The Cell cycle & Cell division Flashcards
What are chromosomes made of? Describe their structure
DNA and a protein called histone
- the DNA molecule is a double helix, running the length of the chromosomes, with sections along its length called genes
When do chromosomes become visible?
- when chromatin condense prior to cell division, aftear each DNA molecule has replicated and made an exact copy of itself
What are sister chromatids?
Two copies of a chromosome
Chromatids are joined at a specialised region, what is this region called?
The cenromere
Different species have different numbers of chromosomes, normal human body cells have how many chromosomes?
46
23 pairs
Define haploid
what is the symbol?
The number of chromosomes in 1 complete set
The simple is ‘n’
Define homologous pairs?
Many organisms receive one complete set of chromosomes (which are identical in size and shape) from each parent and so chromosomes occur in matching pairs (they carry genes for the same characteristics)
define diploid what is the symbol
define triploid what is the symbol
Having 2 compete sets of chromosomes~ 2n
Having 3 complete sets of chromosomes~ 3n
Define ploidy level
Organisms with more than two sets of chromosomes are described as what?
The number of complete sets of of chromosomes in an organism
Polyploid
Why do we have two copies of every gene?
The chromosome sets from the two parents carry genes for the same characteristics so we have two copies of every gene
- They may be however be different versions of the same gene (alleles)
What is the regular patterns of evens that the cell undergoes called?
The cell cycle
Define mitosis?
A type of cell division in which the two daughter cells have the same number chromosomes and are genetically identical to each other and the parent cell
state and explain the 3 main stages in interphase
Interphase is the longest part of the cell cycle with much metabolic activity
- G1: the cell grows and its organelles replicate
- S: The DNA replicates (its quantity doubles)
- G2: proteins such as histones and enzymes are synthesised requring energy from ATP
Describe some of the events that occur in Prophase of Mitosis
Prophase is the longest of the 4 stages of mitosis, whe one of the following changes happen:
- The chromosomes condense. They coil, getting shorter and thicker and become visible as long thin threads. They become distinguishable as a pair of chromatids
- Centrioles are present in animal cells; the pairs separate and move to opposite poles of the cell, organising a partner as they move
Describe some of the events that occur in Metaphase of Mitosis
- At Metaphase, chromosomes attach to the spindle fibres and align on the equator
- If the cell is viewed from the pole then they appear spread out
- If the cell is viewed from the side, the chromosomes appear in a line
Describe some of the events that occur in Anaphase of Mitosis
- Anaphase is a very rapid stage
- The centromere separates and the spindle fibres shorten (contract)
- pulling the now separated chromatids to the poles, centromere first
Describe some of the events that occur in Telophase of Mitosis
This is the final stage of mitosis. Chromatids have reached the poles of the cell and are referred to as chromosomes again
- Chromosomes uncoil and lengthen
- The spindle fibres break down
- The nuclear envelope reforms
- The nucleolus reappears
What happens during cytokinesis of Mitosis?
The division of the nucleus by Mitosis is followed by cytokinesis, the division of the cytoplasm to make two cells
- In animal cells, cytokinesis occurs by constriction of the parent cell around the equator (from the outside inwards)
- In plant cells, droplets of cell wall material, a cell plate, forms across the equator of the parent cell from the centre outwards and they extend an join to form a new cell wall
the differences between Mitosis in animal cells and plant cells
~SHAPE:
Animal cells- cell becomes rounded before mitosis
Plant cells-No shape change
~CENTRIOLES:
Animal cells- present Plant cells- absent
~CYTOKINESIS:
Animal cells-Cleavage furrow develops from the outside inwards
Plant cells- cell plate develops from centre outwards
~SPINDLE:
Animal cells- degenerate at telophase
Plant cells- remains throughout new cell wall formation
~OCCURRENCE:
animal cells- in epithelia, bone marrow etc
Plant cells- in meristems
The significance of mitosis: chromosomes number
- Mitosis produces two cells that have the same number of chromosomes as the parent and as each other
- Each chromosome in the daughter cells is an exact replica of those in the parental cells
- So mitosis produces cells that are genetically identical to the parent, giving genetic stability
The significance of mitosis: Growth, repair & replacement
By producing new cells, an organism increases its cell number and can grow , repair tissues and replace dead cells
- In mammals some tissue are constantly worn away e.g skin so identical cells are able to replace them
The significance of mitosis: Asexual reproduction
- Asexual reproduction produces offspring that are genetically identical to the parent
- It takes place in unicellular organisms
- It takes place in some flowering plants
- There is no genetic variation
What could happen if genes that control the cell cycle are damaged?
Cells may fail to divide or may divide too frequently or at the wrong time
What can radiation do to DNA?
Mutate DNA
What had DNA sequencing identified?
Gene mutations that affect the timing of the cell cycle
What do genes that control the cell cycle act as?
What happens if the genes are mutated?
They act as a brake, preventing the cell cycle from repeating continually
The brake is damaged and the cell Amy go immediately from one round of mitosis to the next and the cells will replicate too fast
What would happen if the distorted mitosis occurred in the bond marrow?
In the colon?
So many immature blood cells accumulate, that they spill out into the general circulation as blood cancers such as leukaemia
- in the colon a tumour would form
Why are the genes called ‘tumour suppressor genes’
Because they prevent rapid replication which would lead to tumour formation
How do some genes have the potential to cause cancer?
If they become mutated or the cell had become infected with a virus
Define Proto-oncogens
Define oncogens
Before the genes are altered l, when they do not cause cancer
Once altered and able to cause cancer they are called oncogens
What is meiosis?
Meiosis takes place in the reproductive organs of plants, animals and some protocitstans. It results in the production of 4 genetically distinct haploid gametes
Why is it necessary to produce haploid cells?
When two haploid gametes fuse at fertilisation, the zygote that is formed has two complete sets of chromosomes, one from each gamete, restoring the diploid condition
What would happen if the chromosome number did not half during gamete formation?
The number of chromosomes would double every generation
Meiosis I: prophase I
- In prophase 1 of meiosis paternal and maternal chromosomes come together in homologous pairs
- The pairing of chromosomes is called synopsis
- Each homologous chromosome pair is called a bivalent
-The chromosomes coil up, condensing to become shorter and thicker, visible as two chromatids - (in animals) centrioles separate and move to the poles of the cells
-They organise the polymerisation of microtubules, which radiate out from them and the spindle forms - Homologous chromosomes associate in their pairs->bivalents
- the chromosomes wrap around each other and remain joined at points called chiasma
- They exchange sections of DNA this is called crossing over and is a source of genetic variation
- This is because it mixes genes from two parents in 1 chromosomes
- The haploid cells have different genetic compositions because of this
~ By the end of prophase 1, the nuclear envelope has disintegrated and the nucleolus has disappeared
Why are there huge numbers of different genetic combinations?
Because crossing over can happen at several places along the length of the chromatid
Meiosis I: Metaphase I
- Pairs of homologous chromosomes arrange themselves at the equator of the spindle in a homologous pair, 1 chromosome from the mother and the other is from the father
- They lie at the equator randomly, with either one facing either pole
- A combination of paternal and maternal chromosomes faces each pole and the combination of chromosomes that goes into the each daughter cell at meiosis 1 is random with respect to which parent they came from >independent assortment
What does independent assortment at meiosis I and II cause?
New genetic combinations, which genes from both parent going into both daughter cells
- With 3 pairs of chromosomes, there are 2^3 possible combinations
- With 44 pairs of chromosomes, there are 2^44 possible combinations
So even if there were no genetic crossing over, there would still be a very large number of different ways genes from the two parents could assort into gametes
Meiosis I: Anaphase I
- The chromosomes in each bivalent separate and, as the spindle fibres shorten, one of each pair is pulled to one pair is pulled to one pole and the other to the opposite pole
- Each pole receives only one of each homologous pair of chromosomes and, because their random arrangement at metaphase, there is a random mixture of maternal and paternal chromosomes
Meiosis I: telophase I
In some species, the nuclear envelope reforms around the haploid group of chromosomes and the chromosomes de-condense and are no longer visible
- But in many species, the chromosomes stay in their condensed form
Meiosis I: Cytokinesis
The division of the cytoplasm, occurs, making two haploid cells
Meiosis II: Prophase II
- Centrioles replicate
- A new spindle forms at right angles to the first
- Nuclear envelope and nucleolus disappear
Meiosis II: metaphase II
The chromosomes align themselves randomly on the equator of the spindle
Each chromosome is a pair of chromatids
This phase produces genetic variation due to the random assortment of chromatids on the equator
Meiosis II: Anaphase II
The centromeres divide and the spindle fibres shorten, pulling the chromatids to opposite poles
Meiosis II: Telophase II
- At the poles, the chromatids lengthen and can no longer be distinguished in the microscope
- The spindle disintegrates and the nuclear envelope and nuclei re-form
The significance of meiosis
- Meiosis keeps the chromosome number from one generation to the next
- Meiosis generates genetic variation in the gametes and therefore the zygotes that they produce
what are the ways in which Meiosis generates genetic variation?
- Crossing over during Prophase I
- Independent assortment at metaphase I and II
so daughter cells have different combinations of chromatids
