Module 2 - Cell Division

What is the cell cycle?

organisms need new cells to grow + repair tissues in body 
consists of three main phases:
interphase - occupies majority of cycle as cells prepare for division 
mitosis - when the nucleus of cell divides into two
cytokinesis - when whole cell and cytoplasn divides in two to produce two identical cells 

What happens during interphase?

during phase cells carry out usual functions whilst preparing for cell division
consists of three phases:
G1 phase - cell grows and makes new proteins to replicate organelles so high amount of protein synthesis occurs
S phase - when DNA is replicated
G2 phase - when cell continues to grow + replicated DNA checked for errors
following mitosis cell may enter G0 where may differentiate

What are cell cycle checkpoints?

cell must past various checkpoinys in order to progress to next stage where checkpoints assess whether each process has been accurately completed - ensures genetic damageis detected + repaired - prevent uncontrolled cell division so important in preventing cancers
three checkpoints include:
G1 checkpoint - checks cell has chemicals needed for replication + for any damage to DNA + has enough enery reserve
S checkpoint - checks all chromosomes have been replicated 
G2 checkpoint - checks DNA has been replicated without errors (if errors occurs cell tries to repair them)
metaphase checkpoint - each chromosome (sister chromatids) checked to ensure attached to spindle 
regulated by CDKs which only allow cell cycle to progress if sufficient cell growth + accurate DNA replication has occured

What are chromosomes?

thread-like structures located inside nucleus - consist of DNA tightly coiled around proteins called histones 

How many chromosomes are in each cell?

have total of 46 chromosomes divided into 23 pairs within each cell
first 22 pairs called autosomes which are identical in both males and females - 23rd pair known as sex chromosomes which differs as male have XY chromosomes and female have XX chromosomes 

What pairs are chromosomes known as?

each pair of chromosomes consist of two homologous chromosomes known as homologous pairs - 1 chromosome in pair is a paternal chromosome inherited from individuals fathers + other maternal chromosome inherited from mother
although chromosomes are identical in terms of genes (which are located in the same location) they carry - may contain different alleles 

What is the structure of chromosomes?

may contain one or two chromatids - during interphase DNA is replicated meaning chromosome goes from having one chromatid to having two - two chromatids in each chromosome held together by centromere where two chromatids within a chromosome known as sister chromatids because they are genetically identical 

What are diploid cells?

contain two copies of each chromosome one copy from each parent
total number of chromosomes described as 2n with n representing one set of chromosomes eg humans have 46 chromosomes in each body cell 23 from mother and 23 from father
diploid number for human cells is 2n=46

What are haploid cells?

contain one copy of each chromosome
total number of chromosomes described as n eg human sex cells have 23 chromosomes so n=23

What happens in the phases that aren’t interphase?

mitosis - nucleus divides + cytokinesis - whole cell and cytoplasm divides

What is the importance of mitosis?

type of cell division in which parent cell divides to produce two gentically identical daughter cells - each daughter cell contains same DNA as original parent cell 
used by multicellular organisms for many processes including:
growth - unicellular zygotes can divide by mitosis to form multicellular organisms which need to get bigger
replacement/repairing of damaged or dead tissues - cells constantly dying and being replaced using mitosis
asexual reproduction - some organisms reproduce using mitosis to form genetically identical offspring eg strawberries + yeast
development of body plans - mitosis is used to form different part of an organism 
production of stem cells - stem cells divide by mitosis

What are the stages of mitosis and what does it form?

Prophase - Metaphase - Anaphase - Telophase
cell divides to form two genetically identical daughter cells - needed for growth and repair of body tissues as well as asexual reproduction

What happens during prophase?

chromosomes condense (become shorter and thicker) + visible under microscope - each chromosome made of two chromatids 
centrioles (bundles of protein) migrate to opposite poles of cell + spindle fibres form between centrioles - each centriole starts forming spindle fibres
nucleolus disappears + nuclear envelope starts to break down leaving chromosomes free in cytoplasm

What happens during metaphase?

chromosomes line up at equator of cell
each chromosome attaches to spindle fibres by their centromere (centrosomes send out microtubules to form a spindle fibre)
at metaphase checkpoint each chromosome checked to ensure attached to spindle
involves protein structure on each chromatid called kinetochore

What happens during anaphase?

centromeres divide to separate each pair of sister chromatids
spindle fibres contract + shortens to pull the chromatids to opposite poles of cell
each chromosome pulled by its centromere causing them to take on a ‘V’ shape

What happens during telophase?

chromatids reach opposite poles of cell where they uncoil to become long + thin chromosomes again 
nuclear envelope forms around each set of chromosomes to form two nuclei and nucleolus starts to reform 

What is cytokinesis?

after steps of mitosis cytoplasm divides in process known as cytokinesis producing two genetically identical daughter cells - organelles shared between two cells having increased in number during interphase 

How can mitosis be observed?

under a light microscope using suitable part of plant eg root tips must be sprouting - a suitable dye such as acetic orcein will stain chromosomes and help make stages of mitosis more visible
can add more later from seneca possibly

How does mitosis occur in plants?

very similar to animal process but plants don’t have centrioles although they still form a spindle - also during cytokinesis vesicles from Golgi apparatus form new cell membrane + new cell membrane + cell wall

What is asexual reproduction?

reproduction which involves production of offspring from single individual where in eukaryotes always involves mitosis
advantageous as much quicker but lack of genetic variation means population less resistant to disease meaning if one organims susceptible all will be so whole population wiped out - more common in plants as cannot move from place to place to find a mate 

How does yeast reproduce?

yeast reproduces asexually by budding where nuclear division by mitosis 
stages 1 - DNA replicates inside the nucleus
2 - area of cell wall weakened causing cytoplasm + cell wall to bulge out forming a bud 
3 - ring of chitin forms at junction of cell + bud - after bud seperates remains as a scar
4 - nucleus migrates to region of bud + undergoes mitotic division but without nuclear membrane breaking up 
5 - one nucleus migrates into bud along with organelles
6 - cytokinesis complete with formation of new cell wall between mother and daughter cells
7 - new daughter cells grows to full size before budding cycle again
unlike plant + animal cells yeast cytoplasm divides unequally leaving one cell to grow while other already full size + nuclear membrane remains intact throughout

What is meiosis?

type of cell division where parent cell divides to form four haploid cells each genetically different from one another 
before meiosis starts DNA is replicated during interphase so each chromosome contains two chromatids 
undergoes 2 divisions: Meiosis I - homologous chromosomes are separated + Meiosis II - chromatids seperated
reduction division where chromosome number halved to form haploid cells which form the gametes

What are the stages of meiosis?

two divisions of meiosis contains: meiosis I - prophase I,metaphase I,anaphase I + telophase I - produces two haploid (n) daughter cells
meiosis II - prophase II,metaphase II,anaphase II + telophase II - produces four haploid (n) daughter cells

What happens during prophase I?

chromosomes condense + homologous chromosomes pair up to form bivalents (maternal chromosomes are blue + paternal are pink)
centrioles migrate to opposite poles of cell where each centriole starts forming spindle fibres 
nucleolus disappears + nuclear envelope starts to break down leaving chromosomes free in cytoplasm
non-sister chromasite of crossing over site ocrossing over where tids join,break and exchange parts in crossing over - chiasmata site of crossing over where homologous chromosomes remain attached until metaphase

What happens in metaphase I?

chromosomes (bivalents) line up along equator of cell in homologous pairs (in humans 23 pairs line up)
each chromosome attaches to spindle by their centromere
paternal + maternal chromosomes in each bivalent position themselves independently

What happens in anaphase I?

homologous chromosomes pairs are seperated + pulled to opposite poles of cell (chromatids stay joined together)

What happens during telophase I?

chromosomes reach opposite poles of cell where they uncoil 
nuclear envelope forms around each set of chromosomes + nucleolus starts to reform
cytoplasm divides to form two cells (cytoplasm)

What happens during prophase II?

chromosomes condense + now visible under microscope
centrioles migrate to opposite poles of cell where each centriole starts forming spindle fibres
nucleolus disappears + nuclear envelope starts to break down 

What happens during metaphase II?

individual chromosomes line up at equator of cell with their chromatids randomly arranged (important if crossing over has occured in meiosis I)
each chromosome attaches to spindle by their centromere

What happens during anaphase II?

centromere divide + separate each pair of chromatids
spindle fibres contract and shorten to pull chromatids to opposite poles of cells

What happens during telophase II?

chromatids reach opposite poles of cell where they uncoil to become long and thin again
nuclear envelope forms around each set of chromosomes to form nuclei + nucleolus starts to reform
cytoplasm divides (cytokinesis) and 4 haploid cells are produced

What is different in the first division of meiosis to mitosis?

in meiosis homologous chromosomes pair up and then seperate into different cells which doesn’t occur during mitosis 
two chromatids formed from single chromosome remain together during first division of meiosis but separate in second division which is similar to mitosis in that chromatids separate so number of chromosomes remains the same

Why is meiosis important?

production of haploid gametes - this allows sexual reproduction to take place
creates genetic variation - increases diversity allowing natural selection to occur

What happens during sexual reproduction?

meiosis needed to produce haploid gamete cells containing half number of chromosomes as a body cell 
where two gametes fuse together to form a zygote 
as each gamete contains half normal number of chromosomes (n) results in zygote cell containing diploid number of chromosomes (2n) where half are from fathers sperm cell and half from mothers egg cell 

What events in meiosis lead to genetic variation?

crossing over (recombination)
independent assortment (random assortment)

What is crossing over?

steps: 1 - during prophase I homologous chromosomes condense + pair up (synapsis) 2 - chromatids of each chromosome twist around one another forming a chiasmata 3 - when chromosomes seperated in anaphase I chromatids break at chiasmata +broken sections are exchanged + reconnects to chromatid from homologous chromosome 
this swaps alleles between homologuos chromosomes to produce different combinations on each chromosome
each cell has a different chromatid increasing genetic variation of offspring 

What is independent assortment?

during metaphase I pairs of homologous chromosomes line up along cell equator where the paternal or maternal chromosomes appearing on left or right random - as a result which chromosomes end up in each daughter cell is also random 

How do you calculate genetic variation?

for number of genetically distinct gametes produced use formula 2 to the power of n where n represents number of chromosome pairs 
for number of genetically different zygotes possible from two parents use same formula but square the answer 

What are 4 key terms of cell organisation?

specialised cells - cells with specific features that allow them to carry out particular function
tissue - group of similar cells working together to carry out particular function
organ - group of similar tissues working together to carry out particular function
organ system - group of organs working together to carry out particular function

What are the examples of level of organisation?

specialised cells - erthrocytes - neutrophils - palisade cells
tissues - epithelium - cartilage - xylem tissue
organs - stomach - pancreas - leaf
organ systems - cardiovascular - digestive - root system

What is the first example of animal tissue?

squamous epithelium - provides a thin lining for organs such as lungs - any type of epithelium that consists of layer of flattened cells
tissue made up of singele layer of squamous epithelial cells as only one cell thick gases can quickly diffuse through tissue

What is the second example of animal tissue?

ciliated epithelium - lines organs sich as trachea where it sweeps mucus away from lungs 
tissue made up of ciliated epithelial cells which use cilia to sweep mucus away/shift material along surfaced of epithelium + goblet cells which release mucus to trap pathogens 

What is the third example of animal tissue?

cartilage - type of connective tissue that acts as a cushion between bones and also provides support to organs such as the ears + nose - function is to protect + strengthen
tissue made up of chondrocyte cells fixed with extracellular matrix

What is the fourth example of animal tissue?

muscle tissue - made of muscle fibres (bundles of elongated cells) these fibres contract (shorten) and relax to move different parts of the body 
three types : smooth found in walls of organs 
cardiac found in heart
skeletal found attached to bones 

What is the first example of plant tissue?

xylem tissue - responsible for transport of water + minerals within plants 
made up of dead xylem vessel cells which have no end walls + organelles which forms continuous column where water can flow - walls of these cells strengthened by waterproof material lignin
consists of up to 4 different types of cell
parenchyma - form a packing tissue between other cells
xylem vessel elements - have wide lumen
tracheids - also transport water but also important at strengthening tissue
fibres - function is purely support 

What is the second example of plant tissue?

phloem tissue - responsible for transport of sugars and amino acids within plants
made up of columns of sieve tube elements and companion cells where sieve tube element cells are seperated by sieve plates with holes so sugars can pass through + contain very few organelles allowing sugar to flow easily
companion cells contain many mitochondria to release energy

What are some example organs?

heart - pumps blood around the body - cardiac muscle requires oxygen for respiration supplied by blood - beating of heart requires coordination done by nervous tissue
leaf - plant organ that carries out photosynthesis + gaseous exchange - within layers has xylem which brings water to the leaf - phloem which transports organic nutrients away - palisade + spongy parenchyma carry out photosynthesis - epidermis which waterproofs the leaf - stomata which allows diffusion of gases out and into the leaf

What are specialised cells?

have features that enable them to carry out their specific function 

How are red blood cells specialised?

erthrocytes responsible for transport oxygen around the body - formed from stem cells in bone marrow as has no nucleus it cannot divide so new cells have to be formed from stem cells to maintain erthrocyte count
flattened biconcave shape - increases the surface area to volume ratio to allow increased diffusion of oxygen 
no nucleus or organelles - provides more room for haemoglobin (molecule that binds to oxygen) increasing oxygen-carrying capacity of cell
cytoplasm has lots of haemoglobin - which binds with oxygen
flexible as has elastic membrane - able to fit through narrow capillaries as allows red blood cell to change shape

How are white blood cells specialised?

neutrophils help to defend body against pathogens produced from same stem cell as rbc - has indentations of nucleus + accumulation of granules which are lysosomes
flexible cell membrane - allows cell to engulf pathogen + can penetrate between junctions of the cells of capillary wall
contain many lysosomes - contains digestive enzymes to break down/destroy engulfed particles
multi-lobed nucleus - allows cells to deform so can squeeze through small gaps to reach sites of infection 

How are sperm cells specialised?

carry genetic information to female gamete
flagellum (u name) - allows cell to swim to egg cell
many mitochondria - supply energy needed for movement
acrosome containing digestive enzymes - digest protective layers around egg cell allows sperm cell to enter it 

How are cells of the organ surface specialised?

squamous epithelial cells cover surface of organs such as lungs and blood vessels
very thin - allows effcient diffusion of gases such as oxygen and carbon dioxide 
permeable - allows diffusion of gases 

How are cells of the organ surface specialised involving cilia specialised?

ciliated epithelial cells cover surface of organs where they can move substances like mucus or egg cells - found in organs such as bronchioles + fallopian tubes 
cilia (hair-like structures) - beat to move pathogens and mucus away from lung or egg towards uterus 

How is the cell for photosynthesis specialised?

palisade cells carry out photosynthesis in leaves of plant 
lots of chloroplasts - absorb light needed for photosynthesis 
thin cell walls - allows carbon dioxide to quickly diffuse into the cell 
tall and thin shape - allows many palisade cells to closely pack together to from continuous layer near surface of leaf 

How are cells in roots specialised?

root hair cells absorb water and mineral ions from soil 
root hair structures - increase the surface area for absorption 
thin,permeable cell wall - allows entry of water and ions 
lots of mitochondria - provide energy for active transport 

How are the cells associated with stomata specialised?

guard cells control opening and closing of stomata - used to allow carbon dioxide to enter the leaves and prevent water loss
come in pairs - allows a gap (stoma.plural stomata) to form between them
change shape when light present - guard cell absorb water to become turgid opening stoma to allow entry of carbon dioxide
change shape when lose water - guard cells shrink and close stoma to prevent water loss
thin outer walls and thick inner walls - allows cell to bend when they are turgid to open the stoma

What forms the plants transport system?

xylem vessels and phloem sieve tubes formed from stem cells in tissue between them called cambium which divides to form phloem cells on the outside and xylem cells on the inside 
cambium is example of meristems which are found in shoot and root tips 
production is stimulated by hormones + balance of different hormones can shift production between where cells that become xylem lose their cytoplasm + deposit lignin in cell walls + phloem formation there is some cytoplasm loss + sieve plates at end of cells

What are stem cells?

undifferentiated cells that can develop into other types of cells - used for growth,development + tissue repair
have 2 key features - can divide by mitosis to produce more undifferentiated cells + can differentiate into specialised cells

What are the four types of stem cells?

totipotent - can differentiate into any cell type and go on to form whole organisms + can form extra-embryonic cells which make up placenta + umbilical cord - only example are zygotes + very early embryos
pluripotent - can differentiate into most cell types but cannot form whole organisms or extra-embryonic cells
multipotent - can only differentiate into a few different cell types
unipotent - can only differentiate into one type of cell

What are the 3 locations of stem cells?

embryonic stem cells - adult stem cells - plant stem cells

What are embryonic stem cells?

found in early stages of embryo development where they can differentiate into cells to form a foetus
in first few divisions of embryo the stem cells are totipotent
after about 7 days stem cells become pluripotent

What are adult stem cells?

found in some adult tissues where they can replace faulty cells
stem cells are multipotent or unipotent 
stem cells in bone marrow replace  worn out erythrocytes and neutrophils

What are plant stem cells?

found in meristematic tissue or meristems at tips of shoots and roots
stem cells are pluripotent 
meristematic tissues also found between xylem and phloem tissues in area called vascular cambium where these stem cells can differentiate into cells of xylem and phloem 

How are stem cells used in research and medicine?

have potential to treat certain diseases therfore works by using stem cells to replace faulty cells - eg parkinsons and alzheimers both caused by loss of nerve cells in brain therefore transplanted stem cells could help regenerate nerve cells + reduce symptoms
stems cells can be used for: testing new drugs - can be tested for toxicity + sude effects before being tested on humans
studying development of organisms - can find out how organisms grow + develop from single cell
identifying causes of disorders - can identify when something goes wrong in organisms development