Chapter 8 (The developing cell)

Question 1

What are chromosomes?

Answer 1

Linear structures
Consist of one molecule of DNA
Found in pairs
During interphase, length of DNA are wrapped around specialised proteins called histones to form chromatin

Question 2

What are chromatids?

Answer 2

Part of a chromosome after it has undergone SCR.

Each sister chromatid is genetically identical to its sister

Question 3

What are homologous chromosomes?

Answer 3

Have same:

• Biochemical structure
• Physical shape and size
• Sequence of genes
• Gene loci (position of genes)
• Centromere position

Have different:

• Origins (one maternal and one paternal)
• Alleles

Question 4

What is the cell cycle?

Answer 4

Cycle of cell growth, maturity and division.
Highly ordered sequence of events
Process that all somatic cells in multicellular organisms use to grow and divide.

produces two genetically identical daughter cells.

Question 5

Length of cell cycle?

Answer 5

Varies between different cell types.
Eg, bacteria - 20 mins
Yeast: 1-3 hours
Hepatocyte: Up to year.

Question 6

Cell theory?

Answer 6

All living things are made of one or more cells
The cell is the smallest unit of life
All new cells from preexisting cells
All existing cells have a common chemical composition: Carbohydrates, proteins, lipids, nucleic acids
Hereditary information is passed from one generation to the next as genes/DNA is passed from parent cell to daughter cell.

Question 7

Key events of cell cycle?

Answer 7

Interphase: 90%
G1
S
G2
M
C

Mitosis 10%
Cytokinesis

Question 8

Importance of mitosis?

Answer 8

• Growth of multicellular organisms - increase no. cells and increase size and mass of tissues.
• Repair of damaged tissues
• Replacement of cells
• Asexual reproduction.

Question 9

What happens at G1?

Answer 9

Growth 1

• Cells increase in size and mass
• All metabolic processes remain active, eg, protein synthesis, hormone production and respiration.
• Organelles replicated (except chloroplasts and mitochondria)
• Increased protein synthesis occurs

Question 10

What happens at G1 Checkpoint?

Answer 10

Cells check:

• Chemicals needed for replication are present
• For damage in DNA before entering S-phase.
• Cell has suitable size
• Cell has sufficient nutrients

If conditions are not correct, the cell enters a resting state (G0) and no longer divides

Question 11

What happens at S phase?

Answer 11

Synthesis

- DNA is replicated using semi-conservative replication

Question 12

What happens in G2?

Answer 12

Growth 2

Chloroplasts and mitochondria increase in size and divide (increase in number)
Cell continues to increase in size and mass
Energy stores are increased
Spindle fibres start to form

Question 13

What happens at G2 checkpoint?

Answer 13

Cells check:
All DNA has been replicated without damage
Cell is off correct size

If this is correct, cell passes into mitotic stage

Question 14

Info on mitosis in cell cycle?

Answer 14

Nuclear division
Sub divided:
PMAT

Question 15

Cytokinesis information?

Answer 15

Equal division of cytosol and organelles to form 2 genetically identical daughter cells.
Process is different in both animal and plant cells.

Question 16

Cells that lose the ability to divide information?

Answer 16

Neurones, erythrocytes, muscle cells
These cells leave the cycle either temporarily or permanently due to:
- Differentiation (cells become specialised and can no longer divide)
- Damaged DNA - cell is no longer viable so enters G0.
Most cells can only divided a finite number of cells.

Question 17

Cell cycle control information?

Answer 17

Sequence and timing of cell cycle controlled by cyclins (specialised proteins)
Cyclins activate enzymes called cyclin-dependent kinases CDKs.
Kinases catalyse the addition of a phosphate group onto a protein (Phosphorylation)
Phosphorylation of a protein changes its tertiary structure of cyclins
Specific CDKs control specific steps in the cell cycle and cell cycle is stopped if errors are detected.

Question 18

Example of cell cycle control?

Answer 18

If DNA is damaged, then p21 (protein) binds to CDK.
This halts cell cycle at G1 stage in interphase.

Mutations to CDK genes can lead to failure to repair DNA -> cancer

Question 19

Principle of mitosis?

Answer 19

Nuclear division to produce 2 genetically identical nuclei
Which are then distributes into two genetically identical daughter cells when the cell cycle is complete and cytokinesis occurs.

Question 20

Interphase information?

Answer 20

Cell is still very active:

• Respiration
• DNA replication
• Protein synthesis
• Organelle replication
• Active transport/bulk transport

Cells grow and synthesise additional proteins, organelles and DNA ready for nuclear and cell division

Question 21

What happens in prophase?

Answer 21

• Longest phase of mitosis
• Chromosomes shorten and thicken
• Each chromosome consists of 2 sister chromatids joined by centromere
• Centrioles move towards poles
• Microtubules form
• Nucleolus and nuclear envelope breaks down.

Question 22

What happens in metaphase?

Answer 22

• Chromosomes line up on equator

- Spindle fibres attach to centromere (made from microtubules)

Question 23

What happens in anaphase?

Answer 23

• Rapid phase
• Spindle fibres contract
• Exerts pulling force on centromeres
• Centromer splits
• Daughter chromosomes pulled to opposite poles, centromere first

Question 24

What happens in telophase?

Answer 24

• Daughter chromosomes reach poles
• Chromosomes uncoil and lengthen
• Spindle fibres disintegrate
• Nuclear envelope reforms around chromosomes at each pole
• Nucleolus reforms

Question 25

Cytokinesis in animal cells?

Answer 25

csm invaginates
Cleavage furrow forms in middle of cell.
csm pinches until 2 sides of csm meet
Cytoplasm divides equally in 2
Organelles are divided equally into 2 new daughter cells

Question 26

Cytokinesis in plant cells?

Answer 26

Only in meristematic tissue (shoot tips and root tips)
Plant cell has cell wall so can’t just invaginate to form cleavage furrow

Vesicles from Golgi apparatus assemble on equator - cell plate
Vesicles fuse with each other and the csm
New sections of cell wall form along middle lamella
Cytoplasm divides equally into 2 new daughter cells

Question 27

What are telomeres?

Answer 27

Region of repetitive DNA sequence at end of eukaryotic chromosomes
They protect chromosomes from DNA damage/from fusion with neighbouring chromosomes

Question 28

What is apoptosis?

Answer 28

Programmed cell death in multicellular organisms (both animals and plants)
Ordered and controlled process

Question 29

What is necrosis?

Answer 29

Unregulated cell death after trauma

Caused by csm rupturing and release of hydrolytic enzymes

Question 30

Sequence of events of apoptosis?

Answer 30

Cell shrinks and chromatin in nucleus condenses (pyknosis)
Enzymes break down the cytoskeleton + cytosol gets denser
csm changes and blebs form.
Karyorrhexis occurs
Cell breaks down into fragments contained within vesicles
Phosphatidylserine binds to receptors on macrophages
Macrophages engulf cell fragments - prevents damage to other cells and tissues.

Question 31

What are blebs?

Answer 31

Bulge of the csm which surrounds a portion of cytoplasm

Question 32

Importance of apoptosis?

Answer 32

Essential for

• organisms undergoing metamorphosis
• Puberty
• Developing fetus
• Production of xylem tissue in plants

Question 33

Control of apoptosis?

Answer 33

Cell signals are received from both inside and outside of cells, ie, intrinsic and extrinsic process.
Signals can include:
- p53 ( tumour suppressor gene produced by cells with damaged DNA )
- Cytokines from cells of the immune system
- Hormones

Question 34

Definition of karyorrhexis?

Answer 34

Fragmentation of a nucleus