6.1.1 (c-d)

Question 1

Define morphogenesis

Answer 1

Morphogenesis: regulation of pattern of anatomical development

Question 2

What is a homeobox gene?

Answer 2

• DNA has different genes regulating morphogenesis that are called homeotic genes
• A subset of homeotic genes is called homeobox genes – which has a homeobox of 180 base pairs

Homeobox genes – are regulatory genes controlling body development (i.e. the positioning of body parts) (group of genes containing a homeobox)

Question 3

What is a homeodomain?

Answer 3

Within the DNA there is a homeobox gene – which codes for a protein

The homeobox (180 bp length) section of the homeobox gene, codes for the homeodomain (which is within the synthesised protein)

Homeodomain: binds to DNA and switches other genes on or off by regulating transcription, so any protein with a homeodomain is a transcription factor

Question 4

What is the hierachy with regards to:

Homeotic genes, homeobox genes, hox genes?

Answer 4

1. Homeotic Genes: all the genes regulating morphogenesis (an organism’s shape)
2. Homeobox Genes: A subset of homeotic genes that codes for a protein with a homeodomain – making it a transcription factor
3. Hox Genes: A type of homeobox gene only found in animals

Question 5

What is are Hox genes?

Answer 5

• Homeobox genes only present in all bilaterian animals
  
  • Suggesting hox genes existed in the common ancestor of all bilaterian animals
• Responsible for the correct positioning of body parts
• Hox genes are found in clusters next to each other on chromosomes
• Humans have 39 hox genes – the order of the genes on a chromosome determines which effects are expressed when

Question 6

How do Hox genes ensure body parts are correctly positioned, and what occurs if it goes wrong?

Answer 6

• If mutations are found in hox genes, body parts develop and position in the wrong places on the body – known as homeotic mutations
• The order of the hox genes on a chromosome align with expression patterns along an embryo
  
  • I.e. the anterior (head) genes are at one end, and the posterior (tail) genes are at the other end – spatial linearity
• There is also a temporal order – the head genes are expressed first, down to the tail genes

Hox genes code for homeodomains which switch different genes on or off to express them

Question 7

What are the roles of mitosis and apoptosis in controlling body development?

Answer 7

• Mitosis and apoptosis are both control by homeobox and Hox genes
  
  • The proliferation of cells is necessary for growth
  • The programmed death of cells is to remove unnecessary cells to ensure correct development
    
    • E.g. when growing hands and feet, tissue grows between fingers and toes, these must be removed
    • T lymphocytes that recognise self-antigens would give rise to autoimmune diseases if not killed

Question 8

Describe the process of apoptosis?

Answer 8

1. Enzymes breakdown cytoskeleton
2. Cell shrinks and organelles packed together with chromatin fragments
3. Plasma membrane breaks up to form cell fragments (vesicles) with the cell contents
4. Vesicles undergo phagocytosis

Question 9

What is a factor affecting the expression of regulatory genes?

Answer 9

Stress – when there is homeostatic imbalance in the body

Can be caused by external (temperature, light intensity) or internal (hormone release, psychological stress)

Question 10

Over generations, why have there been little change in terms of mutations in homeobox genes? [2]

Answer 10

• Important genes, mutations would alter body plan affecting organisms survival rate
• The organisms with homeotic mutations would be selected against due to survival, hence the loss of those genes