Chapter 6 Cell Division

Question 1

What is the cell cycle

Answer 1

a order of events that take place in the cell,
resulting in the division of the cell and formation of two new genetically identical daughter cells.

Question 2

Interphase

Answer 2

During interphase:
DNA is replicated and checked for errors in the nucleus
protein synthesis occurs in the cytoplasm
mitochondria grow and divide
chloroplasts grow and divide in plant and algal cells
metabolic processes of cells occur

Stages of Interphase:
G1 - the first growth phase - the cell increases in size, organelles are synthesised and produced.
S - synthesis phase: DNA is replicated in the nucleus
G2 - the second growth phase: the energy stores are increased and replicated DNA is checked for errors.

Question 3

Checkpoints in the cell cycle

Answer 3

G1 checkpoint - at the end of G1 phase before S phase. Checks for: - cell size, nutrients, growth factors, DNA damage. if the cell satisfies the requirements of this checkpoint it continues to the S phase else it enters the G0 phase - resting state.

G2 checkpoint - at the end of G2 phase before mitosis. Checks for: - cell size, DNA replication, any DNA damage.

Spindle fibre checkpoint - at metaphase - checks all chromosomes are attached to spindles and have aligned.

Question 4

The importance of mitosis

Answer 4

essential for growth and repair of tissues.
also necessary for asexual reproduction.
Bacteria do not have a nucleus and reproduce through other means - binary fission.

Question 5

Chromosomes before Mitosis

Answer 5

the DNA is replicated during interphase. Each chromosome is converted into two identical DNA molecules called chromatids.
There is a replication from one chromatid per chromosome to two.

Question 6

Prophase during Mitosis

Answer 6

Chromatin fibres begin to coil and condense.
Protein microtubules form spindle shaped structures linking the poles of the cells. The fibres are necessary to move the chromosomes into the correct position before division.
Two centrioles migrate to opposite poles of the cell.
The spindle fibres attach to specific areas on the centromere and start to move the chromosomes to the centre of the cell
By the end of prophase the nuclear envelope has dissappeared

Question 7

Metaphase

Answer 7

Chromosomes are moved by spindle fibres to form a plane in the centre of the cell called the metaphase plate, and then they are held in position. The metaphase plate is at the equator.

Question 8

Anaphase

Answer 8

The centromeres holding together the pairs of chromosomes in each chromosome divide during anaphase. THe chromatids are separated - pulled to opposite poles of the cell by the shortening spindle fibres.
The ‘V’ shape of the chormatids is as a result of them being dragged by their centromeres through the liquid cytosol

Question 9

Telophase

Answer 9

In telophase the chromatids have reached their poles are now called chromosomes. The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them. The chromosomes start to uncoild and the nucleolus is formed.
Then cytokinesis begins

Question 10

Cytokinesis in Animal cells

Answer 10

a cleavage furrow forms arround the middle of the cell. The cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle forming two cells.

Question 11

Cytokinesis in Plant cells

Answer 11

Plant cells have cell walls so a cleavage furrow cannot form. Instead vesicles from the Golgi apparatus begin to assemble in the same place as where the metaphase plate was formed. The vesicles fuse with each other and the cell surface membrane, dividing the cells in two.

Question 12

Prophase 1 - Meiosis 1

Answer 12

Chromosomes condense, the nuclear envelope disintergrates, the nucleolus disappears and spindle formation begins just like in prophase in mitosis.
Homologous chromosomes pair up, forming bivalents, when chromsomes entangle causing crossing over.

Question 13

Metaphase 1 - Meiosis 1

Answer 13

Homologous pairs of chromosomes assemble along the metaphase plate.
Independent assortment occurs when the homologous pairs are orientated on the metaphase plate. The maternal and paternal chromosomes end up facing either pole leading to many combinations and genetic variation.

Question 14

Anaphase 1 - Meiosis 1

Answer 14

homologous chromosomes are pulled to either poles.
sections of DNA in sister chromatids will cross over once they break apart after being entangled. This will form recombinant chromatids. Genentic variation rises.

Question 15

Telophase 1 - Meiosis 1

Answer 15

chromosomes assemble at each pole and the nuclear membrane reforms. Chromosomes uncold. The cell undergoes cytokinesis and divides into two cells. The reduction of chromosome number from diploid to haploid is complete.

Question 16

Prophase 2 - Meiosis 2

Answer 16

The chromosomes condense and become visible again. THe nuclear envelope breaks down again and spindle formation begins.

Question 17

Metaphase 2 - Meiosis 2

Answer 17

the individual chromosomes assemble on the metaphase plate. Due to crossing over the chromatids are no longer identical so there is independent assortment agian and more genetic variation.

Question 18

Anaphase 2 - Meiosis 2

Answer 18

results in chromatids of the individual chromosomes being pulled to opposite poles and division ateh the centromeres just like in anaphase of mitosis.

Question 19

Telophase 2 - Meiosis 2

Answer 19

The chromatids assemble at the poles of telophase 2. The chromsomes uncoil and form chromatin again. The nuclear envelope reforms again. The nucleolus becomes visible. Cytokinesis results in division of the cells forming 4 daughter cells in total. They are also genetically different from each other and from the parent due to crossing over and independent assortment.

Question 20

Erythrocytes/ Red blood cells

Answer 20

flattened biconcave shape - to increase surface area to volume ratio. Allows them to transport oxygen around the body. They don’t have nuclei or many other organelles, which increases the space for haemoglobin. Flexible to squeece through narrow capillaries.

Question 21

Neutrophils

Answer 21

play a role in the immune system.
kmulti lobed nucleus makes it easier for them to squeeze through small gaps to get to the site of infection. The granular cytoplasm contains many lysosomes that contain enzymes used to attack pathogens.

Question 22

Sperm cells

Answer 22

are male gametes.
function: to deliver genetic information to the female gamete.
have many mitochondria for energy to swim. Tail to swim.
Acrosomes contains digestive enzymes which digest the protective layers around the egg, allowing the sperm to penetrate.

Question 23

Palisade cells

Answer 23

present in the mesophyll.
contain large amounts of chloroplasts to absorb large amounts of light for photosynthesis.
Rectangular box shapes so they can be packed closer together.
thin cells wals - increasing rate of diffusion of carbon dioxide.
large vacuole - to maintain turgor pressure.
chloroplasts can move within the cytoplasm to absorb more light

Question 24

Root hair cells

Answer 24

present in the surfaces of the roots near the growing tips.
root hairs increase the surface area of the cell to maximise uptake of water and minerals from the soil.

Question 25

Pair of guard cells

Answer 25

on the surfaces of leaves, form small openings called stomata.
necessary for CO2 to enter for photosynthesis.
when guard cells lose water and become swollen as a result of osmotic forces, they change shape and the stoma closes to prevent further water loss from the plant. The cell wall of a guard cell is thicker on one side so the cell does not change shape symettrically.

Question 26

What are the 4 main types of tissues in animals

Answer 26

Nervous tissue - adapted to support the transmission of electrical impulses.
Epithelial tissue - adapted to cover the body surfaces - internal and external.
Muscle tissue - adapted to contract.
Connective tissue - adapted either to hold other tissue together or as a transport medium.

Question 27

Squamous epithelium

Answer 27

made of squamous epithelial cells and is sometimes known as pavement epithelium due to its flat appearance. It is very thin due to the squat of flat cells that make it up and also because it is only one cell thick. It is present when rapid diffusion across a surface is essential. It forms the lining of the lungs and allow rapid diffusion of oxygen into the blood.

Question 28

Ciliated epithelium

Answer 28

hair like structures called cilia on the surface that move in a rhythmic manner.
They line the trachea. Goblets are also present, releasing mucus to trap any unwanted particles present in the air, preventing bacteria from reaching the alveoli once in the lungs.

Question 29

Cartilage

Answer 29

a connective tissue found in the outer ear, nose and at the end of bones and between bones. It contains fibres of elastic and collagen. It is firm and flexible. Made of chondrocyte cells embedded in an extracellular matrix. prevents ends of bones rubbing together causing damage.
Many fish have whole skeletons made of cartilage, not bone.

Question 30

Muscle

Answer 30

a tissue that contracts to move bones. Different types of muscle fibres:
- skeletal muscle fibres - muscles attached to bones containing myofibrils
- connective tissue - thin white stips.

Question 31

The epidermis - plants

Answer 31

single layer of closely packed cells covering the surface of plants. usually covered by a waxy, waterproof cuticle to reduce the loss of water. contains stomata.

Question 32

Xylem tissue - plants

Answer 32

a vascular tissue responsible for transport of water and minerals throughout plants. The tissue is made of elongated dead cells. The walls of the cells are strengthened with lignin providing structural support.

Question 33

Phloem tissue

Answer 33

a vascular tissue reponsible for transporting organic nutrients such as sucrose, from leaves and stems where it is made to where it is needed. Made of columns of sieve tube cells separated by perforated walls called sieve plates.

Question 34

Organ

Answer 34

a collection of tissues that are adapted to perform particular functions in an organism.

Question 35

Organ system

Answer 35

large multicellular organisms have coordinated organ systems. Each is composed of a number of organs working together to carry out major functions in the body.

Question 36

Stem cells

Answer 36

begin as undifferentiated cells and unspecialised. The have the potential to differentiate and specialise. They undergo cell division again and again and are the source of cells for growth and repair. Once stem cells have become specialised they go to G0 phase so they stop dividing.

Question 37

Stem cell potency

Answer 37

Totipotent - can differentiate into any type of cell. A fertilised egg, or zygote and the 8 or 16 cells are totipotent.
Pluripotent - can form all tissue types but are not whole organisms. Present in early embryos and are the origin of the different types of tissue with in an organism.
Multipotent - can only from a range of cells within a certain type of tissue.

Question 38

Replacements of red and white blood cells

Answer 38

Due to the lack of a nucleus and organelles they only have a short lifespan of around 120 days. They therefore need to be replaced constantly. Stem cell colonies in the bone marrow produce approximagtely three billion erythrocytes per kilogram of body mass per day to keep up with the demand.
Neutrophilis live for only about 6 hours and they stem cells produce 1.6 billion per kg per hour. This increases during an infection.

Question 39

Sources of animal stem cells

Answer 39

Embryonic stem cells - they are present at the very early stage of and are totipotent.
Tissue ( adult) stem cells - these are present throughout life. They are found in bone marrow. They are multipotent although they can be aritificially triggered to become pluripotent. Stem cells can be harvested from the umbilical cords of newborn babies. The advantage of this source are the plentiful supply of umbilical cords and that invasive surgery is not needed.

Question 40

Source of plant stem cells

Answer 40

present in meristematic tissue in plants. is found wherever growth is occurring in plants for example at the tips of root and shoots.
is also located between phloem and xylem tissues called vascular cambium.
the cells in the vascular cambium differentiate into cells present in the xylem or phloem.

Question 41

uses of stem cells.

Answer 41

Heart disease- muscle tissue in the heart is damaged as a result of heart attack, normally irreparable- this has been tried experimentally with some success already.
Type 1 diabetes – with insulin dependent diabetes the body’s own immune system destroys the insulin producing cells in the pancreas; patients have to inject insulin for life – this has been tried experimentally with some success already.
Parkinson’s Disease – the symptoms are caused by the death of dopamine producing cells in the brain; drugs currently only delay the progress of the disease.
Alzheimer’s disease – brain cells are destroyed as a result of the build-up of abnormal proteins; drugs currently only alleviate the symptoms.
macular degeneration – this condition is responsible for causing blindness in the elderly and diabetics; scientists are currently researching the use of stem cells in its treatment and early results of very encouraging.
birth defects – scientists have already successfully reverse previously untreatable birth defects in model organisms such as mice.
spinal injuries – scientists have restored some movement to the Hind limbs of rats with damaged spinal cords using stem cell implants.