B6 - Cell Division Flashcards
What is the cell cycle
The cell cycle is the process that all body cells use to grow and divide
It starts with a cell that has already been produced by cell division and ends with this cell dividing to produce 2 genetically identical daughter cells.
Two ways eukaryotic cells divide
Mitosis
Meiosis
Mitosis
Division into two daughter cells that are genetically identical to each other and to the parent cell
Meiosis
Division into four unique daughter cells with half the chromosomes of the parent cell.
Haploid cell
One copy of each chromosome
Diploid cell
Two copies of each chromosome
Importance of the cell cycle
- produces genetically identical daughter cells
- growth of tissue
- replacement of work out / damaged cells
- repair of body tissues
- asexual reproduction
Stages of the cell cycle
Interphase (preparation phase)
- G1 (growth phase 1)
- S (DNA synthesis)
- G2 phases (growth phase 2)
M phase (dividing phase)
- mitosis
- cytokinesis
G1 stage
First growth phase of cell, synthesis of proteins, organelles replicate e.g mitochondria, ribosomes etc. cell increases in size.
End of G1 checkpoint.
S stage
Synthesis phase. Replication of each chromosome in the nucleus. They are now called sister chromatids (joined at the centromere)
G2 stage
Second growth phase, cell continues to grow in size. Duplicated DNA is checked for errors. Energy stores (i.e ATP molecules) are increased
End of G2 checkpoint
Cell cycle checkpoints
G1 checkpoint
G2 checkpoint
Spindle assembly / metaphase checkpoint
G1 checkpoint
Checks
- cell is the correct size
- nutrients / chemicals are present
- growth factors present
- any damage to DNA
G2 checkpoint
Checks for:
- cell is the correct size
- DNA has been replicated without damage
G0 phase (resting state)
Phase where the cell leaves the cell cycle either temporarily or permanently. Because DNA may be damaged.
Spindle assembly checkpoint / metaphase checkpoint
Checks for chromosomes attachment to the spindle
What happens if DNA is not checked
- mutations in the DNA sequence
- faulty DNA produced
- error in copying daughter cells
- daughter cells will not receive identical genetic information
- proteins not made or do not function properly
Stages of the Mitotic phase
Mitosis
Cytokinesis
Homologous chromosomes
- a pair of chromosomes - one maternal and one paternal
- the chromosomes carry the same gene but may carry different forms of the genes. An alternate form of the same gene is called an allele
- e.g gene is eye colour, alleles could be brown, green, blue etc
Sister chromatids
A sister chromatid refers to the identical copies formed by the DNA replication of a chromosome, with both copies joined together by a common centromere.
Difference between sister chromatids and homologous chromosomes
A homologous pair of chromosomes contain one maternal and one paternal chromatid. They carry the same genes although may have different alleles of these genes, position (loci) and size are the same. Members of a homologous pair pair up during meiosis.
Chromosome structure
- only visible during cell division
- each chromosome consists of two chromatids joined somewhere along its length at the centromere
- genetic information (genes / alleles) Carried on each chromatid is identical.
Mitosis is made up of 4 steps
PMAT
- prophase
- metaphase
- anaphase
- telophase
Prophase
- Chromosomes condense and thicken (becoming visible)
- consists of sister chromatids joined at the centromere
- two centrioles migrate to opposite poles of the cell (in animal and some plant cells)
- spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell.
- nuclear envelope disappears
Metaphase
- brief phase
- individual sister chromatids (chromosomes) are moved by the spindle fibres to align at the metaphase plate / equator at the centre of the cell
- sister chromatids are attached to the spindle fibres by the centromere
Anaphase
- centromeres holding the pairs of chromatids in each chromosome divide.
- sister chromatids separate
- spindle contracts (fibres shorten)
- each chromatid is pulled by their centromere to opposite poles of the cell.
Telophase
- chromatids have reached opposite poles of the cell. They uncoil and become long and thin again.
- they are now called chromosomes
- spindle fibres disappear
- nuclear envelope reforms and enclose around the chromosomes at each pole
Cytokinesis
- this is the phase where the cell surface membrane and cytoplasm divides.
- in animal cells a ‘cleavage furrow’ forms
- in plant cells ‘cell plate’ forms.
- this results in 2 genetically identical daughter cells being formed.
Why does meiosis need to have double the number of steps as mitosis
- to halve chromosome number
- to separate homologous pairs and sister chromatids
- because chromosomes are two chromatids at the start.
What feature of the DNA molecule is changed as a result of mutation
The order of the bases
Possible effects that mutation could have on the structure and function of a protein
- different primary / secondary / tertiary structure
- protein shorter due to deletion or long due to insertion / duplication
- protein unchanged due to silent mutation
- function is lost / worse / better
Difference between cells produced during mitosis and meiosis
- in meiosis there are 4 unique daughter cells produced, however in mitosis there are only 2 identical daughter cells
- in meiosis there each daughter cell has half the number of chromosomes as the parent cell. However in mitosis each daughter cell has the same number of chromosomes as the parent cell.
What is a stem cell
- A stem cell is a cell that has not yet become a specialised cell (undifferentiated)
- can divide many times by mitosis
- each new cell has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle by differentiation
What is a unipotent stem cell
Can not differentiate, but are capable of self renewal (progenitor cells, muscle stem cells)
Multipotent stem cells
Can differentiate into a number of closely related cell types within a certain type of tissue (haematopeotic stem cells in bone marrow give rise to different types of blood cells)
Pluripotent stem cells
Pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells.
Totipotent
Totipotent stem cells that are able to differentiate into any type of cell found in body including into extra embryonic cells such as those in the placenta. These cells are found in the embryo at an early stage called the blastocyst.
Embryonic stem cells
- found in embryos and can develop into almost every cell type under the right conditions in a lab
- present at a very early stage of embryo development
- embryonic stem cells are Totipotent before 7 days and pluripotent after the blastocyst forms.
Adult tissue stem cells
- adult stem cells are stem cells found in adult tissues (such as bone marrow, brain, muscle, liver stem cells). These cells can only differentiate into the same type of cell as the tissue they came from.
- present throughout life from birth
- found in specific areas (bone marrow is multipotent)
- could be artificially triggered to become pluripotent.
- can be harvested from umbilical cords of newborn babies
- in animals, adult stem cells are used to replace damaged cells.
Stem cells found in bone marrow
- are multipotent adult stem cells
- this means they can only differentiate into erythrocytes (red blood cells), monocytes and neutrophils and lymphocytes.
What are erythrocytes
They are red blood cells, the main function of which is the transport of oxygen around the body (and also the transport of carbon dioxide)
Why can red blood cells not divide
They lack a nucleus, so new erythrocytes are constantly being formed from bone marrow stem cells in order to maintain the red blood cell count in the blood. This process is called erythropoiesis
Adaptations of an erythrocyte
- changing the cell into a biconcave shape, which has a larger surface area. Allowing for more oxygen to be absorbed
- the building up of haemoglobin in the cytoplasm. It releases this when oxygen concentrations decrease below a certain level
- the ejection of the nucleus and other organelles to create more room in the cytoplasm for haemoglobin.
- an elastic membrane allowing erythrocytes to change shape and squeeze through narrow capillaries.
What are neutrophils?
- neutrophils are the first white blood cells to arrive at an infection site on or in the body.
- they exit the blood through the tiny gaps in capillary walls and collect around foreign bodies (pathogens)
- they then destroy these by engulfing them (phagocytosis) and digesting them using their hydrolysis enzymes.
Adaptations of neutrophils
- a flexible shape and flexible nuclear membrane allowing neutrophils to fit between capillary wall cells and to form pseudopodia (the extensions that engulf foreign bodies during phagocytosis)
- containing many lysosomes that contain digestive enzymes that destroy invading cells.
Where are plant stem cells founded
Meristems
Plant stem cells
Plant stem cells are Totipotent throughout their life meaning they can differentiate into any type of plant cell.
What is the cambium
- the cambium lies between the xylem and phloem. It is a layer of meristem cells. They divide to produce new xylem and phloem.
How are the xylem and phloem formed
- formed from stem cells that are found in the cambium
How can stem cells be used to treat Alzheimer’s
With Alzheimer’s, nerve cells in the brain die in increasing numbers. This results in severe memory loss. Researchers are hoping to use stem cells to regrow healthy nerve cells in people with Alzheimer’s.
How can stem cells be used to treat Parkinson’s
Patients with Parkinson’s suffer from tremors that they can’t control. The disease causes the loss of a particular type of nerve cell found in the brain. These cells release a chemical called dopamine, which is needed to control movement. Transplanted stem cells may help to regenerate the dopamine-producing cells.
Meiosis
Meiosis is the process by which sex cells are made in the reproductive organs. It involves the reduction division of a diploid germaline cell into four genetically distinct haploid nuclei.
Importance of meiosis
- takes place in sex organs
- gametes produced here
- important to have genetically different gametes
- this promotes genetic variation and allows for natural selection to take place.
Stages of meiosis
Meiosis I
Meiosis II
Meiosis I
Introduces genetic diversity by randomly dividing a cell’s genes in two. It results in two haploid cells.
Meiosis II
Is similar to mitosis. It splits each chromosome into its two chromatids and places one in each daughter cell. It results in four haploid gametes.
How does genetic variation occur during meiosis
- during meiosis I, homologous pairs of chromosomes swap alleles. This is called ‘crossing over’.
- the chromosomes from each pair are randomly allotted to the daughter cells by independant assortment
Advantage of meiotic division
It promotes genetic variation
Prophase 1
- chromosomes condense getting shorter and fatter
- chromosomes arrange themselves into homologous pairs - forming bivalents.
- centrioles move to opposite ends of the cell forming the spindle fibres
- nuclear envelope dissolves
- crossing over of genetic information occurs
Metaphase 1
- the bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres.
- the maternal and paternal chromosomes in each pair position themselves independently of the others; this is independent assortment.
- this means that the proportion of paternal or maternal chromosome that end up on each side of the equator is due to chance
Independent assortment
- when homologous pairs line up along the equator of the cell during metaphase 1 and get separated during anaphase 1 of meiosis 1 it is completely random which chromosome from each pair ends up in the daughter cell.
- this gives rise to new combinations of alleles
Anaphase 1
- spindle fibres contract and split the bivalent, homologous chromosomes pairs separate. There is no separation of the centromere.
- each homologous chromosomes moves to opposite sides of the cell.
- the result is that 23 chromosomes move to one pole, and 23 chromosomes move to the other pole.
Telophase 1
- chromosomes assemble at each pole and decondense
- nuclear envelope may reform
- cell undergoes cytokinesis to form two haploid daughter cells.
Crossing over
- happens in prophase 1
- the homologous chromosomes are held together at points called chiasmata
- crossing over of genetic material between non-sister chromatids can occur at these chiasmata.
- as a result of this exchange of genetic material between non-sister chromatids can occur at these chiasmata. This leads to new gene combinations forming on chromatids
Prophase 2
- chromosomes pair up and re-condense
- nuclear envelope breaks down again
- spindle fibres reform
- centrioles move to opposite poles of the cell.
Metaphase 2
- chromosomes are aligned on the equator by the spindle fibres
Anaphase 2
- centromere divides
- pairs of sister chromatids are separated
- each new daughter cell inherits one chromatids from each chromosome
- spindle fibres contract
Telophase 2
- chromatids uncoil and decondense
- spindle fibres break down
- nuclear envelopes reform
- the cell undergoes cytokinesis
