D2 - Cell and Nuclear Division, Water Potential

Cytokinesis

The division of a cells cytoplasm to form two cells. It occurs after mitosis and happens differently in plant and animal cells.

Cytokinesis in Plant Cells

A new cell wall is made across the equator, with a plasma membrane on both sides. This divides the cell in two.

Cytokinesis in Animal Cells

The cytoplasm is divided by moving the plasma membrane. Movement is due to actin and myosin proteins adjacent to the membrane. Tension is exerted to form the cleavage furrow, with the membrane pulled inward so it eventually splits the cell.

Cytokinesis can be equal or unequal

• Cytoplasm of the mother cell is divided into equal halves

• Unequal division results in small cells that can grow and further divide with a nucleus

• Examples are oogenesis (egg production) in humans, or budding in yeast

Mitosis - For Continuity

Daughter cells receive all the chromosomes and genes of the mother cell. Same chromosome number. Used in asexual reproduction to produce genetically identical offspring or in organisms for body cells

Meiosis - For Change

Diploid nucleus divides into haploid nucleus, halving the chromosome number. Allows for haploid gamete production. Allows for genetic diversity because every haploid cell has different combination of alleles.

Condensation

When DNA is packed up to form shorter and fatter chromosomes. Includes DNA being wrapped twice around 8 histone proteins. These are then linked together and supercoiled

Microtubules

Narrow structures assembled from many molecules of tubulin (a globular protein). More tubulins can be added to extend the microtubules.

Microtubule Motors

Cause movement by removing tubulin subunits from the end of microtubules and shortening them.

Kenetichores

Microtubule motor anchored to chromatid’s centromere to which microtubules attach to during anaphase. They allow chromosomes to be pulled to opposite poles.

Stages of Meiosis

1. Early prophase

2. Late prophase

3. Metaphase

4. Anaphase

5. Early telophase

6. Late telophase

Early Prophase

• Microtubules grow from centrioles to form spindles

• Chromosomes condensing

Late Prophase

• Spindle microtubules extend from each pole to the equator

• Each chromosome includes identical sister chromatids

Metaphase

• Nuclear membrane broken down

• Chromosomes migrate to equator

• Microtubules attach to kenetichores

Anaphase

• Sister chromatids separate becoming separate chromosomes

• Kenetichores shorten spindle microtubules, pulling to opposite poles

Early Telophase

• Chromosomes reach the poles and nuclear membranes form around them

• Spindle microtubules break down

Late Telophase

• Chromosomes uncoil

• Cytokinesis divides cells

Viewing Stages of Mitosis

• Interphase - no condensation

• Prophase - condensation

• Metaphase - aligned to equator

• Anaphase - V shaped and moving to poles

• Telophase - condensing at poles

Meiosis

A reduction division because it halves the number of chromosomes. Mother cell divides a diploid nucleus producing four cells with haploid nuclei

Diploid Nucleus

• Has 2 sets of chromosomes

• Contains homologous chromosomes, with same genes in same sequence but potentially different alleles

Haploid Nucleus

Has only one set of chromosomes

Represented by the letter n

Includes gametes like sperm and egg to be used during fertilization to produce one diploid cell.

Meiosis Stages

Meiosis Prophase I

• Chromosomes pair up, each pair are homologous

• Spindle microtubules grow from poles

Meiosis - Metaphase I

• Random orientation of homologous chromosomes on the equator

• Spindle microtubules attach

Meiosis - Anaphase I

• Homologous chromosomes pulled to opposite poles, halving the chromosome number

• Each chromosome still includes 2 chromatids

Homologous chromosomes vs Sister chromatids

Sister chromatids are identical copies of a single chromosome.

Homologous chromosomes are pairs of chromosomes that are similar in size, shape, and gene content. They carry the same genes in the same order, but may have different versions of those genes.

Chromosome vs Chromatid

• A chromosome is a long, thread-like structure carrying genetic information in the form of DNA

• A chromatid is one of the two identical halves of a replicated chromosome, joined at the centromere

Meiosis - Prophase II

• Two haploid cells produced by 1st meiosis division

• DNA does not have to replicated because Each chromosome still has 2 chromatids

Meiosis - Anaphase II

• Kinetochores pull chromatids to the poles

• Sister chromatids not identical because of exchange of alleles

Meiosis - Telophase II

• Chromosomes decondense inside new nuclear membranes

• 4 haploid cells produced

• Every nucleus produced is genetically different

Trisomy 21 (Down Syndrome)

Caused by an error in meiosis when pair 21 of homologous chromosomes fails to separate in anaphase 1 (non-dysjunction), moving to same pole.

The zygote has 3 copies of chromosome 21.

Trisomy

Having 3 chromosomes instead of 2. Usually results in death of gamete or or early-stage embryo

Meiosis Generating Variation

Creates genetic diversity (different combinations of alleles) in 2 ways:

1. Random orientation of bivalents

2. Crossing over

Random Orientation of Bivalents

Bivalent: Pair of homologous chromosomes, one from each parent.

Orientation in metaphase determines to which pole each chromosome moves to.

Crossing Over

Homologous chromosomes pair up in the very early stages of meiosis and non-sister chromatids exchange lengths of DNA at the chiasma.Produces new combinations of alleles. Random places of genetic exchange.

Solvents

Liquids that can dissolve other substances to make solutions

Solutes

Are dissolved substances in solutions

Solvation

The process of dissolving

Types of Solutions

Hypertonic - Higher solute concentration

Hypotonic - Lower solute concentration

Isotonic - Same solute concentration

Net Movement of Water

Up the solute concentration gradient, from the lower to the higher solute concentration.

Dynamic Equilibrium

When the environment of a cell is isotonic the water molecules continue moving but equally so there is no net movement.

Cells in Hypotonic Solution

Water enters cell.

Volume of cytoplasm increases, swelling the cell until bursting.

Cells in Isotonic Solution

Dynamic equilibrium with cell volume not changing

Cells in Hypertonic Solution

Water exits the cell.

Volume is reduced but plasma membrane area stays the same so appears shrivelled.

Regulation of Water Movement

• Unicellular organisms in freshwater take on water by osmosis and expel it using contractile vacuoles

• Cells surrounded by other cells must be kept in an isotonic environment

• Humans use kidneys to regulate solute concentrate of extracellular fluids

Cell Walls in Hypotonic Solution

• Water enters cell

• Increase in cytoplasm volume creating turgor pressure against wall

Cell Walls in Hypertonic Solution

• Water exits cell

• Gap develops between cell wall and plasma membrane due to decreased voluume

• Called plasmolysed

Isotonic Solutions in Medicine

• Intravenous fluids: injected into veins via syringe or bag, must be close to isotonic to avoid osmotic cell damage

• Tissues/organs used in transplants must be bathed in isotonic solutions to prevent damage