Cell division n a lil mutation Flashcards
Explain DNA replication as semi-conservative using evidence from the Messelson-Stahl experiment.
Both original parental strands contain only heavy 15^N. Each 15^N strand is used as a template for replicating a new 14^N strand.
As the replication is semiconservative, in the first generation, the isolated DNA showed only 1 single band of intermediate density.
Where each replicated hybrid DNA molecule was composed of one new 14^N strand and one parental 15^N strand.
Explain the role of DNA nucleotides, DNA polymerase, DNA primase and Okazaki fragments in DNA replication
Helicase: unwinds the double helix starting from the origin of replication by breaking hydrogen bonds between the complementary base pairs on parental strands.
DNA primase: attaches to the unwound chain and catalyses the synthesis of RNA primer to provide free 3’OH ends for DNA polymerase III.
DNA polymerase III: elongates the new daughter strand in the 5’ to 3’ direction by catalysing phosphodiester bond formation between the incoming deoxyribonucleotides and the free 3’OH end of the daughter strand.
Free deoxyribonucleotides: are incorporated with complementary base pairing to parental DNA strands.
Okazaki fragments: in a replication fork, the leading strand is synthesised continuously while the lagging strand is synthesised discontinuously to form Okazaki fragments.
DNA polymerase I: RNA primers are removed and replaced by deoxyribonucleotides by DNA polymerase I.
DNA ligase: Nicks between Okazaki fragments are filled in by DNA ligase by forming phosphodiester bonds between Okazaki fragments.
Explain how gel electrophoresis is used to analyse DNA, including how the DNA fragments on a gel are made visible and the role of DNA markers in identifying DNA fragments of different sizes.
Nucleic acid dyes bind to DNA to form a complex, which can then be visualised by exposing the gel to UV light.
DNA has a negative charge and will migrate towards the anode when placed under an electric current.
The agarose gel acts as a molecular sieve to separate DNA based on size.
DNA fragments that are long will migrate slower and hence travel a shorter distance than those which are shorter.
DNA marker: Using appropriate DNA molecular weight standards, the size (in basepairs, bp) of unknown DNA bands can be determined.
State the importance of mitosis in growth, repair and asexual reproduction.
Increase in cell number for growth OF onion
Replace worn-out cells OF onion
Asexual reproduction OF onion to genetically identical offspring
NOTE: what cells? specific (stem cells: various blood cells)
DNA content and chromosome number change in graph explain
Unit au
Flat: meiosis I/II, mitosis (interphase and cytokinesis not part of meiosis/mitosis)
Down: DNA was divided equally between (2 daughter cells produced.)
Up: Fertilisation has occurred, the gamete fused with another gamete to restore diploid number of chromosomes in zygote
Drawing of mitosis and meiosis
Mitosis
Horizontal, no crossing over
Meiosis
prophase I: 2 centrosome suns at poles, dotted-line envelope, partnered same size chromosomes (crossing over incomplete)
Metaphase I: Chromosomes lined up at centre, lines from centromeres to centrosome suns (crossing over complete)
Anaphase I: separate horizontal
Telophase I: cleavage, dotted-line envelope
Prophase II: 2 centrosome suns at poles, dotted-line envelope, partnered same size chromosomes (crossing over complete)
MII to TII: same but vertical
End Cy: long and short curly strands
Label cytoplasm!!
DNA content and chromosome number change in graph describe: meiosis mitosis
Mitosis
Chromosome number: 2n to 4n (start of A) to 2n (after Cy)
DNA: x sloping up to 2x (start of S to end of S) to x (after Cy)
Meiosis
Chromosome number: 2n to n (end of Cy (1)) to 2n (start of A) to n (after Cy (2))
DNA: x sloping up to 2x (start of S to end of S) to x (after Cy (1)) to x/2 (after Cy (2))
Mitosis
Prophase:
(Replicated) chromosomes increasingly shorten and thicken by supercoiling to become 2 chromatids held together at the centromere.
Nuclear envelope breaks down (nucleolus disappears)
Centrosome divides and 2 centrioles replicate to form 2 centrosomes
Metaphase:
2 centrosomes move to opposite ends of the cell.
Microtubules of the cytoplasm start to form spindle fibres, radiating out from the centrioles.
Each chromosome is attached to a microtubule of the spindle and arranged at the equator of the spindle.
Anaphase:
Spindle fibres shorten
Centromeres separate
Chromatids are pulled by centromeres to opposite poles.
Telophase:
Nuclear envelope reforms around both groups of chromosomes at opposite ends of the cell.
Cytokinesis
Chromosomes ‘decondense’ by uncoiling, becoming chromatin again.
Nucleolus reforms in each nucleus
Meiosis
Prophase I:
Chromosomes replicate and condense to form 2 chromatids held together by a centromere.
Centrioles duplicate. Homologous chromosomes pair up as they continue to shorten and thicken.
Centrosomes start to move apart. Homologous chromosomes repel each other. Sites where chromatids have broken and rejoined cause crossing over at chiasma.
Nucleolus and nuclear envelope disappear.
Metaphase I:
2 centrosomes move to opposite ends of the cell.
Microtubules of the cytoplasm start to form spindle fibres, radiating out from the centrioles.
Each homologous pairs attach to microtubules of spindle by their centromeres and are arranged at the equator of the spindle
Anaphase I:
Chromosomes of each homologous pair move to opposite poles of the spindle, but individual chromatids remain attached by their centromeres. (horizontal)
Telophase I: Homologous chromosomes arrive at opposite poles Chromosomes uncoil Nuclear envelope reform around nuclei Spindle breaks down
Prophase II:
Chromosomes increasingly shorten and thicken by supercoiling to become 2 chromatids held together at the centromere.
Nuclear envelope breaks down (nucleolus disappears)
Centrosome divides and 2 centrioles replicate to form 2 centrosomes
Spindle apparatus re-formed at right angles to the original spindle
Metaphase II:
Chromosomes line up at the equator of the spindle, attached by their centromeres.
Anaphase II:
Centromeres divide
Chromatids move to opposite poles of the spindle, centromeres first
Telophase II:
Nuclear envelope form around 4 groups of chromatids so 4 nuclei are formed (½ the chromosome number of the original parent cell)
Chromosomes uncoil and disperse as chromatin
Nucleoli reform
Cytokinesis definiton
in-tucking of cell surface membrane at the equator of the spindle, ‘pinching’ the cytoplasm in half.
Plant vs animal cytokinesis
Animal cells:
Cleavage furrow form
Plant cells:
Formation of vesicles of new cell wall materials
Formation of cell plate
identify and explain causative factors (e.g. genetic, chemical carcinogens, radiation, loss of immunity) which can increase the chances of cancerous growth.
Ionising radiation: X-rays & radiation (damaging ions in nucleus, break up DNA)
Non-ionizing radiation: UV light (wrong nucleotide pairing, mutation)
Chemicals: tobacco smoke
Cause damage to DNA molecules of chromosomes, resulting in mutation: change in the amount or chemical structure of DNA of a chromosome.
Mutations of different types build up in the DNA of body cells over time.
A single mutation is unlikely to be responsible for triggering cancer.
Explain the significance of mitotic cell cycle and the need to regulate it tightly
Dysregulation of checkpoints of cell division can result in uncontrolled cell division and cancer, when the cell cycle operates without a molecular control system controlled by specific genes.
Describe the development of cancer
development of cancer as a multi-step process that includes accumulation of mutations,
State what is meant by homologous pairs of chromosomes.
Matching pairs of chromosomes that can possess different versions of the same genes, one pair from the male parent and one pair from the female parent.
