Homeobox GenesBody organisation

Cell Differentiation

• Cell differentiation is the development of non-specialised cells into cells with specialised functions.– Examples: muscle cells, liver cell, red blood cells

• As organisms grow and develop from fertilised eggs; organs and tissues develop to produce a characteristic form. The process is called morphogenesis.

• Both processes are controlled by gene expression

What is gene expression?

Gene expression is the activation of a gene that results in a polypeptide or protein being made.

The expression of some genes (regulatory genes) results in the production of a protein that can turn on or switch off other genes, these are called Transcription factors

Body plans• Every organism has a unique body pattern because

of the influence of HOMEOTIC genes.• These specify how different areas of the body

develop their individual structures, eg. Arms, legs etc

• HOMEOTIC or HOMEOBOX genes were discovered when geneticists studying fruit flies found mutants with legs growing where their antennae should be and 2 sets of wings instead of 1.

Homeotic Genes• Homeotic genes are regulatory genes that

determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis.

• These seem to be the master genes of development, they act as switches for other genes

• Mutant Antennapedia gene

Normal Mutant with legs growing out of head

• Antennapedia complex (group of Homeobox genes)• 5 genes that affect the anterior part of the fly• When mutated, legs grow in the place of antennae

7

• Bithorax gene complex (3 homeobox genes affecting thoracic development)a. Normal – wings on 2nd thoracic segment and 2 halteres

on 3rd thoracic segment (far left photo, halteres in white)

b. Mutant – 3rd segment has wings so 2 sets of wings and no halteres

Halteres

Homeobox• In Drosophila (fruit flies) the

specific DNA sequence within a homeotic gene that regulates patterns of development is the homeobox.

• The same or very similar homeobox sequences have been found in many other eukaryotic organisms

• Homeotic genes code for homeotic proteins that function as transcription factors which switch on other genes

• The Homeobox is a DNA sequence within the homeotic genes which contains 180 base-pair sequences, coding for a 60 amino acid polypeptide.

• The protein binds to an area of the DNA and initiates the transcription of another set of genes. It acts like a switch.

Homeobox (HOX genes)The HOX genes encode important transcription factors. The transcription factors cause proteins to be made that specify cell fate and identify – the embryonic pattern along the primary axis (anterior/posterior) – as well as the secondary axis (genital and limb bud)– They have a major role in development of CNS, axial skeleton, positioning of

limbs as well as the gastrointestinal and urogenital tract.

Homeotic genes involved in spatial pattern control and development contain a conserved 180-bp sequence known as homeobox. This encodes a 60-amino-acid domain that binds to DNA.

– The Hox proteins regulate other “executive” genes that encode transcription factors or morphogen signals, as well as operating at many other levels, on genes that mediate cell adhesion, cell division rates, cell death and cell movement.

In Humans as in most vertebrates there are 4 homeobox gene clusters (39 HOX genes), located on chromosomes 7p14, 17q21, 12q13 and 7q31.

Drosophila has eight Hox genes arranged in 2 clusters on a single chromosome.

A. Drosophila's eight Hox genes in a single cluster and 39 HOX genes in humans.B. Expression patterns of Hox and HOX genes along the anterior-posterior axis in invertebrates and vertebrates.

• Three lines of evidence support the idea that Hox gene complexity has been instrumental in the evolution and speciation of animals with different body patterns1. Hox genes are known to control body

development2. General trend for simpler animals to have fewer

Hox genes and Hox gene clusters3. Comparison of Hox gene evolution and animal

evolution bear striking parallel

Hox genes

Hox genes

• Found in all animals• Genetic mutation of Hox genes may have been

critical events in the formation of new body plans

• Number and arrangement of Hox genes varies among different types of animals

• Increases in the number of Hox genes may have led to greater complexity in body structure

Hox genes in the Animal Kingdom

• Fruit flies have only one Antennepedia-bithorax complex

• Humans and many other vertebrates have 4 similar Hox gene clusters

• They probably arose through gene duplication

• Hox genes shape the number and appearance of body segments (repeated structures) along the main body axes of both vertebrates and invertebrates

How is a multicellular organism made?

• See Page 114 Text book

• Even before fertilisation an egg has a gradient of proteins that help to establish its polarity (which end becomes the head or anterior and which is the tail, posterior)

• After fertilisation “Maternal Effect” genes cause more proteins to be made that reinforce this polarity and also establish the dorsal (back) and ventral (belly) orientation

• Polarity is this formation of the axis on which the embryo differentiates

• Once the orientation is in place other genes are switched on

• Segmentation occurs driven by Segmentation genes

• Finally the Homeotic Selector genes are switched on

• These control the final specialised development of each segment

• There are two gene families in fruit flies (Drosophila), one controls the development of head and thorax, the other controls the development of thorax and abdomen

Drosophila development• The fertilised egg establishes

the pattern for the adult body plan by establishing protein gradients. (a)

• After fertilization, the zygote develops into a blastoderm. A series of nuclear divisions without cytoplasmic division produces many free nuclei in a syncytial blastoderm (b and c)

• Individual cells are created after the nuclei line up along cell membrane to form a cellular blastoderm (d)See page 114

Gastrulation involves cells migrating to the interior, 3 cell layers formed- ectoderm, mesoderm and endoderm (e)

Segmented body plan develops (f) driven by segmentation genesMd, Mx and Lb segments merge to form head3 thoracic segments T1-38 abdominal segments A1-8

Larva – free livingPupa – undergoes metamorphosisAdult form determined by expression ofHomeotic selector genesEgg to adult in 10 days

A Homologous Group of Homeotic Genes Is Found in All Animals

• Vertebrate Hox genes are homologous to those that control development in simpler organisms such as Drosophila

• Homologous genes are evolutionarily derived from the same ancestral gene and have similar DNA sequences

• Hox genes in mice1. Follow the colinearity rule (are expressed in the

same sequence as in simpler animals) 2. Have a key role in establishing anteroposterior

axis and controlling the development of the body plan

Homeotic genes in Mus

• The mouse has Hox genes on 4 different chromosomes

• Hox genes are similar to those found in invertebrates but spread across more chromosomes

Four general phases for body formation

1. Organize body along major axes

2. Organize into smaller regions (organs, legs)

3. Cells organize to produce body parts

4. Cells themselves change morphologies and become differentiated

Hox genes determine the number and types of vertebrae in animals

• Hox c-6 determines that the chicken has a shortish neck and the 7 vertebrae shown will develop ribs

• Snake: Hox c-6 is expanded dramatically toward the head and toward the rear so the snake has no neck and all these vertebrae develop ribs.

Positional information during development

• Each cell in the body must become the appropriate cell type based on its relative position.

• Each cell receives positional information that tells it where to go and what to become.

• Cells may respond by1. Cell division, 2. cell migration, 3. cell differentiation or 4. cell death (apoptosis)

See pages 116-117 for details of Apoptosis

Position or Spatial Organization is Everything• 2 main mechanisms are used to communicate positional information• Morphogens , eg retinoic acid; this activates homeobox genes in the

correct sequence. High concentrations of Vitamin A(precursor to retinoic acid) taken in pregnancy can interfere with the correct sequence of homeobox activation and hence affect the development of the embryo and cause birth defects

• Cell adhesion

Cell adhesion• Each cell makes its own cell adhesion molecules

(CAMs) • Positioning of a cell within a multicellular organism is

strongly influenced by the combination of contacts it makes with other cells and with the extracellular matrix.

For more information go to:

• http://www.pbs.org/wgbh/evolution/library/03/4/l_034_04.html

• http://www.dnaftb.org/dnaftb/37/concept/index.html

• http://learn.genetics.utah.edu/archive/bodypatterns/index.html

• Or search for information yourself!