Cell Reproduction Flashcards
Mitosis
• 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
Structure of DNA (Mitosis)
• 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
The Cell Cycle (Mitosis)
• 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
Interphase (Mitosis Stages)
• 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
Prophase (Mitosis Stages)
• 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
Metaphase (Mitosis Stages)
• METAPHASE – second stage of mitosis
• Centromere of chromosomes attach to spindle fibres
• Pairs line up along the EQUATOR (the middle) of the cell
Anaphase (Mitosis Stages)
• 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.
Telophase (Mitosis Stages)
• 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
Cytokinesis (Mitosis Stages)
• 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
Stem Cells
• 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
Cancer
• 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.)
Chromosome Numbers (Meiosis)
• 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)
Meiosis
• 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
Interphase
• Prior to meiosis the DNA is replicated as it is in mitosis and the cell grows.
Meiosis I
• 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.
