the cell cycle, voice of the genome

Elements of plant cell replication

Only meristem cells can undergo mitosis

Only divide using centrioles - plant cells make the spindle from the cytoplasm

Involves pinching of the cytoplasm while plant cells form a cell plate across the equator of the cell

Phases of interphase

M

G1

S

G2

What happens during G1 of interphase

Rapid growth

High metabolic rate within cell

New organelles formed

Cell size increases - requires structural proteins and enzymes - high rates of protein synthesis

What happens during S phase of interphase

New DNA synthesises in the nucleus

Histones built up and chromosomes are made from two chromatids

Quantity of DNA doubles

What happens during M phase of interphase

Mitosis

What happens during G2 phase of interphase

Accumulation of energy stores

Organelles divide

Chromosomes start to condense

Prophase

Chromosomes become shorter and thicker

Nucleoli disappear - chromatids and centromere are visible

Centrioles move to opposite poles of cell

Micro tubules radiate out from centrioles - form an aster

Nuclear membrane breaks down

Spindle forms from microtubules - spindle fibres

What happens during metaphase

Chromosomes attach to spindle fibres at centromere

Chromosomes align along the equator

What happens during anaphase

Centromeres divide and fibres shorten - pull chromatids (chromosomes) to opposite poles

Telophase

Chromatids called daughter chromosomes

Chromatids lengthen and are no longer visible

Nuclear envelope reform and nucleoli reappear

Each cell now had the same mass of DNA - total mass halves during cell division

Cytokinesis

Stage where the structure of the cell divides

All organelles are evenly distributed around each nucleus

Plant cells - spindle fibres at equator move out and form a phragmoplast

Organelles congregate and a new cell wall grows across the middle, separating the 2 cells

Animal cells - cell surface membrane tucks in and creates a cleavage furrow

The cleavage furrow deepens until cell separates

Meiosis overview

Organisms that undergo sexual reproduction produce specialised cells called gametes that undergo fertilisation to produce a cell that had a diploid nucleus called a zygote

The zygote has received half its genetic material from each parent and it normally genetically unique

This cell divided via mitosis until large enough for the cells to differentiate

This cell mass then develops into a foetus, which can eventually produce haploid gametes

Homologous pairs characteristics

1. Exactly same length

2. Centromere in the same position

3. Same number of genes

4. Genes arranged in the same linear order

How are gametes produced

Meiosis

When haploid cells containing half the normal number of chromosones are found

What happens at the crossing over of chromosomes

Chromatids break and rejoin at sights of attraction called crossing over which forms a chiasma (plural = chiasmata)

What is the result of chromosomes crossing over

4 chromosomes with different combinations of maternal and paternal genes

Overview of meiosis - homologous chromosomes and doubling

2 pairs of homologous chromosomes

Each chromosome replicates forming a chromosome of two chromatids joined by a centromere

Overview of meiosis - crossing over of chromatids

Homologous chromosomes pair up and exchange genetics material by crossing over

Contributes to the genetic variation that results from meiosis

Overview of meiosis - independent assortment of chromosomes

Chromosomes pair up randomly

The first division pulls one chromosome (2 chromatids) to each new cell

Overview of meiosis - first and second division

Within each new cell, the chromosomes move apart to each side of the cell as the centromere splits

This results in 4 gamete cells each with one chromosome from each pair

All cells are different in terms of the combination of alleles

Importance of meiosis

Offers mechanism for genetic variation

Each gamete carries only one form of a particular gene

Crossing over allows exchange of genetics information

Orientation of chromosome is random after 1st division

Independent assortment of chromosomes is a huge contributor of inherited characteristics

What do sperm need to be able to do

Independently move through the oviduct (flagellum and mitochondria)

Recognise the egg and move towards it (chemical signalling)

Fuse the haploid nucleus with the haploid nucleus of the egg (acrosome containing digestive enzymes)

The egg

Larger cell which cannot move independently

Movement through fallopian tubes is achieves through muscular contractions and the action of cilia

The cytoplasm contain haploid nucleus as well as lipid droplets and lysosomes

The zona pellucida is a jelly-like coating that surrounds the ovum

The corona radiata is a layer of cells surrounding the ovum that is made from proteins and carbohydrates including hyaluronic acid

Process of fertilisation

Sperm migrates through coat of follicle cells and binds to receptors molecules in the zona pellucida

The binding induces the acrosome reaction in which the sperm releases hyaluronidase into the zona pellucida

Zona pellucida is broken down by these enzymes allowing the sperm to reach the plasma membrane of the egg

The nucleus and other components of the sperm enter the egg

Cortical granules form a barrier called fertilisation membrane which now functions as a block of polyspermy

Initial mitotic division

Diploid cells starts the process of replication by mitosis using existing energy reserves to speed up the process

All cells produced during this time are classed as totipotent which means they have the potential to develop into an individual human

Once the zygote consists of around 200-300 cells it changes from a solid ball to a hallow ball called a blastocyst

The outer layer of cells will form the placenta while the inner mass of around 50 pluripotent embryonic stem cells will go on form the embryo

Subsequent miotic division

After gastrulation, the developing embryo differentiates into 3 layers

Ectoderm - endoderm - mesoderm

Chemical signals activate gene expressions forming mRNA, which produce specific protein for each cell type

Cells change from being pluripotent to being multi-potent, as they continue to differentiate

In the adult body, only some cells remain multipotent, such as bone marrow which can become bone or blood cells

3 layers of developing embryo

Ectoderm

Endoderm

Mesoderm

Pluripotent

Cells that can differentiate into any other cell

Multipotent

Cells that can differentiate into only certain cells

Ectoderm

Skin cells and central nervous system

Endoderm

Digestive tract, thyroid and lungs

Mesoderm

Muscles, bone, connective tissue, circulatory system

What can cells in the endoderm (internal layer) turn into

Lung cells (alveolar cell)

Thyroid cells

Digestive cells (pancreatic cell)

What can cells in the mesoderm (middle layer) turn into

Cardiac muscle cells

Skeletal muscle cells

Tubule cells of the kidney

Red blood cells

Smooth muscle cells (in gut)

What can cells in the ectoderm (external layer) turn into

Skin cells of epidermis

Neutron on brain

Pigment cells

How to harvest stem cells

1. Trachea is removed from dead donor patient

2. It is flushed with chemicals to remove all existing cells

3. Donor trachea ‘scaffold’ coated with stem cells from the patients hip bone marrow cells from the airway lining added

4. Once cells have grown (approx. 4 days) donor trachea is inserted into patients bronchus

Stem cells sources and uses

Umbilical cord blood

Adult stem cells

Embryo tissue

Induced pluripotent stem cells

Force differentiated cells to become induced pluripotent stem cells, almost identical to embryonic stem cells, and avoid controversy surrounding harvest technique

Non-reproductive cloning

This is likely to be a huge area of interest with cloning in the future

It involves the production of a huge number of cloned cells

These will not be rejected as foreign, which minimises risks of transplant surgery

Potential for ‘home grown’ donor tissue

Totipotent stem cells can differentiate to form any organ or tissue

Possibility of regrowth if transplant is not possible

Therapeutic cloning

Regeneration of damaged heart muscle following cardiac arrest

Reversing effects of diseases affecting nervous system

Repair of spinal and possible brain tissue following trauma

Other uses of stem cells

Drug research

Developmental biology

Transplants

Stem cell uses - drug research

Stem cells are identical, therefore any genetic effects are removed during drug trials, multiple cells can be generated

Stem cell uses - developmental biology

Studying stem cells can allow biologists to understand the processes of cell differentiation

Stem cell uses - transplants

Could grow human skin from stem cells

Removing the need to harvest skin from other locations on the body

It may be possible to grow organs for transplantation in isolation

Tissue cultures

Cutting only generates a small number of artificial clones, for larger numbers or when dealing with valuable plant, we use tissue culture to generate clones, this is possible because some plant cells remain totipotent

Micropropagation using callus tissue culture

A small piece of tissue (explant) is removed from the shoot tip

This is now placed on a nutrient growth medium

The cells divide by mitosis and form a mass of undifferentiated cells called a callus

Single cells are then removed and placed on a third medium containing root growth hormone

Small plants can then be transferred to a greenhouse before being planted outside

This is used broadly in plant research, genetic modification and conservation of endangered species of plants

Ethical concerns of stem cells

Stem cells from bone marrow or as a result of being induced (iPS cells) are non-controversial

Opinions on using stem cells from human embryos is quite mixed, as this may be seen as a potential human

The harvesting practices for umbilical stem cells were not declared, leading to concern from new parents

In the uk, parliament has the final say on regulation of medical practices

Embryonic stem cells can be provided by ‘spare’ embryos from fertility research, but this still carries the same level of controversy for many individuals

Control of development

Control over everything that happens to a cell lies with the nucleus, in 1934, Joachim Hammerling demonstrated this convincingly using giant algal cells

In conclusion the rhizoid containing the nucleus, determines the genetics of the hat, regardless of the stalk that the hat grows from