Cell Division Flashcards
What are chromosomes?
- DNA, protein, little bit of RNA
- DNA is made up of small sections called genes
- found in matching pairs called homologous chromosomes in diploid cells
How many chromosomes do humans have?
23 pairs of homologous chromosomes
Each homologous pair has one chromosome inherited from the mother (maternal), and one chromosome inherited from the father (paternal)
What is a diploid and haploid cell?
- total number of chromosomes in a somatic body (cell) is the DIPLOID number, as there are two of each type of chromosome
- gametes are haploid cells, as they have one of each type of chromosome (half the diploid number)
What happens during interphase?
- DNA replicates (during s-phase)
- organelles reproduced so each daughter cell has the same number
- protein synthesis occurs, including producing the enzymes required for cell division (DNA helicase, DNA polymerase)
- ATP synthesis occurs in mitochondria during aerobic respiration
What happens during each phase of interphase?
G1:
- cell gets bigger and more rounded
- organelles replicated
- protein and ATP synthesis
S-phase:
- DNA is replicated inside the nucleus
- DNA content doubles
G2:
- protein synthesis continues
- centrioles replicate (only inside animal cells)
What is mitosis?
The division of the nucleus that occurs in 4 phases
- prophase
- metaphase
- anaphase
- telophase
What happens during prophase?
- chromatin condenses so it is now possible to see the replicated chromosomes
- centriole pairs migrate towards opposite poles of the cell to make the mitotic spindle
- nucleolus inside nucleus breaks down and disappears
-nuclear envelope made of two phospholipid bilayer now breaks down and disappears - replicated chromosomes attach to the mitotic spindle by their centromeres
Longest stage
What happens during metaphase?
- replicated chromosomes that are attached to the mitotic spindle now move along the spindle fibres, until all the chromosomes align down the equator of the cell
What happens during anaphase?
- the mitotic spindle fibres now contract (shorten), splitting the centromere
- the split centromeres get pulled to opposite poles of the cell, with a sister chromatid attached
- the sister chromatids end up at opposite poles of the cell
Fastest phase
What happens during telophase?
- nuclear envelopes now reform inside the cell around the separated sister chromatids (this cell has two nuclei in it now, each haploid)
- the nucleolus reappears inside each new nucleus
- the spindle fibres break down
What happens during cytokinesis in animal cells?
Contractile proteins in the cytoplasm shorten, pulling the cleavage furrow inwards, when the membrane fuses two genetically identical diploid daughter cells are formed
What happens during cytokinesis in plant cells?
A cell plate forms across the equator of the parent cell from the inside outwards, and a new cellulose cell wall is laid down
What are differences between plant and animal cells in mitosis?
- animal cells become more rounded before mitosis, whilst plant cells do not change shape due to the strong cellulose cell wall
- animal cells contain centrioles, whereas plant cells have no centrioles
- in cytokinesis in animal cells, a cleavage furrow forms from the outside inwards and pinches the cytoplasm (due to contracting proteins), whereas in plant cells, the cell plate develops from the centre outwards, forming a new cellulose wall
- in animal cells, the cell spindle degenerates at telophase as two new nuclei form, whilst in plant cels, the spindle remains throughout new cell wall formation
- in animal cells, mitosis occurs in epithelial tissue, bone marrow, and other sites, whereas in plant cells mitosis occurs at the meristems (tips of roots and shoots)
Why is mitosis significant?
- produces two daughter cells with the same number of chromosomes as the parent cell
- each chromosome is an exact replica of one or the original parent chromosomes, so mitosis produces cells that are genetically identical to the parent cell, ensuring genetic stability
What are the functions of mitosis?
- growth of an organism
- repair of damaged tissue
-replacement of dead cells
How do asexual plants reproduce?
Produces offspring by mitosis, so offspring are genetically identical to the identical to the parent
No genetic variation = they are clones
Large numbers of genetically identical offspring are produced by strawberry runners, daffodil bulbs and potato tubers
What are the advantages of asexual reproduction?
- offspring are genetically identical to the parent, so they are able to survive successfully if the environmental conditions remain stable
- less time and energy are required as there is no need to produce gametes and find a reproductive partner
- population can rapidly increase in number if environmental conditions are stable
What are the disadvantages of asexual reproduction?
- lack of genetic variation means offspring cannot adapt to changing environmental conditions
What are carcinogens?
Radiation, some chemicals, some viruses that can mutate DNA and affect the genes that control the cell cycle
What do proto-oncogenes do?
Regulate the cell cycle by signalling when cells should divide
What are oncogenes?
A mutated proto-oncogene that leads to uncontrollable cell division meaning tumour formation
What types of tumour can form?
Benign = non-cancerous, won’t invade nearby tissues
Malignant = cancerous, can spread to other tissues
What are tumour suppressant genes?
A DNA base sequence that codes for the correct primary structure of a tumour suppressant protein (correct tertiary structure) so the protein acts as a ‘brake’ on the cell cycle, causing it to slow down = if mutation in DNA, then this will not occur, and there will be unregulated cell division causing cancer cells
What is meiosis?
Produces gametes in sexually reproducing organisms, resulting in four genetically different daughter cells
Involves two stages of cell division-
Meiosis I = chromosome number is halved
Meiosis II = haploid cells divide again, producing four genetically different daughter cell (similar to mitosis)
What happens during prophase I?
- chromosomes condense into visible sister chromatids joined at the centromere
- homologous chromosomes pair up into a bivalent
- chromatids from homologus pairs wrap around each other and then partially repel each other, but remain joined at points called chiasmata (where homologous chromosomes can exchange equivalent sections of DNA in crossing over)
- nuclear envelope disintergreates, nucleolus disappears, centrioles move to opposite poles of the cell and spindle microtubles form
What happens in 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
What happens in anaphase I?
- spindle fibres contract and shorten, separating bivalents and chromosomes are pulled apart to oppose poles
- each pole receives one chromosome from each homologous pair, resulting in a random mix of maternal and paternal chromosomes due to independent assortment of bivalents in metaphase I
What happens in telophase I?
- in some species, the nuclear envelope reforms around the haploid chromosomes, which uncoil and lengthen. In many species the chromosomes remain condensed
What is cytokinesis in meiosis I?
The cytoplasm is divided and produces two haploid cells
What happens in prophase II?
- centrioles migrate to the poles and the new spindle develops at right angles to the previous one
- happens 2x as there are two haploid cells
What happens in metaphase II?
- chromosomes align on the equator of the cell, attached to the spindle fibres at their centromeres
- independent assortment occurs with chromatids randomly facing either pole
What happens in anaphase II?
- spindle fibres contract and shorten
- centromeres split and sister chromatids are pulled to opposite poles
What happens in telophase II?
- chromosomes uncoiled and lengthen and are no longer visible under the microscope
- spindle breaks down
- nuclear envelope and nucleolus reform
What happens in cytokinesis of meiosis II?
Four haploid daughter cells are produced
What are sources of genetic variation in meiosis?
- crossing over (only in 10-15% of cells)
- independent assortment
- random fusion of haploid genes
What is crossing over?
- during prophase I, chromosomes condense and homologous pairs for a bivalent in a process called synapses
- chromatids in the bivalent join at chiasmata, where parts of the chromosome may be exchanged
What is independent assortment?
- during metaphase I (and metaphase II if crossing over has occurred) chromosomes randomly distribute themselves at the equator of the cell
- this random distribution leads to the independent assortment of chromosomes
How to work out the number of combinations produced by independent assortment?
n
2
n = pairs of chromosomes
What is the random fusion of haploid genes?
- 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
What are the advantages of sexual reproduction?
- allows offspring to adapt to a changing environment, allowing natural selection to take place
- spores produced by fungi and seeds by plants are widely dispersed to help reduce intraspecific competition
What are the differences between mitosis and meiosis?
- in mitosis, there is only one cell division, whereas in meiosis there are two cell divisions
- in mitosis two genetically identical daughter cells are produced, whereas in meiosis, four genetically different daughter cells are produced
- in mitosis, chromosomes do not associate in homologous pairs (bivalents), whereas in meiosis chromosomes do associate in homologous pairs (bivalents) in prophase I
- in mitosis crossing over does not take place, whereas in meiosis crossing over does take place
