Cell Division, Cell Diversity and Cellular Organisation

Question 1

How do different types of cells divide

Answer 1

Eukaryotic cells enter the cell cycle and divide by mitosis or meiosis
Prokaryotic cells replicate by binary fission
Viruses do not undergo cell division as they are non living

Question 2

What are the three key stages of the cell cycle

Answer 2

Interphase (G1, S, G2)
Nuclear division (mitosis or meiosis)
Cytokinesis

Question 3

Describe interphase

Answer 3

Longest stage in the cell cycle
G1- Where protein synthesis occurs to make the proteins involved in synthesising organelles
The organelles replicate
Cell is checked that it is the correct size, has the correct nutrients, growth factors and no damaged DNA (if doesn’t pass these checks, replication will not continue)
S phase - Where DNA is replicated
G2- Cell continues to grow , energy stores increase and the newly replicated DNA is checked for copying errors

Question 4

Describe mitosis

Answer 4

Mitosis creates two identical diploid cells and is used for growth, tissue repair and asexual reproduction in plants, animals and fungi
Four key stages
Prophase
Metaphase
Anaphase
Telophase

Question 5

What happens in prophase

Answer 5

Chromosomes condense and become visible. In animal cells the centrioles separate and move to opposite poles of the cell
Centrioles create spindle fibres which are released from both poles which create a spindle apparatus, this will attach to the centromere and the chromatids on the chromosome in later stages
Plants have a spindle apparatus but lack centrioles

Question 6

What happens during metaphase

Answer 6

The chromosomes align along the equator of the cell
The spindle fibres are released from the centrioles and attach to the centromere and chromatids
The spindle assembly checkpoint occurs in this stage. There is a check to ensure every chromosome has attached to a spindle fibre before mitosis can proceed into anaphase

Question 7

What happens during anaphase

Answer 7

Spindle fibres start to shorten and move towards centrioles and pull the centromere and chromatids they are bound to towards opposite poles
This causes the centromere to divide into two and the individual chromatids are pulled to each opposite pole
This stage requires energy in the form of ATP which is provided by respiration in the mitochondria

Question 8

What happens during telophase

Answer 8

The chromosomes are now at each pole of the cell and become longer and thinner again
The spindle fibres disintegrate and the nuclear membrane reforms

Question 9

Describe cytokinesis

Answer 9

The cytoplasm splits in and forms two genetically identical cells
In animals, a cleavage furrow forms in the middle of the cell and the cytoskeleton causes the cell membrane to draw inwards until the cell is split in two
In plant cells, the cell membrane splits into two new cells due to the fusing of vesicles from the golgi apparatus, the cell wall forms new sections around the membrane to complete the division into two cells

Question 10

How to observe mitosis

Answer 10

Thin slice of the root tip is places on a microscope slide and broken down with a mounted needle
A stain is added to make the chromosomes visible and the cover slip is pushed down to squash the top to achieve a single layer of cells so that light can pass through

Question 11

Formula for mitotic index

Answer 11

number of cells in mitosis
X100
total number of cells

Question 12

Describe meiosis

Answer 12

Two nuclear divisions (Meiosis I and Meiosis II both include PMATC) in this process which result in four genetically different haploid daughter cells

Question 13

What is the difference between haploid and diploid

Answer 13

Haploid (n) = one copy of each chromosome
Diploid (2n) = two copies of each chromosome

Question 14

How are genetic differences introduced in meiosis

Answer 14

Independent assortment
Crossing over

Question 15

Describe crossing over

Answer 15

during prophase I, the homologous chromosomes pair to form bivalents
Crossing over genetic material can occur between the non sister chromatids of bivalents to form a chiasma (where crossing over occurs)
Breaks can occur in the genetic material where the chromatids cross over and parts of chromatids are then exchanged between the homologous pairs
This results in new combinations of alleles in the resulting gamete

Question 16

Describe independent assortment

Answer 16

During metaphase I, the homologous pairs of chromosomes line up opposite each other on either side of the equator
It is random on which side of the equator that the paternal and maternal chromosomes of each pair aligns
As there are 23 different homologous pairs, there are 2^23 different ways these pairs could assort themselves
As a result, each gamete receives different combinations of the maternal and paternal chromosomes

Question 17

How to find out the number of possible combinations for organism

Answer 17

2^n
where n is the number of homologous pairs

Question 18

How are multicellular organisms organised?

Answer 18

Cells
Tissue
Organ
Organ system
Entire Organism

Question 19

Structure and function of erythrocytes

Answer 19

Biconcave shape to increase surface area for diffusion and to increase the cell flexibility for it to fit through narrow capillaries. These cells have no nucleus so there is more space to hold haemoglobin to increase the transport of oxygen

Question 20

Structure and function of neutrophils

Answer 20

Has a lobed nucleus and granular cytoplasm, these cells are flexible to enable them to surround pathogens and engulf them. They contain lysosomes filled with the hydrolytic enzyme lysozyme. Neutrophils are made from stem cells in the bone marrow

Question 21

Structure and function of sperm cells

Answer 21

Flagellum contains many mitochondria to release energy for locomotion to enable the sperm cell to move towards the egg cell, the acrosome contains digestive enzymes to digest the wall of the egg cell so sperm can penetrate and fertilise the egg cell

Question 22

Structure and function of palisade cells

Answer 22

Located in the mesophyll tissue layer of leaves. They are rectangular, tightly packed cells that contain many chloroplasts to absorb and maximise light energy for photosynthesis. They have thin cell walls to reduce the diffusion distance of carbon dioxide

Question 23

Structure and function of guard cells

Answer 23

These pair of cells have flexible walls, more so on one side which results in the cells bending when turgid to open stomata and closing when flaccid and this helps control water loss by transpiration

Question 24

Structure of squamous epithelial cells

Answer 24

Usually only single layer of flat cells is in contact with the basement membrane of the epithelium. This provides a short diffusion distance e.g. these form the lining of the lungs

Question 25

Structure of ciliated epithelial cells

Answer 25

These cells have hair like projections that sway to move substances such as mucus out of the lungs or an egg in the oviduct. Goblet cells are also located within the epithelium and these cells release mucus to trap molecules such as trapping dust in the trachea

Question 26

Structure of cartilage

Answer 26

A connective tissue that is firm and flexible, located in the outer ear, nose and the ends of bones. It provides structural support and it prevents the bones from rubbing together which would damage them. It is made up of elastin and collagen fibres and chondrocyte cells within an extracellular matrix

Question 27

The structure of muscle

Answer 27

Composed of tissues that can contract and relax to create movement.
Skeletal muscles cause the skeleton to move and are made up of myofibrils containing the proteins actin and myosin.
Smooth muscle is located within organs.
Cardiac muscle is within the heart
Muscles have multiple fibres connecting with connective tissues in between

Question 28

Structure of Xylem

Answer 28

The cells that make up the part of the vascular bundle in plants responsible for transporting water and mineral ions.
The tissue is made up in elongated, hollow dead cells with lignin in the walls to strengthen and waterproof the walls. Xylem tissues are made from stem cells in the meristem

Question 29

Structure of phloem

Answer 29

The cells that make up the part of the vascular bundle responsible for transporting organic substances made in photosynthesis. It is made of sieve tube element cells which have perforated end walls and are lacking most organelles to make transport of sugars easier and companion cells which contain organelles to provide resources for the sieve tube elements. Phloem sieve tubes are made from the stem cells in the meristem

Question 30

What are stem cells

Answer 30

Undifferentiated cells that can self renew (continually divide) and become specialised

Question 31

What are totipotent cells

Answer 31

Cells that can divide and produce any type of body cell.
During development, totipotent cells translate only part of their DNA, resulting in cell specialisation. Totipotent cells occur only for a limited time in early mammalian embryos

Question 32

What are pluripotent cells

Answer 32

Cells are found in embryos and can become almost any type of cell
They are used in research, using them to treat human disorders however sometimes the treatment doesn’t work or they create tumours

Question 33

What are multipotent cells

Answer 33

cells can differentiate into a limited number of cells
found in mature mammals and can divide to form a limited number of different cell types

Question 34

What are unipotent cells

Answer 34

Can only differentiate into one type of cell
Cells found in mature mammals and can divide to form a limited number of different cell types

Question 35

Uses of stem cells

Answer 35

Research and medicine
-repairing damaged tissues
-treatment of neurological conditions such as alzheimer’s and parkinson’s
-research into developmental biology