Cell Reproduction

Question 1

Mitosis

Answer 1

• DNA is found in the nucleus of every cell in the body.
• The DNA in the nucleus determines the type of proteins that the cell can make, which determine the structure of the whole body.
• Enzymes are also proteins so DNA determines the type of enzymes the body makes and therefore the chemical reactions within the body.
• It is vital that when a cell reproduces, each new cell gets exactly the same DNA as the parent cell.
• Mitosis is a process of nuclear division that produces two daughter cells from one parent cell that have identical sets of genetic material:
- Daughter cells will have the same number of chromosomes (46) as the parent cell.
- This is referred to as the DIPLOID number or 2n chromosome number.
• Purposes of cell division:
- Method of growth
- Replacement of damaged, diseased or dead cells
- Each new cells must contain the same genetic information as the parent cell.
• Biologists describe Mitosis is a number of stages:
- Interphase
- Prophase
- Metaphase
- Anaphase
- Telophase

Question 2

Structure of DNA (Mitosis)

Answer 2

• All of the deoxyribonucleic acid (DNA) molecules in each nucleus is estimated to total 2-3 metres long. Their width is two-millionths of a millimetre.
• In each nucleus there are 46 molecules/strands of DNA
• The DNA strands are bound to proteins called HISTONES and wrapped around them so they can fit in the small space.
• In a cell that is not dividing the DNA coiled around the histones forms a tangled network called CHROMATIN.
• Once the DNA has replicated in mitosis, the coiled chromatin condenses even further (‘super-coils’) into CHROMOSOMES
• Chromosomes are commonly referred to as a ‘X’ structure.
• However, chromosomes only take this form when the strand of chromatin has replicated, super-coiled and joined with its pair.
• A single ‘super-coiled’ ‘half X’ before it pairs up can also be called a CHROMOSOME (or a chromatid)
• After a strand of chromatin has replicated, super-coiled and joined up with its pair, together they are referred to as SISTER CHROMATIDS
• A (‘X’ shaped) chromosome is made up of the two chromatids which are joined in the centre by a CENTROMERE

Question 3

The Cell Cycle (Mitosis)

Answer 3

• The CELL CYCLE – events that take place from one cell division and the next
• Mitosis refers to the division of the nucleus and is only one part of the cell cycle.
• 2 main stages:
• Interphase:
- G1 – Normal functions, protein synthesis, cell growth and duplication of cell organelles.
- S – DNA replication
- G2 – growth and preparation for cell division
• Mitosis – prophase, metaphase, anaphase and telophase
- Cell divides into 2 daughter cells
• After cell division some cells may continue the next cycle G1 phase while other cells leave the cell cycle and stop dividing for days, years or even for life.
- These cells are in G0 phase

Question 4

Interphase (Mitosis Stages)

Answer 4

• INTERPHASE – is the period between nuclear divisions
• During this time, the cell goes through G1, S and G2 phases of the cell cycle.
• During S phase the DNA molecules make exact copies of themselves in the nucleus.
- Called DNA REPLICATION
- At this point the DNA molecules take the form of thread-like chromatin
• Also during this phase the cell grows and produces duplicates of all the cell organelles

Question 5

Prophase (Mitosis Stages)

Answer 5

• PROPHASE – is considered the first stage of mitosis
• During early prophase:
- Two pairs of CENTRIOLES become visible
+ Centrioles move to the opposite ends of the cell (called POLES)
+ Microtubules begin to radiate from the centrioles
- Nucleolus disappears and nuclear membrane breaks down
- Duplicated threads of chromatin DNA condense (super-coil) and become visible as chromosomes
• By late prophase:
- Centrioles have reached opposite poles of cell
- Microtubules have joined to form a framework of fibres called a SPINDLE
- Nuclear membrane has completely broken down
- Chromatid pairs (X chromosomes) migrate towards centre of the cell

Question 6

Metaphase (Mitosis Stages)

Answer 6

• METAPHASE – second stage of mitosis
• Centromere of chromosomes attach to spindle fibres
• Pairs line up along the EQUATOR (the middle) of the cell

Question 7

Anaphase (Mitosis Stages)

Answer 7

• ANAPHASE – third phase of mitosis
• Centromere divides to separate the sister chromatids
• Individual chromatids (‘half X’ chromosomes) begin to move to opposite poles of the cell, pulled by the retracting spindle.
• Note: even though the two chromatids are now independent of each other they are still called chromosomes.

Question 8

Telophase (Mitosis Stages)

Answer 8

• TELOPHASE – fourth and last phase of mitosis
• Two sets of diploid (46 or 2n) single-chromatid (‘half X’) chromosomes form tight groups at each pole of the cell
• Nuclear membranes form around each group and nucleolus appears in each new nucleus.
• Chromosomes uncoil and become chromatin threads again
• Spindle fibres disappear and centrioles divide

Question 9

Cytokinesis (Mitosis Stages)

Answer 9

• CYTOKINESIS – is the process of dividing the cytoplasm to form two cells.
• Telophase is the last phase of mitosis, however while the events of telophase are occurring, cytokinesis begins.
- Cytokinesis is not a phase of mitosis but occurs at the end of telophase in the Mitotic phase of the Cell Cycle.
- Mitosis is commonly referred to as cell division, however it technically refers to just the division of the nucleus.
• The cell membrane moves inwards creating a FURROW between the two nuclei in the cytoplasm
• The furrow deepens until it cuts cytoplasm into two parts
• The cell membrane reforms around each new cell
• Both cells now consist of their own nucleus with identical number and type of chromosomes as the parent cell
- Therefore two daughter cells have been produced ¡ These daughter cells now enter Interphase

Question 10

Stem Cells

Answer 10

• Stem cells are undifferentiated cells, capable of repeated cell division.
• This means they are unspecialised, and can undergo the process of mitosis
• There are a few levels of stem cells:
- Embryonic stem cells are pluripotent and can differentiate into any human body cell
- Adult stem cells are multipotent and can only develop into cells of the same tissue type. E.g., A haemopoetic stem cells can become any type of blood cell

Question 11

Cancer

Answer 11

• All cancers are different, but they result in abnormal and uncontrolled division of cells
• A tumour is a mass of tissue (cells)
• They can be either malignant: spread to other parts of the body
• Or benign: a growth that does not spread
• The control of cellular replication (mitosis) is from the DNA
• Mutations (changes) to the DNA can cause many cancers
• Prevention:
- Education:
• Sunsmart: reducing UV exposure
• Stop smoking ads etc.
- Legislation:
• Laws about smoking in public and children
• Standards are X ray exposure and safety equipment
• Early Detection:
- Cervical Cancer: papsmears
- Breast Cancer: mammograms
- Bowel Cancer: Blood test and colonoscopy
- Prostate Cancer: rectal examination, blood test, biopsy
• Carcinogens are substances that can cause damage to the Dan and are associated with cancers:
- UV light (sunlight can cause skin cancer)
- X rays
- Radiation (uranium can cause leukemia)
- Viruses (HPV can cause cervical cancer)
- Other chemicals (alcohol, asbestos, tar, solvents etc.)

Question 12

Chromosome Numbers (Meiosis)

Answer 12

• All human SOMATIC (body) cells have 46 chromosomes – this is referred to as the DIPLOID chromosome number (2n)
• The sex cells, or GAMETES, have 23 chromosomes – this is referred to as the HAPLOID chromosome number (n)

Question 13

Meiosis

Answer 13

• Meiosis is a special process of nuclear division which results in the production of sperm or ova
• Meiosis involves two divisions and results in four daughter cells
• Each daughter cell contains only half the original number of chromosomes, the haploid number (23 or n).
• Meiosis avoids doubling the amount of chromosomes in an offspring with each generation of fertilisation
- A zygote should have 23 chromosomes from each parent which equals the correct amount – 46 chromosomes.
• Meiosis takes place in the sex organs – the testes for sperm and the ovaries for ovum.
• This process of gamete formation is called GAMETOGENESIS
• Production of sperm is called SPERMATOGENESIS
• Production of ovum is called OOGENESIS
• Meiosis involves 2 nuclear divisions, and goes through the same phases as mitosis, only twice.

• First Meiotic Division:
- Interphase
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I

• Second Meiotic Division:
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II

Question 14

Interphase

Answer 14

• Prior to meiosis the DNA is replicated as it is in mitosis and the cell grows.

Question 15

Meiosis I

Answer 15

• Prophase I:
- Already replicated chromatin threads condense to form (‘X’ shaped) chromosomes (pairs of chromatids joined by centromere)
- Chromosomes gradually move to equator and find their HOMOLOGOUS PAIR
- Nucleolus and nuclear membrane disappear
- Spindle forms
- Crossing over occurs

• Metaphase I:
- Chromosomes line up at equator, attached to spindle fibres by their centromeres.
- However, this time each chromosome is lines up side by side with it’s HOMOLOGOUS PAIR.

• Anaphase I:
- Spindle fibres contract
- During this stage the centromeres do not divide and pairs of chromatids remain intact.
- Pairs of homologous (‘X’ shaped) chromosomes move apart (still attached by
centromere), with one member of each pair (consisting of 2 chromatids) moving to each pole.
- Each pole has one complete set of 23 chromosomes (46 chromatids).

• Telophase I:
- Chromosomes uncoil into chromatin, nuclear membrane forms, spindle disappears.
- Cytoplasm divides but no cytokinesis.

Question 16

Meiosis - Second Division

Answer 16

• The purpose of the second meiotic division is to separate the pairs of chromatids so that each gamete has only 23 chromatids (‘half X’ chromosomes).
• No interphase II – DNA does NOT replicate again.

Question 17

Meiosis II

Answer 17

• Prophase II:
- New spindle forms at right angles to the original spindle direction
- Chromosomes move towards equator

• Metaphase II:
- Chromosomes line up at equator

• Anaphase II:
- Centromeres divide so that each chromatid is now a separate chromosome
- New chromosomes move to opposite poles

• Telophase II:
- Nuclear membrane forms and cytoplasm starts to divide
- Cytokinesis occurs producing 4 daughter cells with half the original number of chromosomes.

Question 18

Crossing Over

Answer 18

• During Prophase I matching regions on homologous chromosomes may break and then reconnect to the other chromosome.
- This is called CROSSING OVER
- The point where two chromatids cross is called a CHIASMA
• This results in new combinations of genes along the chromosome (so that the chromosomes passed onto the offspring are not identical to those of the parents) and creates GENETIC VARIATION between gametes.
- The result of having a new combination of alleles is termed RECOMBINATION.

Question 19

Stem Cells

Answer 19

Unspecialised cells that are capable of dividing by mitosis for indefinite periods (known as PROLIFERATION) and cell differentiation (giving rise to specialised cells).

Question 20

Cell Differentiation

Answer 20

• Where the stem cells become specialised and develop the characteristics and functions of particular types of cells.
• As the stem cells proliferate, different genes become activated by internal signals from other genes, or by external signals (such as chemicals or certain molecules), which leads to differentiation.
• Because of their ability to differentiate into any type of cell, stem cells could potentially provide an unlimited source of adult cells.

Question 21

Three Types of Stem Cells

Answer 21

TOTIPOTENT stem cells:
- After the egg is fertilised and becomes a zygote, it is totipotent.
- Totipotent stem cells have the potential to specialise into any type of cell necessary for embryonic development, including the embryo and all the embryonic membranes (ie. placenta).
- The totipotent stem cell undergoes several rounds of division, and after about 6 days specialises to form the blastocyst.

PLURIPOTENT stem cells:
- The inner cell mass of the blastocyst is made up of pluripotent stem cells.
- Pluripotent stem cells can give rise to many, but not all, cell types necessary for foetal development.
- They can give rise to foetal tissues, but not placental tissues.

MULTIPOTENT stem cells:
- Each pluripotent stem cell undergoes further specialisation to become multipotent stem cells.
- Multipotent stem cells can give rise to cells that have a particular function.
- There are many types of multipotent stem cells, such as muscle stem cells which would give rise to all the types of muscle cells (skeletal, smooth and cardiac).
- Multipotent stem cells are also found in adults.
• Eg. Blood stem cells are found in red bone marrow which constantly replenishes the supply of blood cells.

• Stems cells have the potential to be used to replace damaged or degenerated tissues.

Question 22

Sources of stem cells

Answer 22

• Umbilical cord and placental stem cells – once baby is born these can be extracted from discarded tissue.
• Embryonic stem cells – cultured from (unused) frozen embryos (at blastocyst stage) obtained from IVF clinics.
• Adult stem cells – multipotent stem cells from many kinds of tissue. Advantage is patients own cells could be used for treatments.

Question 23

Spermatogenesis

Answer 23

• SPERMATOGENESIS – is the production of SPERMATOZOA (Sperm)
• It occurs in the seminiferous tubules of the testes
• The process takes about 72 days and occurs continuously after puberty.
• Seminiferous tubules are lined with SPERMATOGONIA (singular = spermatogonium) which are immature ‘mother’ sperm cells that contain the diploid number (46) of chromosomes (46 single chromatin threads).
• At puberty spermatogonia begin dividing by mitosis to provide a constant source of new cells for the production of spermatozoa.
• Some spermatogonia are pushed away from the membrane of the tubules and in towards the centre (the lumen) where they undergo a period of growth and maturation to become PRIMARY SPERMATOCYTES.
• During this ‘interphase’ the DNA replicates to produce 46 (‘X’ shaped) chromosomes
• Primary spermatocytes undergo the first stage of meiosis to become SECONDARY SPERMATOCYTES which are haploid (23 ‘X’ shaped chromosomes).
• The secondary spermatocytes undergo the second meiotic division which produces four SPERMATIDS.
• Spermatids are haploid (23 ‘half X’ chromosomes)
- Or 23 single chromatin threads  half the amount of a normal somatic cell
• Spermatids then lastly mature into SPERMATOZOA
- During this time most of the cytoplasm is lost and a tail containing contractile material forms.
• Spermatozoa are nourished by SERTOLI CELLS
- Which also act as phagocytes by engulfing the cytoplasm which is lost.

Question 24

Spermatozoa Structure

Answer 24

• Sperm are 0.06 mm long and they are made of a head, middle piece and tail.
- Head – contains nuclear material and fluid filed vesicle at tip. Fluid contains enzymes which are capable of breaking down layer of cells surrounding ovum so that fertilisation can take place.
- Middle – contains mitochondria where respiration takes place to provide the sperm with energy for movement. Thin layer of cytoplasm.
- Tail – capable of violent swimming motions to propel the sperm forward.
• Because of the little cytoplasm, sperm are only viable for ~72 hours once they leave the testes. They receive their nourishment from the semen in which they are suspended.

Question 25

Oogenesis

Answer 25

• Oogenesis – is the production of OVA (singular = ovum)
• It occurs in the ovaries and takes approximately one month; the development of a mature ovum coincides with the monthly Ovarian cycle.
• Before a female baby is born, millions of egg ‘mother’ cells called Oogonia develop in the ovaries. Oogonia are diploid (46 single chromatin threads) and divide by mitosis)
• By birth, each ovary contains several thousand oogonia which have undergone a growth and maturation period to become Primary Oocytes
- During this time the oogonia goes through interphase where the DNA replicates and becomes 46 (‘X’ shaped) chromosomes
• The primary oocytes begin Prophase I but pause halfway through. They stay paused in this state until puberty.
• The primary oocytes are surrounded by a single layer of cells which form the Primary Follicle
• At puberty, the process of follicle growth and maturation begins
• As the follicle matures, the primary oocyte completes the first meiotic division to produce two haploid cells (with 23 ‘X’ shaped chromosomes).
• However, these two cells are not equal in size. They both receive half the chromosomes, but one receives nearly all the cytoplasm while the other receives very little cytoplasm.
- The larger is referred to as the secondary oocyte and goes onto become the ovum
- While the smaller is called the first polar body and eventually disintegrates.
• The secondary oocyte immediately commences the second meiotic division but pauses again at Metaphase II
• At this point ovulation occurs, where the follicle ruptures and the ovum and its polar body are expelled.
• The secondary oocyte enters the uterine tube
- If it is fertilised by a spermatozoa, then in completes meiosis.
• The second meiotic division of the secondary oocytes also produces two haploid cells of unequal size (with 23 ‘half X’ chromosomes)
- The larger one with more cytoplasm is called the Ootid and eventually matures into the Ovum
- The smaller one is the Second Polar Body
• The first polar body may also undergo a second meiotic division to produce two more polar bodies
- All the polar bodies disintegrate
• Oogenesis produces a single ovum from each primary oocyte, while spermatogenesis produces four spermatozoa from each primary spermatocyte.

Question 26

Cancer

Answer 26

• All cancers are different, but they result in abnormal and uncontrolled division of cells
• A tumour is a mass of tissue (cells)
• They can be either malignant: spread to other parts of the body
• Or benign: a growth that does not spread
• The control of cellular replication (mitosis) is from the DNA
• Mutations (changes) to the DNA can cause many cancers