D2 cell division, gene expression, water potential

what is cytokinesis? how does it work in animal and plant cells

• def: process where cytoplasm of parent cell is split to make 2 daughter cells

• animals:

  • actin/myosin makes contractile ring to pinch cell membrane

  • forms a cleave furrow (with aid of fluid nature of double membrane)

• plants:

  • need to assemble cell plate (made from fusion of vesicles containing cell wall mateirals)

  • plate grows outward until fusing with existing cell wall

describe an example of unequal division of cytoplasm in humans

oogenesis

• primary oocyte —> 2 rounds of division —> secondary oocyte + first polar body

• if fertilisation occurs, secondary oocyte —> cell division —> mature ovum + second polar body

• *mature ovum has majority of cytoplasmic contents —> necessary nutrients for early development

describe budding

• unequal cytokinesis

• asexual reproduction =  outgrowth of a genetically identical daughter cell or ‘bud’ from the parent cell. The bud starts small and grows in size until it forms a fully developed cell that can function independently from the parent cell.

• daughter cell is smaller than parent and gets less cytoplasm

• parent cell left with small scar

mitosis vs. meiosis functions

mitosis

• somatic cells

• makes diploid genetically identical daughter cells

• for growth or replace damaged cells

meiosis

• makes 4 genetically uniquhe daugter nuclei

• also beings with diploid cell

• has 2 rounds of nuclear division

describe shared features of mitosis/meiosis

1. need DNA first (in interphase)

2. both involve movement of chromosomes (microtubule + microtubule motors to move them)

how does DNA look like before and after nuclear division

When the cell is not undergoing nuclear division, the DNA is loosely packed around the histone proteins.

At the beginning of nuclear division the chromatin is supercoiled and condensed by histone proteins. This results in tightly coiled shapes that we recognise as chromosomes. Supercoiling of DNA allows for efficient separation of replicated DNA during nuclear division.

describe mitosis stages

• makes 2 genetically identical diploid daughter cells

1. prophase

• chromatin condenses into chromosomes (already undergone replication, made of 2 genetically identical sister chromatids)

• nuclear membrane breaks down

• spindle fibres form

• plants use MTOC to organise spindle fibres, animals use centrosomes —> MTOCs migrate to each pole of the cell

2. metaphase

• sister chromatids line up on metaphase plate

• spindle fibres (bound to centromere of sister chromatids) moves them into position

3. anaphase

• spindle fibres shorten to split centromere and pull sisters apart to either pole

4. telophase

• chromosomes decondense, nuclear membrane reforms

• spindle fibres disintegrate

describe steps in meiosis I

• produces 4 genetically unique daughter haploid nuclei

• is reduction division

Meiosis I

P1:

• nuclear mmebrane disintegrates

• MTOCs migrate to opposite poles

• spindle fibres form

• chromatin condenses into chromosomes —> sets of sister chromatids join into a bivalent/tetrad. crossing over may occur (Exchanging equivilant DNA segments between non-sister chromatids)

M1:

• spindle fibres attahc to centromeres of homologous chromosomes to pull bivalents to centre of cell

• maternal & paternal homologues show random orientation to poles (orientation of maternal is independent of other paternal)

A1:

• spindle fibres shorten to separate bivalent

• pulls homologous chromoosmes to opposite poles

• sister chromatids still connected at centromere

T1:

• homologous chromosomes reach poles of the cell and decondense

• nuclear membrane forms

• spindle fibres breakdown

now, is cytokinesis, and interkinesis (resting)

list steps in meiosis II

Prophase II: During prophase II the DNA recondenses, the nuclear membrane disintegrates, MTOCs migrate to opposite poles and the spindle fibres start to form.

Metaphase II: In metaphase II spindle fibres attach to the centromeres, lining up sister chromatids in the centre of the cell. Sister chromatids show random orientation towards the poles.

Anaphase II: In anaphase II, spindle fibres shorten, splitting the centromere and pulling sister chromatids apart towards opposite poles. Once sister chromatids are separated, they are called chromosomes.

Telophase II: In telophase II the chromosomes reach the opposite poles of the cell and  decondense. A nuclear membrane forms around each nuclei.

Following meiosis II, cytokinesis will occur to create four haploid daughter cells.

what is non-disjunction caused by? what does it result in?

• caused by:

  • failure of pairs of homologous chromosomes to separate in anaphase I

  • failure of sister chromatids to separate in anaphase II

  • leads to gametes with 1 extra or missing chromosome

what is trisomy 21?

a condition that results from an extra copy of chromosome 21. Down syndrome occurs in approximately 1 in every 700 live births worldwide and results in individuals with a range of intellectual and physical disabilities. This includes poor muscle tone, heart defects and delayed development

what is an example of monosomy?

• monosomy X (turner’s syndrome)

• normally, females have XX, males have XY

• non-disjunction happened in mother/father sex chromosomes not separating properlI

• a gamete is missing an X chromosome

• offspring only get 1 X chromosome from a parent

what are 3 ways meiosis results in genetic diversity

1. crossing over

• in prophase I:

  • may occur several times in the same bivalent

  • occurs at different places each time meiosis occurs

  • happens anywhere along chromosome

  • sister chromatids no longer genetically identical —→ gamete inherits diff. combinations of alleles

2. random orientation/independent assortment

• in metaphase I or II:

  • homologous chromosomes line in equator and are orientated randomly (position don’t matter)

3. random fertilisation

• the randomness of which egg/sperm fuse

define cell proliferation

• the process of cellular division and replication

• for growht, cell replacement, etc

what are 3 stages in cell proliferation

1. growth

• cell proliferation increases cell number and organism size & complexity

• plants: growth occurs in meristems —> cells at tip remain differentiated, so cells behind them can specialize and differentiate into specific cell types

• humans: cell division happens every 24 hours in early embryonic development

2. cell replacement

• ex. skin cells

3. tissue repair

• cells divide and migrate to site of injury

what are. 3stages in cell cycle

interphase (G1, S, G2)

• G1: in cytoplasm, cell grows in size and carries out regular metabolic fx, mito and chloroplasts replicate through binary fission, cell doubles in size

• G2: in nucleus, DNA replication

• G2: in cytoplasm, cell continues growing, synthesizes microtubles and proteins needed for cell/nuclear division, checks that DNA replication was accurate

mitosis

cytokinesis

how is cell cycle controlled?

by cyclins

• a family of proteins that regulate cell cycle

• they bind and activate CDKs (enzymes that phosphorylate proteins for cell cycle)

• only active in some stages of cell cycle (need threshold)

ex. cyclin E binds to CDK before S-phase, activating DNA replication

a mutation can occur in ____ or _____ which are genes involved in the cell cycle

1. proto-oncogenes

• genes coding for proteins promoting cell growth

• a mutation results in uncontrolled division

2. tumour suppressor genes

• genes coding for proteins that slow down/prevent cell division

• when mutated, leads to uncontrolled cell division

contrast 2 types of tumours

1. benign

• abnormal growth of cells

• non cancerous

• grows slowly

• has well defined borders

2. malignant

• lacks well defind border

• can spread through metastasis in bloodstream/lymphatic system (forms secondary tumour that’s difficult to treat)

formula for mitosis

mitotic index

• measures the % of actively dividing cells in a population

• from 0-1

• actively dividng cells w/visible chromosomes/total number of cells

what are promoters?

• noncoding regions of DNA where RNA polymerase binds to

• typically loacted at 5’ end of a gene

• allows initiation of transcription

what do transcription factors do?

• proteins that influence gene expression (Affects interaction of RNA polymerase and promoter)

• encourages RNA polymerase to bind to promoter or blocks it

• the availability of it affects how easily RNA polymerase binds to promoter

what is enhancer? what does it work with

• regions of DNA that regulate when and to what extent a gene. is expressed

• noncoding

• unlike promoters, enhancers can be located far away from genes

• works with activator proteins (Type of transcription factor), where binding forms a complex that interacts with promoter region

can multiple genes transcribed at the same time?

yes, since 1 enhancer can interact w/multiple promoters

what is operon

• a collection of genes with the same promoter

• these genes are usually transcribed together

what determines how long an mRNA persists?

• usually, the longer = translated more

• will eventually be broken by nucleases

lifespan depends on:

• chemical modifications (guanine cap at 5’, poly-A tail, all increases stability of mRNA)

  • short tails are likely to be translated since they are vulnurable to degradation —> can vary the length to vary how much each protein is made

• presence of stabilising proteins (interfears with nucleases to extend lifespan, as well as help binding of protective proteins)

define epigenesis

the process where cells/organisms develop from an undifferentiated zygote through interactions between DNA and environmental factors

• basically, how stem cells differentiate into different tissues

• environmental factors can modify gene expression factors without changing DNA sequence (by activating/silencing genes) to alter phenotype

• the study of how behaviour and environment affects gene transcription

describe DNA methylation

• methylation of cytosine in a promoter represses transcription —> prevents binding of transcription factors

• results in silencing of genes

• level may depend on age, diet, etc

describe histone protein structure

• has core domain + tail

• tails (N-terminus) protrude outwards

• has net positive charge, mainly in the tails

• helps attract DNA to keep it together

describe the different structures that nucleosomes can be in based on level of transcription

1. heterochromatin

• when DNA is supercoiled around histones

• DNA less accessible to RNA poly.

• less transcription

2. euchromatin

• when DNA is loosely packed

• more accessible to RNA polymerase

• higher transcirption

what are ways histone tails can be modified?

1. acetylation

• decreases charge of histone protein, reducing attraction between histone and DNA

• easier for RNA polymerase to access DNA

• increased gene expression

2. methylation

• of AA in histone tails

• represses/activates trancsription

both alter nucleosome structure

define epigenetic inheritance

• def: inheritance of non-genetic information that can influence gene expression and phenotypic traits

• for this to occur, epigenetic changes (DNA methylation, histone modificaiton) must occur in germline cells

list 3 environmental effects on gene expression

1. exposure to air pollution

• affects DNA methylation

2. diet

• high folic acid = increased methylation

3. temperature

• the sex of reptiles

what are epigenetic tags?

• tags like methyl groups that regulate gene expression

• after fertilisation, most tags are removed

discuss effect of lactose on lac operon expression

lac operon = cluster of 3 genes in bacterial DNA coding for proteins for lactose digestion

bacteria use lactose as a backup energy source

when lactose absent:

• lac repressor binds to operator region

• prevents attachment of RNA polymerase to lac operon promoter

• represses transcription

when present:

• lactose bindst o repressor, detaching it from promoter, allowing RNA polymerase to bind and transcribe it

discuss effect of tryptophan on expression of bacterial genes

• it affects tryptophan operon = a cluster of 5 genes in bacterial DNA, necessary for synthesis of tryptophan AA

• when tryptophan absent:

  • RNA polymerase binds to tryptophan operon to transcribe genes for tryptophan

  • bacteria can produce tryptophan when not available

• when present:

  • tryptophan binds to repressor protein, which binds to generator region of operon

  • inhibits transcription fo operon

when can water dissolve the substance

when force of attraction between ions and water is greater than force of attraction between oppositely charged ions

hyper vs. hypotonic solution

hyper = high solute concentration (high osmotic concentration)

hypo = low solute concentraiton (low osmolarity)

water flows from…

hypotonic —> hypertonic until isotonic

what happens if cell is put in hypertonic solution:

• will be net movement of H2O out of cell

• cell shrinks (crenated or plasmolysis)

what happens if cell is placed in hypotonic solution

• will have net movement of water into cell

• cell swells

• animal cells may lyse

how to determine isotonic [solute] of a plant tissue?

meausre % change in tissue mass and length of plant tissues placed in different solution concentrations

1. same mass/length of plant tissue is put in different solute concentrations

2. then, removed and patted dry

3. data is collected

discuss H2O movement on plants

1. influx of H2O = accumulates turgor pressure, making cytoplasm exert pressure against cell wall. however, the cell is turgid since the wall present busting

2. efflx of H2O = plant cell plasmolyses

define water potential

def: measure of the pot. energy of water/unit volume H2O, measured in kPa. pure water has 0kPa

water potential is defined by what 2 things

1. solute potential

• ability for H2O to move given the concentration of solutes

• the more solutes in a solution, the less free water (less potential to move)

2. pressure potential

• pressure that H2O exerts on cell membrane

• can be positive (increase water pot. when exerted outwards from cell) or negative (xylem suction)

negative water pot means..

harder for water to move

water moves by osmosis….in terms of water pot.

from high water pot to low water pot.

describe what happens when plants placed in hypotonic solution in terms of water pot.

1. solute pot of tissue is more negative than the solution

2. H2O moves into tissue (less negative water pot. —> more negative)

3. the additional water in tissue cells tell them to appply turgor pressure against cell wall

4. thus, the pressure pot. is positive, since pressure inside cell is higher than outside cell