root apical meristem, at the roots

  • Lateral meristem for increase in diameter

Examples of cell replacement cell proliferation

• On the skin there are dead cells that are lost (at the epidermis, bc cells produce keratin so cells dry out and reach surface)

• At dermis the new cells are produced, and rate is high

• Whole skin is not cell proliferation bc new cells is not greater than number of cells dying

Example of tissue repair through cell proliferation

Skin is good at tissue repair and wound healing bc of cell proliferation

Cell cycle function

Cell proliferation is achieved through the use of the cell cycle

3 general phases of cell cycle

Interphase wich is a metabolically active phase, mitosis to split the nucleus and then cytokinesis to split the cytoplasm

Phases within interphase

Gap1 (G1) phase, Synthesis (S) phase, Gap2 (G2) phase

What happens during G1 phase

Cell growth through protein synthesis, number of organelles increases, and there is the production of cell components

Can be a very long period

What happens during S phase

DNA replication and synthesis to prepare for another division

g2 phase

• Checking chromosomes for errors and fixing those mistakes

• Making sure there are enough mitochondria and chloroplasts

• Might start the condensation of DNA

What is the G0 phase?

G0 is after cytokinesis, the cell cycle can stop

The cell stops dividing and differentiates and becomes specialised by turning genes on and off

How is the cell cycle controlled?

Through the use of cyclins, which are dif proteins that when the threshold level is reached move to that stage

What are cyclins

Proteins used to go from phase to phase of the cell cycle, have threshold levels

What kind of genes also control cell division and the cell cycle?

Proto-oncogenes and Tumour-suppressor genes

What does a mutation in proto-oncogenes cause?

Causes it to mutate into oncogenes who don’t control the cell cycle but promote cell proliferation

• like just pressing the gas pedal, flooring it

What do mutations in tumour-suppressor genes cause?

The checks that tumour-suppressor genes exercise on DNA to correct faulty DNA no longer occurs

• No brakes, no checks to make sure its ok and no taking it slow

Definition tumour

The accumulation of an abnormal mass of cells as the consequence of uncontrolled cell division

Different kinds of tumours

Benign tumours and malignant tumours (primary and secondary)

Benign tumours definition and behavior

• The growth of abnormal cells that are not cancerous

• The cells grow closely and don’t spread to other parts of the body therefore don’t cause harm unless are pressing into neighbouring organs

• In rare cases large benign tumours can turn into malignant tumours if very large

Malignant tumours def and behaviour

• Cells that divide and grow more rapidly than benign tumours, cancerous

• Don’t have well-defined borders

• More likely caused by cells with hormonal stimulation to divide cells (e.g. breasts, ovaries, tested)

Definition metastasis

The process of the spread of malignant tumours to other parts of the body through the bloodstream and or the lymphatic system

Primary tumours

Primary tumours are original malignant tumours before metastasis

Secondary tumours

Malignant tumours after spreading throughout the whole body through a process called metastasis

More difficult to reach and more widespread, so a more advanced stage

How to calculate mitotic index

Number of cells in mitosis / number of cells in picture

Use of mitotic index

The ratio of cells undergoing mitosis to the total number of cells in that tissue can be a measure of cell division and can be prognostic tool for predicting response to chemotherapy