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

Genetics of differentiated cells

When cells differentiate they follow instructions within their nucleus

Dollys - this demonstrated that even a differentiated cell nucleus contains the information required for developments of a multicellular organism

Does gene expression vary

Because we are able to produce multiple different types of differentiated cells, the genes in the nucleus that are expressed must vary

As we can only observe the result of gene expression (the protein is present or it is not)

This is hard to test

Work by Dawid and Sargent provided support for this idea

Dawid and Sargent (1983)

mRNA was extracted from differentiated and undifferentiated cells (measure of which genes are being expressed)

mRNA from the differentiated cells was used to make complementary (cDNA), using reverse transcriptase

The cDNA (differniated cells) was mixed with the mRNA (undifferentiated cells), under conditions that allowed hybridisation

After separation some cDNA strands had not hybridised, showing that the two cell types were expressing some genes that were identical, but also different genes

cDNA (differentiated cells) cannot hybridise with mRNA (undifferentiated cells)

Differentiated and undifferentiated cells are expressing different genes

The epigenome

Determines which genes are transcribed in a given cell, it consists of chemical markers that are attached to the DNA and histones

Different types of cells have their own unique ‘setting’ of genes which allows individual differentiation

Controls of gene expression

methylation

epigenetic markers

Controls of gene expression - methylation

Genes can be prevented from being transcribed by Methylation of DNA

This occurs when methyl groups bind to the promotor region (first part of a gene)

This causes chemicals to be attraction to this region and results in chromatin remodelling which changes the DNA strand

The DNA becomes more tightly bound, preventing transcription to mRNA

The addition of a methyl groups bind had silences the gene preventing expression

Many human cancers are a result of loss of methylation, where genes are no longer silences and become expressed

Promoter region

First part of a gene

Control of gene expression - epigenetic markers

This process allows cells that have differentiated to only express the genes that are required for that particular cell

For example the nucleus of the liver cell will be able to express all necessary proteins required for normal liver function

However, all the other genes can be switched off by methylation or other mechanisms, ensuring that these genes are not expressed

Switching genes off or on - the lac operon

Bacteria such as E. Coli are capable of synthesising a variety of different enzymes, depending on their environment

E. Coli that are not previously exposed to lactose in milk cannot use it immediately as a respiratory substrate, as they have small amounts of the 2 enzymes required

B-galactosidase catalyses hydrolysis of lactose

Lactose permease: transports lactose into the cell

After a short time - E. Coli starts to synthesise large quantities of these enzymes as a result of induction by the presence of lactose (inducer)

Process is controlled by genes in a section of DNA

Two types of histone modification

Acetylation

Methylation

Histone modification - acetylation

Addition of an acetyl (CoCH3) group - activates chromatin and allows transcription

Genes of the operon

P - O - Z - Y

Structural genes of the operon

Y and Z

Control site genes of operon

P - O

What is the P in the operon

Promoter region

RNA polymerase bind here to start transcription of Z & Y

What is the O of in the operon

Operator region

Switches Z and Y on and off

What is the Z in the operon

Codes for B - galactosidase

What is the Y in the operon

Codes for lactose permease

What happens when lactose isnt present in the operon

Regulator gene is expressed and produces repressor protein

One binding site on repressor protein binds to operator region, covering promotor region where RNA polymerase would attach

RNA polymerase cannot bind to promotor region and neither gene Z or Y is expressed

What happens when lactose is present in the operon

Lactose binds to the other binding site on repressor protein, changing the shape

Repressor protein cannot bind to operator region

RNA polymerase binds to the promotor region and genes Z and Y are expressed

What if gene expression goes wrong

If we do not have correct expression/inhibition of genes, then cells can develop differently than expected

FOP - rare condition where bones develop in unusual places

FOP - is the result of a gene mutation which controls secretion of proteins required for bone growth in white blood cells

This gene (ACVR1) is not turned off in white blood cells so when these cells attempt to treat damage they migrate to the damaged area and secrete the protein

Presence of this protein results in the cells expressing other genes that complete the transformation into bone cells

Organisation of the organism - cells

Individual cells are specialised for a specific function due to differences in gene expression

Organisation of the organism - tissue

A collection of similar cells that preform a common function they can be attached or separate from each other

Organisation of the organism - organ

A collection of tissues working together to allow a specific function

Organisation of the organism - organ systems

A number of organs working together to preform life function

How do cells in tissue work together

Cells that make up tissue must be able to recognise other similar cells

This is achieved through recognition proteins called adhesion molecules that are expressed on a cell surface membrane

Adhesion molecules on one cell are able to bind to complementary shaped adhesion molecules on another cell, allowing them to bind and work together

These adhesion molecules are synthesised as part of the process that differentiates cells during gene expression

Cells from a tissue which become separated can reform into tissues due to their adhesion molecules

Gene expression and development

In order for cells to differentiate appropriately gene expression must be coordinated, in order to produce increasing specialised cells

Genetic maps showing ‘master genes’ are well known for many species allowing cells to become increasingly specialised

As cells divide both the genome and epigenome is copied into new cells, allowing subsequent cells to continue to differentiate

Gene expression and development in plants

Cells in the plant meristem specialise to develop into a flower, containing four key structures

Sepals - petals - stamens - carpels

Three genes are involved in producing these structures, depending on their expression of genes A, B and C

Four main structures in flowers

Sepals

Petals

Stamens

Carpels

What are the sepals for in a flower

Protection

What are petals in flowers for

Decoration

What are stamens in flowers for

Male gametes

What are carpels in flowers for

Female gametes

What genes of A, B or C are in sepals

Gene A only

What genes of A, B or C are in Petals

Genes A & B

What genes of A, B or C are in Stamens

Genes B and C

What genes of A, B or C are in Carpels

Gene C only

Apoptosis

Defined as; programmed cell death taking place after a set number of mitotic divisions

The process is controlled through a series of biochemical reactions and differs greatly from necrosis (cell death following trauma)

This may seem wasteful, but it is often easier for plants and animals to grow large structures and then have them ‘pruned’ away by apoptosis. This ensures that different tissues can be shaped to work effectively in the adult organism

Mammal toes and fingers are webbed in early stages of development, but usually become seperated due to apoptosis