chapter 19 p3

Question 1

Translational control:
These following mechanisms regulate the process of protein synthesis:

Answer 1

degradation of mRNA - the more resistant the molecule the longer it will last in the cytoplasm, that is, a greater quantity of protein synthesised.

binding of inhibitory proteins to mRNA prevents it binding to ribosomes and the synthesis of proteins.

activation of initiation factors which aid the binding of mRNA to ribosomes (the eggs of many organisms produce large quantities of mRNA which is not required until after fertilisation, at which point initiation factors are activated).

Question 2

Translational control:

Answer 2

Protein kinases
Protein kinases are enzymes that catalyse the addition of phosphate groups to proteins.
The addition of a phosphate group changes the tertiary structure and so the function of a protein.
Many enzymes are activated by phosphorylation.
Protein kinases are therefore important regulators of cell activity.
Protein kinases are themselves often activated by the secondary messenger cAMP.

Question 3

Post-translational control involves modifications to the proteins that have been synthesised. This includes the following:

Answer 3

• addition of non-protein groups such as
  carbohydrate chains, lipids, or phosphates
• modifying amino acids and the formation of bonds such as disulfide bridges
• folding or shortening of proteins.
• modification by cAMP - for example, in the lac operon CAMP binds to the cAMP receptor protein increasing the rate of transcription of the structural genes.

Question 4

Living organisms come in

Answer 4

all shapes and sizes from tulips to mosquitoes to humans. It is the same small group of genes, however, that control the growth and development of these vastly different living forms.

Question 5

morphogenesis.

Answer 5

The regulation of the pattern of anatomical development

Question 6

These genes were discovered by

Answer 6

scientists investigating strange mutations observed in fruit flies such as legs on the head in place of antennae or extra pairs of wings.
Fruit flies are small flies belonging to the genus Drosophila that feed and reproduce on rotting fruit.
They are small, easy to keep, and have a short life cycle so have always been a popular choice for use in genetic studies.

Question 7

Homeobox genes are

Answer 7

a group of genes which all contain a homeobox.

Question 8

The homeobox is a

Answer 8

section of DNA 180 base pairs long coding for a part of the protein 60 amino acids long that is highly conserved (very similar) in plants, animals, and fungi.
This part of the protein, a homeodomain, binds to DNA and switches other genes on or off, Therefore, homeobox genes are regulatory genes.

Question 9

The common ancestor of the mouse and human is

Answer 9

thought to have lived about 60 million years ago.
Mutations have been accumulating ever since and evolution has led to two very different organisms.
Many of the homeobox genes present in the mouse and human, however, still have identical nucleotide sequences.

Question 10

one of the homeobox genes.

Answer 10

Pax6
When mutated it causes a form of blindness (due to underdevelopment of the retina) in humans.
Mice and fruit flies also have this gene and disruption of the gene causes blindness in these organisms as well.
These findings suggest that Pax6 is a gene involved in the development of eyes in all three species.

Question 11

Hox genes:

Answer 11

• (often used interchangeably with homeobox genes) are one group of homeobox genes that are only present in animals.
• They are responsible for the correct positioning of body parts.
• In animals the Hox genes are found in gene clusters - mammals have four clusters on different chromosomes.
• The order in which the genes appear along the chromosome is the order in which their effects are expressed in the organism.
• Human beings have 39 Hox genes in total that are all believed to have arisen from one ancient homeobox gene by duplication and accumulated mutations over time.

Question 12

The layout of living organisms:
p1

Answer 12

Body plans are usually represented as cross-sections through the organism showing the fundamental arrangement of tissue layers.
Diploblastic animals have two primary tissue layers and triploblastic animals have three primary tissue layers.
A common feature of animals is that they are segmented, that is, the rings of a worm or the less obvious back bone of vertebrates.

Question 13

The layout of living organisms:
p2

Answer 13

• These segments have multiplied over time and are specialised to perform different functions.
• Hox genes in the head control the development of mouthparts and Hox genes in the thorax control the development of wings, limbs, or ribs.
• The individual vertebrae and associated structures have all developed from segments in the embryo called somites.
• The somites are directed by Hox genes to develop in a particular way depending on their position in the sequence.

Question 14

body layout diagram

Answer 14

Question 15

The body shape of most animals shows symmetry.

Answer 15

Radial symmetry is seen in diploblastic animals like jellyfish. They have no left or right sides, only a top and a bottom.

Bilateral symmetry which is seen in most animals means the organisms have both left and right sides and a head and tail rather than just a top and bottom.

Asymmetry is seen in sponges which have no lines of symmetry.

Question 16

Mitosis and apoptosis: p1

Answer 16

Mitosis, which results in cell division and proliferation, and apoptosis (Figure 6), which is programmed cell death, are both essential in shaping organisms.
The role of mitosis is to increase the number of cells leading to growth.
The role of apoptosis is not so immediately obvious. Consider how a sculptor works to give shape to a block of wood or stone.
The shape is revealed as material is removed bit by bit.
This is one of the ways in which apoptosis shapes different body parts - by removing unwanted cells and tissues.

Question 17

Mitosis and apoptosis: p2

Answer 17

Cells undergoing apoptosis can also release chemical signals which stimulate mitosis and cell proliferation leading to the remodelling of tissues.
A sculptor working with clay will often add and remove clay during the reshaping process.
Hox genes regulate both mitosis and apoptosis.

Question 18

process of apoptosis

Answer 18

Question 19

Factors affecting the expression of regulatory genes:
p1

Answer 19

• environment, both internal and external.
• Stress can be defined as the condition produced when the homeostatic balance within an organism is upset.
• This can be due to external factors such as a change in temperature or intensity of light.
• Internal factors can change due to the release of hormones or psychological stress.
• These factors will have a greater impact during the growth and development of an organism.

Question 20

Factors affecting the expression of regulatory genes:
p2

Answer 20

• Drugs can also affect the activity of regulatory genes.
• An example of this was the drug thalidomide.
• Thalidomide was given to pregnant women in the 1950s and 1960s to treat morning sickness.
• It was later discovered that it prevented the normal expression of a particular Hox gene.
• This resulted in the birth of babies with shortened limbs.
• Thalidomide is currently used in the treatment of some forms of cancer.
• The property of this drug that has previously caused problems during pregnancy is being exploited to stop the development of some tumours.
• It is believed that thalidomide prevents the formation of networks of capillaries which are necessary for some tumours to grow and develop.

Question 21

Ontology doesn’t mimic phylogeny:

Answer 21

• The theory of recapitulation states that as organisms develop from fertilised egg to embryo they repeat the evolutionary process that they have been through.
• In biology, this can be summarised by the phrase ontology (the development of an organism] mimics phylogeny (the evolutionary history of an organism).
• This theory is not accepted by modern biologists but a modern theory states that ‘oncology (the study of cancer) recapitulates ontology.
• This refers to the discovery that genes originally expressed in the development of the embryo are expressed again by cancerous cells.