6.1.3 genetic control of body plan development

Question 1

define apoptosis

Answer 1

programmed cell death

Question 2

define conserved

Answer 2

remained in all descendent species throughout evolutionary history

Question 3

define homeobox sequence

Answer 3

sequence of 180 base pairs (excluding introns) found in genes that are involved in regulating patterns of anatomical development in animals, fungi & plants

Question 4

define hox genes

Answer 4

• subset of homeobox genes, found only in animals
• involved in formation of anatomical feature in current locations in body plan

Question 5

describe homeodomain sequences

Answer 5

• homeobox sequence consisting of 180 DNA base pairs encoding 60-amino acid sequence within a protein
• can fold into certain shape & bind to DNA, regulating transcription of genes beside them
• transcription factors
• act within cells nucleus

Question 6

what is the shape the homeodomain-containing proteins fold into called
- what does it consist of?

Answer 6

= H-T-H

consists of:
- 2 alpha helices (H) connected by 1 turn (T)
- part of the homeodomain amino acid sequence recognises the TAAT sequence of enhancer region (initiates/enhances transcription) of gene to be transcribed

Question 7

evidence for homeobox genes sequences being very similar

Answer 7

• scientists demonstrated that the homeobox sequence in the homeotic genes of the fruit fly exists in the mouse
• the base sequences in these homeobox sequences are very similar in both species
• showed that these gene sequences are crucial for regulation of development & differentiation of organisms

Question 8

where are hox genes found

Answer 8

only in animals

Question 9

evidence for homeobox sequences being conserved

Answer 9

• molecular evidence shows homeobox genes are present in cnidaria, which means these genes arose before the palaeozoic era (began 541 million years ago)
• indicates these genes arose in an early ancestor

Question 10

what do hox genes regulate

Answer 10

• development of embryos along anterior-posterior axis
• control which body parts grow where

Question 11

what happens if hox genes mutate

Answer 11

• abnormalities happen
• eg. antennae on head of fruit fly developing as legs, mammalian eyes developing on limbs

Question 12

how are hox genes arranged

Answer 12

• arranged in clusters
• each cluster may contain up to 10 genes
• in tetrapods (4 limbed vertebrates) there are 4 clusters

Question 13

describe hox genes in early embryonic development

Answer 13

• active
• expressed in order along anterior-posterior axis
• sequential & temporal order of genes expressions corresponds to sequential & temporal development of body parts (colinearity)

Question 14

what do hox genes encode

Answer 14

• homeodomain proteins, which act in nucleus as transcription factors
• can switch on cascades of activation of other genes that promote mitotic division, apoptosis, cell migration & help regulate cell cycle

Question 15

are hox genes similar across different classes of animals

Answer 15

yes
eg. fly can function with chicken hox gene inserted in place of its own

Question 16

how are hox genes regulated?

Answer 16

hox genes are regulated by other genes: gap genes & pair-rule genes

Question 17

how many times do body cells divide before dying

Answer 17

around 50 times = Hayflick constant

Question 18

5 steps during apoptosis

Answer 18

1. enzymes break down cell cytoskeleton
2. cytoplasm becomes dense with tightly-packed organelles
3. cell surface membrane changes & small protrusions (blebs) form
4. chromatin condenses, nuclear envelope breaks down & DNA breaks into fragments
5. cell breaks into vesicles which are ingested by phagocytic cells, so cell debris doesn’t damage other cells/tissue

–> quick process

Question 19

how is apoptosis controlled?

Answer 19

• many cell signals
• some signalling molecules released by cells when genes involved in regulating cell cycle/apoptosis respons to internal cell stimuli & external stimuli (eg. stress)
• signalling molecules include: cytokines (from immune system), hormones, growth factors & nitric oxide
• proteins released into cytoplasm where they bind to apoptosis inhibitor proteins, allowing apoptosis to occur

Question 20

how can nitric oxide induce apoptosis

Answer 20

• makes inner mitochondrial membrane more permeable to hydrogen ions & dissipating the proton gradient

Question 21

describe apoptosis & development

Answer 21

• extensive cell division of cell types prevented by apoptosis (without release of hydrolytic enzymes)
• during limb development, apoptosis causes separation
• apoptosis removes harmful/ineffective t-lymphocytes during development of immune system

Question 22

why should the rate of cells dying equal the rate of cells being produced by mitosis?

Answer 22

• not enough apoptosis leads to formation of tumours
• too much apoptosis leads to cell loss & degeneration