1.6 Cell division Flashcards
Why do cells divide?
Growth - multicellular organisms increase their size by increasing their number of cells via mitosis
Asexual Reproduction - certain eukaryotic organims my reproduce asexually via mitosis (e.g. vegetative reproduction)
Tissue Repair - replacing dead/damaged cells
Embryonic Development - a fertilized egg (zygote) will undergo mitosis and differentiation in order to develop into an embryo
If an organisms grows larger, it needs
more cells, AND cells need to have a copy of the organism’s DNA
Cell division allows for __________ of the organisms by producing more cells, and it also allows for more _____________ to occur
growth, cell differentiation
Mitosis occurs most frequently in
developing embryos
New cells are also needed on a regular basis to ___________ dead, damaged, or infected cells
replace
Cell division (specifically through mitosis) is also used in
asexual reproduction (self-replication)
Cell cycle
series of events through which cells pass to divide and create two identical daughter cells
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Every eukaryote has genes on…
chromosomes - storage units of DNA in the nucleus
Each chromosome has a partner…
(homologous chromosomes) - one from each parent. Both copies are required for the cell to function.
Pair of chromosomes with the same gene sequence, loci, chromosomal length, and centromere location
Different species have different chromosome numbers:
Humans = 23 pairs (n = 23)
Diploid number (2n) = 46
Gametes
(sex cells - sperm and eggs) are haploid (n).
They have a half set of chromosomes, as they will pair up with the other half in fertilization.
centromere
the part of a chromosome that links sister chromatids
centrioles
organise spindle microtubules
Spindle microtubules
(also referred to as spindle fibres)
Sister chromatids
are duplicated chromosomes attached by a centromere
After anaphase when the sister chromatids separate they should then be referred to as daughter chromosomes
Why do chromosomes supercoil?
Human cells are on average 10𝝻m in diameter and the nucleus within each is less than 5𝝻m in diameter.
Human chromosomes are 15mm to 85mm (15,000𝝻m to 85,000𝝻m) in length
Chromosomes need to be stored compactly to fit within the nuclei of cells
This problem becomes more acute during mitosis when chromosomes need to be short and compact enough that they can be separated and moved to each end of the cell
How chromosomes supercoil?
Strain is places on a DNA helix by overwinding or underwinding of the helix
This causes the DNA molecule to coil back on itself becoming shorter and wider
Note: in eukaryotes, proteins called histones aid the process
Interphase
part of the cell cycle that does not involve division; majority of life cycle
G1 (Gap 1)
Metabolic reactions
Increase volume of cytoplasm
Organelles produced and replicated
Proteins synthesised
S (Synthesis)
DNA replicated
G2 (Gap 2)
Metabolic reactions
Increase the volume of the cytoplasm
Organelles produced and replicated
Proteins synthesised
G0 (Gap 0)
“Resting phase”
Temporary or permanent phase
Cells that do not divide
Carry out regular functions
Metabolic reactions
Prophase
DNA supercoils chromatin condenses and becomes sister chromatids, which are visible under a light microscope
The centrosomes move to opposite poles of the cell and spindle fibres begin to form between them
The nuclear membrane is broken down and disappears
Metaphase
Spindle fibres from each of the two centrosomes attach to the centromere of each pair of sister chromatids
Contraction of the microtubule spindle fibres cause the sister chromatids to line up along the centre of the cell.
Anaphase
Continued contraction of the microtubule spindle fibres cause the separation of the sister chromatids
The chromatids are now referred to as chromosomes
Chromosomes move to the opposite poles of the cell
Telophase
The chromosomes uncoil de-condense to chromatin (and are no longer visible under a light microscope).
Chromosomes arrive at the poles.
Microtubule spindle fibers disappear
New nuclear membranes reform around each set of chromosomes
Mitosis
division of the nucleus
Cytokinesis
division of the cytoplasm
Occurs concurrently with telophase
Different in plant and animal cells
Animal Cells
A ring of contractile proteins (microfilaments; actin and myosin) immediately inside the plasma membrane at the equator pulls the plasma membrane inward
The inward pull on the plasma membrane produces the characteristic cleavage furrow
When the cleavage furrow reaches the centre of the cell, it is pinched apart to form two daughter cells
Plant cells
During telophase, membrane-enclosed vesicles derived from the Golgi apparatus migrate to the centre of the cell
Vesicles fuse to form tubular structures → merge (with the addition of more vesicles) to form two layers of plasma membrane (i.e. the cell plate)
Cell plate continues to develop until it connects to the existing cell’s plasma membrane
Vesicles deposit, by exocytosis, pectins and other substances in the lumen between the daughter cells to form the middle lamella (“gluing” to cells together)
Both daughter cells secrete cellulose to form their their new adjoining cell walls
Mitotic index
ratio between number of cells in mitosis in a tissue and the total number of observed cells
mitotic index= # cells in mitosis/total # of cells
Cyclins
a family of proteins that control the progression of cells through the cell cycle
Cyclin in cell cycle
- Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches it threshold.
- Cyclins bind to enzymes called cyclin-dependent kinases
- These kinases then become active and attach phosphate groups to other proteins in the cell.
- The attachment of phosphate triggers the other proteins to become active and carry out tasks (specific to one of the phases of the cell cycle).
Normal cell cycle - role of cyclins
The timing and rate of cell division are important and controlled by proteins and enzymes
The cell cycle in eukaryotic cells is driven by a combination of two substances
- Cyclins are proteins that bind to enzymes called cyclin dependent kinases
- Different cyclin/CDK combinations control different activities during different stages of the cell cycle
Progression through parts of the cell cycle are affected in…
various ways by specific cyclins
different type of cyclins throughout cell cycle
Cyclin D
Triggers cells to move from G0 to G1 and from G1 into S phase
Cyclin E
prepares the cell for DNA replication in S phase.
Cyclin A
activates DNA replication inside the nucleus in S phase.
Cyclin B
promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis.
Tumours
abnormal growth of tissue that develop at any stage of the life in any part of the body.
A cancer
a malignant tumour and is names after the part of the body where the cancer (primary tumour) first develops.
A mutation
a change in an organism’s genetic code. A mutation/change in the base sequence of a certain genes can result in cancer.
Mutagens
agents that cause gene mutations. Not all mutations result in cancers, but anything that causes a mutation has the potential to cause a cancer.
Mutagens can be:
chemicals that cause mutations are referred to as carcinogens
high energy radiation such as X-rays
short-wave ultraviolet light
Some viruses
If a mutation occurs in an
oncogenes it can become cancerous. In normal cells oncogenes control of the cell cycle and cell division.
Formation of a tumor
- mutation in a oncogene
- malfunction in the control of the cell cycle
- uncontrolled cell division
- tumour formation
Several mutations must occur in
the same cell for it to become a tumour causing cell. The probability of this happening in a single cell is extremely small.
Factors (other than exposure to mutagens) that increase the probability of tumour development include:
The vast number of cells in a human body – the greater the number of cells the greater the chance of a mutation.
The longer a life span the greater the chance of a mutation.
