Cell Division Flashcards
Describe chromosome structure
- When a cell is not dividing, the DNA exists as chromatin within the nucleus. Chromatin consists of DNA and histone proteins and is found in the nucleus of eukaryotic cells
- When cell division begins, chromatin condenses to form chromosomes
- Chromosomes consist of DNA, protein and a little RNA. The DNA is made up of many small sections called genes
- In diploid cells, chromosomes are found in matching pairs called homologous chromosomes
- At the start of cell division, the DNA replicates (during interphase) and the chromosomes are visible as 2 identical sister chromatids joined at the centromere
How many pairs of homologous chromosomes do humans have?
23 pairs
What does each homologous pair consist of?
One chromosome inherited form the mother (maternal) and one chromosome inherited from the father (paternal)
Finish the sentence: Gametes (sex cells) have one of each type of chromosome and are __________
Haploid (n)
What activities occur in a cell during interphase?
- DNA replicates
- Organelles are produced
- Protein synthesis occurs
- ATP synthesis occurs
State (in order) the 4 phases of mitosis
- Prophase
- Metaphase
- Anaphase
- Telophase
Describe prophase
- Chromatin condenses into visible sister chromatids joined at the centromere
- Centrioles move to opposite poles, forming protein microtubules and developing the spindle
- The nuclear envelope disintegrates, the nucleolus disappears and chromosomes move towards the cell equator
Which is the longest stage of mitosis?
Prophase
Describe metaphase
Chromosomes align along the cell equator and attach to spindle fibres at the centromere
Describe anaphase
- Rapid stage in which spindle fibres contract and shorten, the centromere splits and sister chromatids are pulled apart
- Chromatids are drawn to opposite poles with the centromere leading
Describe telophase
- Chromosomes uncoil and lengthen
- Spindle fibres break down
- Nucleolus and nuclear envelope reform
Describe cytokinesis in animals
Constriction of the parent cell from the outside inwards, forming a cleavage furrow
Describe cytokinesis in plants
A cell plate forms across the equator of the parent cell from the inside outwards and a new cellulose wall is laid down
State 5 differences in mitosis between plants and animals
(Shape, centrioles, cytokinesis, spindle, occurrence)
- Animals: cells become rounded before mitosis, Plants: no shape change
- Animals: centrioles are present, Plants: centrioles absent
- Animals: during cytokinesis, cleavage furrow develops from the outside inwards, pinching the cytoplasm, Plants: cell plate develops from the centre outwards, forming a new cell wall
- Animals: spindle degenerates at telophase, Plants: spindle remains throughout new cell wall formation
- Animals: in epithelial tissue, bone marrow and other sites, Plants: in meristems (tips of roots and shoots)
Finish the sentence: Mitosis leads to ________ of an organism, _______ of damaged tissues and ____________ of dead cells
Give an example form plants and animals
Growth, repair, replacement
Plants: mitosis occurs in root tips and growing points of shoots
Animals: in human skin, mitosis replaces dead surface cells with genetically identical cells from below
Finish the sentence: Most animals and plants also produce _______ _________, which are not identical to the parent cell, allowing for ________ ___________
Haploid gametes, sexual reproduction
State 3 advantages of asexual reproduction
- Offspring are genetically identical to the parent, so they are able to survive successfully if the environmental conditions are stable
- Less time and energy are required as there is no need to produce gametes and find a reproductive partner
- The population can rapidly increase in number if environmental conditions are stable
Give a disadvantage of asexual reproduction
- Lack of genetic variation means offspring cannot adapt to changing environmental conditions
_____________ such as radiation, certain chemicals and some viruses can mutate DNA and affect the genes that control the cell cycle
Carcinogens
What regulates the cell cycle by signalling when cells can divide? What happens if these genes mutate?
Proto-oncogenes. If these genes mutate, uncontrollable cell division can occur, leading to tumour formation. These mutated genes are called oncogenes.
State and describe the 2 types of tumour that can form
- Benign - non cancerous and won’t invade nearby tissues
- Malignant - cancerous and can spread to other tissues
Meiosis produces __________ in ___________ reproducing organisms, resulting in __ genetically different daughter cells
Gametes, sexually, 4
Describe prophase I
- Chromosomes condense into visible sister chromatids joined at the centromere
- Homologous chromosomes pair up in a bivalent
- Chromatids from homologous pairs wrap around each other and partially repel each other but remained joined at the chiasmata. A chiasmata is where homologous chromosomes can exchange equivalent sections of DNA. This swapping is called crossing over.
- Nuclear envelope disintegrates, nucleolus disappears, centrioles move to opposite poles and spindle microtubules form
Describe metaphase I
- Chromosomes align themselves either side of the equator in their homologous pairs, attached to spindle fibres at their centromeres
- Homologous pairs of chromosomes (bivalents) align randomly along the equator, leading to independent assortment
- This results in a random combination of paternal and maternal chromosomes in each daughter cell
Describe anaphase I
- Spindle fibres contract and shorten, separating bivalents and chromosomes are pulled to opposite poles
- Each pole receives one chromosome from each homologous pair. This results in a random mix of maternal and paternal chromosomes due to independent assortment in metaphase I
Describe telophase I
- In some species, the nuclear envelope reforms around the haploid chromosomes, which uncoil and lengthen. In many species, chromosomes remain condensed
- Cytokinesis then occurs - when the cytoplasm is divided, producing 2 haploid gametes
Describe prophase II
Centrioles migrate to the poles and the new spindle develops at right angles to the previous one
Describe metaphase II
- Chromosomes align on the equator, attached to spindle fibres at their centromere
- independent assortment occurs with chromatids randomly facing either pole
Describe anaphase II
- Spindle fibres contract and shorten
- Centromeres split and sister chromatids are pulled to opposite poles
Describe telophase II and cytokinesis
- Chromosomes uncoil and lengthen and are no longer visible under a microscope
- Spindle breaks down
- Nuclear envelope and nucleolus reform
Cytokinesis produces 4 haploid daughter cells
In what 3 ways does meiosis introduce genetic variation?
- Crossing over
- Independent assortment
- Random fusion of haploid gametes
Describe how meiosis introduces genetic variation through crossing over
- During prophase I, chromosomes condense, and homologous pairs form a bivalent, this process is called synapsis
- Chromatids in the bivalent join at chiasmata, where parts of the chromosome may be exchanged (crossing over)
Describe how meiosis introduces genetic variation through independent assortment
- During metaphase I and II, chromosomes randomly distribute themselves at the equator of the cell
- This random distribution leads to independent assortment of chromosomes. With 3 pairs of chromosomes there are 2^3 = 8 possible combinations of maternal and paternal chromosomes
Describe how meiosis introduces genetic variation through random fusion of haploid gametes
- At fertilisation, the diploid chromosome number is restored when two random haploid gametes fuse to form a zygote
- Half of the chromosomes in the new cell come from the maternal gamete and half come from the paternal gamete
State 2 advantages of sexual reproduction
- Allows offspring to adapt to a changing environment, this allows natural selection to take place
- Spores produced by fungi and seeds by plants are dispersed widely to help reduce intraspecific competition
Name 6 differences between mitosis and meiosis
Mitosis
1. Only 1 cell division
2. Two daughter cells are produced
3. Chromosomes so not associated with in homologous pairs (bivalents)
4. The chromosome number in each daughter cell is diploid
5. Crossing over does not take place
6. Daughter cells are genetically identical to each other and to the mother
Meiosis
1. 2 cell divisions
2. 4 daughter cells produced
3. Chromosomes associated with in homologous pairs (bivalents)
4. The chromosome number in each daughter cell is haploid
5. Crossing over does take place a prophase I
6. Daughter cells are genetically different
