2.6-Cell Division and Diversity (copy)

what is the purpose of mitosis

growth and repair of tissues

the cell cycle

1. G₁-first growth phase

2. S- Synthesis phase

3. G₂-second growth phase

4. M-mitotic phase

G1- Growth Phase

• cell prepares to replicate DNA by synthesising mRNAs and proteins required to execute further steps

• cell grows larger, organelles are copied

S- Synthesis phase

• all genetic information is copied by DNA replication

• Process generates sister chromatids

sister chromatids

• identical pairs of chromosomes attached to each other by a centromere

centromere

specialised sequence of DNA that links sister chromatids and is important throughout mitosis

G2- Growth Phase

• cell undergoes additional growth, replenishes energy stores and prepares and reorganises cytoplasmic components for division

  • some organelles duplicated

  • cytoskeleton dismantled

Stages of Mitosis

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase

• chromosomes condense

• nuclear envelope breaks down

• organelles break up and move towards edges

• mitotic spindle (made of microtubules) forms- extend from pole to pole

Metaphase

• mitotic spindle facilitates movement of chromosomes

  • align along centre of cell - metaphase plate

  • sister chromatids still attached to each other

• Spindle Checkpoint-ensures anaphase won’t proceed until all chromosomes are at metaphase plate and microtubules by kinetochore

Anaphase

• when chromosomes are properly aligned, anaphase proceeds

• sister chromatids separate at centromere- pulled to edge of cell

  • now chromosomes

Telophase

• spindle disappears

• new nuclear envelope forms around chromosomes

• chromosomes decondense

• cytokinesis occurs

Cytokinesis in Plant Cells

• dividing plant cells create a structure known as a cell plate

  • made of vesicles containing plasma membrane and cell wall components

  • cell plate enlarges until it merges with cell walls

  • divides cell in two and allows cell wall to be regenerated

Stages of Meiosis

1. prophase I

   • DNA condenses into chromosomes- consists of 2 sister chromatids joined by centromere

   • chromosomes arranged side by side in homologous pairs- bivalents

   • centrioles migrate to poles to form spindle

   • nuclear envelope breaks down and nucleolus disintegrates

2. Metaphase I

   • bivalents line up along equator of the spindle

   • maternal and paternal chromosomes position themselves independently of the others- independent assortment

3. Anaphase I

   • homologous pairs are separated as microtubules pull whole chromosomes to opposite ends of the spindle

4. Telophase I

   • chromosomes arrive at opposite poles

   • spindle fibres break down

   • nuclear envelopes form around two groups of chromosomes and nucleoli reform

5. Cytokinesis

   • cytoplasm divides- organelles distributed and cell surface membrane pinches inwards

6. Prophase II

   • nuclear envelope breaks down and chromosomes condense

   • spindle forms perpendicular to old one

7. Metaphase II

   • chromosomes line up in a single file along the equator

8. Anaphase II

   • centromeres divide and individual chromatids are pulled to opposite poles

     • four groups of chromosomes with half the number of chromosomes as original

9. Telophase II

   • nuclear membranes form around each group of chromosomes

10. Cytokinesis

    • cytoplasm divides to create 4 haploid cells

crossing over

• process by which non-sister chromatids exchange alleles

• results in new combination of alleles on the two chromosomes

process of crossing over

• homologous chromosomes pair up and are close to each other

• non-sister chromatids cross over at chiasmata and get entangled

• entanglement places stress on DNA molecules, so a section of chromatid from one chromosome may break and re-join with chromatid from another chromosome

independent assortment

• production of different combinations of alleles in daughter cells due to random alignment of homologous pairs along the equator

the process of independent assortment

1. in prophase 1, homologous chromosomes pair up and in metaphase 1 they are pulled towards to equator of the spindle

2. each pair can be arranged with either chromosome on top- random

3. orientation of one homologous pair is independent by the orientation of another pair

4. homologous chromosomes are separated and pulled apart to different poles

calculating combinations of chromosomes

• 2ⁿ

  • n →haploid number of chromosomes

• for a zygote:

• 2²ⁿ

function and adaptations of erythrocytes

• function→ carry oxygen for respiration around the body

• biconcave shape- increase SA:V= higher rate of diffusion

• small diameter- large SA:V ratio

• flexible cytoskeleton → can change shape to fit through capillaries

• no nucleus, mitochondria, ER, little cytoplasm→ more space for haemoglobin

function and adaptations of neutrophils

• function→ destroy pathogens through phagocytosis and enzyme secretion

• twice the size of erythrocytes→ stay in bloodstream

• multilobed nucleus→ caused by flexible nuclear membrane- helps cell penetrate cell junctions

• flexible→ allows to engulf and destroy pathogens

• move via chemotaxis→ directed migration of cells towards a chemoattractant

function and adaptations of sperm cells

• function- reproduction→ fertilise an egg in order to initiate development of an embryo

• nucleus in head has half the number of chromosomes (haploid)

• acrosome in head contains digestive enzymes→ break down outer layer of egg cell

• lots of mitochondria- release energy through respiration for tail movement

• has tail which rotates to propel the cell forwards

function and adaptations of root hair cells

• function→ absorb water and mineral ions from soil

• root hair increases SA- higher rate of osmosis

• thinner walls- shorter diffusion path

• permanent vacuole contains concentrated cell sap- maintains water potential gradient

• lots of mitochondria- energy for active transport

• no chloroplasts

structure and function of ciliated epithelium

• function- move substances across surface of tissue

• have cilia which beat in a coordinated way to shift material along the surface of epithelium tissue

• goblet cells secrete mucus- helps trap dust, microorganisms etc

function and adaptations of squamous epithelium

• function- provide a surface covering or outer layer e.g. in alveoli, blood vessels

• single layer of flattened cells on basement membranes

• thin cross section- short diffusion pathway

• permeable- easy diffusion of gases

function and adaptations of palisade cells

• function- carry out photosynthesis to produce glucose and oxygen

• large number of chloroplasts- more light absorption for photosynthesis

• tall and thin shape- allows light to penetrate deeper before encountering another cell wall, also means cells can be densely packed together

function and adaptations of guard cells

• function→ control opening of stomata to regulate water loss and gas exchange

• inner cell walls thicker (facing air outside leaf) and outer cell walls thinner (facing adjacent cells)- difference in thickness allows cell to bend when turgid

• cytoplasm has high density of chloroplasts and mitochondria

what is a stem cell

a cell that can divide by mitosis to differentiate into a specialised cell

ability to differentiate→ potency

types of potency

• totipotency→ can differentiate into any cell type found in an embryo, as well as extra-embryonic cells

• pluripotency→ can differentiate into any cell found in an embryo

• multipotency→ adult stem cells

development of erythrocytes

• formed from bone marrow stem cells:

  1. multipotent cell divides to form proerythrocytes

  2. changes have occurred therefore cell can only form erythrocyte

  3. haemoglobin builds up in cytoplasm

  4. nucleus is ejected

  5. further changes (e.g. haemoglobin, biconcave) develop to form a mature erythrocyte

development of neutrophils

• formed from multipotent stem cells in bone marrow

• indentations in nucleus form→ lobed structure

• granules accumulate→ lysosomes containing enzymes

the development of xylem and phloem

• in roots and stems, stem cells at inner edge of cambium differentiate into xylem and cells at outer edge differentiate into phloem

• cambium cells that differentiate to form xylem lose cytoplasm, deposit lignin in cell walls and lose end walls

• cambium cells that differentiate into phloem lose some of their cytoplasm and organelles, develop sieve plates at ends of cells

use of embryonic stem cells in medicine

• stem cells used in therapeutic treatment of disease due to ability to differentiate

• can treat diseases e.g. Alzheimer’s, Parkinson’s

• embryos used are often waste embryos from IVF

  • have potential to develop into human beings→ ethical objections

use of multipotent adult stem cells in medicine

• research is being done to see if adult stem cells can be used to treat disease e.g. leukemia, and injuries e.g. skin burns

• less controversial than embryonic→ donor can give permission

• need to be close match to bloody type and antigens etc→ risk of rejection