6 - cell division Flashcards
What is the interphase?
Referred to as ‘growth period’
3 stages: G1, S, G2
What occurs during G1?
Organelles are synthesised and produced => replicated
Cell increases in size
CHROMOSOMES DO NOT REPLICATE IN G1
What occurs during the S phase?
DNA replication
What occurs during G2?
Cell continues to increase in size
Energy stores increase
Cell checks duplicated chromosomes and repairs error
What occurs during G0 and what are reasons for it?
DNA damaged, cell not viable and cannot divide
Permanent cell arrest
Most cells only divide for limited time > become secescent
Cells specialise and can no longer divide - leaves to perform function and does not re-enter the cell cycle
Define mitosis
Nuclear division in the mitotic phase of the cell cycle
What are chromosomes and chromatids?
Chromosomes - coiled and condensed DNA ready for division
Chromatids - 2 identical copies of a chromosome (DNA) held by a centromere (1 chromosome is converted into 2 chromatids)
1 chromosome > 2 chromatids per chromosome (after replication) > 1 chromatid (after mitotic division)
What are the roles of centromeres?
Necessary to ensure chromatids stay together, are precisely manouvred and segregated equally.
What occurs during prophase?
1) Chromatin fibres (complex of proteins DNA and RNA) coil and condense to form chromosomes. Nucleolus breaks down and nuclear membrane begins to degrade.
2) Protein microtubules form spindle fibres, which are used to move chromosomes into correct positions
3) Centrioles migrate to opposite poles of the cell - cylindrical bundles of protein which help spindle formation
4) Spindle fibres attach to centromere and begin to move chromosomes
5) Nuclear envelope disintegrates by end of prophase.
What occurs during metaphase?
Chromosomes moved by spindle fibres to form plane across the cell
What occurs during the anaphase?
Chromatids separate as centromeres holding chromosomes together divide - they are pulled to opposite poles of the cell by shortening the spindle fibres
- The V-shape appears as centromere drags chromatids through liquid cytosol.
What occurs during the telophase?
Chromatids reach poles - now called chromosomes
New set of chromosomes assemble at each pole
2 new nuclear envelopes form
What occurs during cytokinesis?
Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells.
Define haploid.
1/2 the normal chromosome number (1 chromosome of each type)
Define diploid.
Normal chromosome number - 2 of each type, one from each parent
Define allele.
Gene variants, different versions of the same gene
What occurs during ‘Prophase 1’?
Spindle fibres form from centrioles
Nucleolus disappears, and nuclear membrane disintegrates
Homologous chromosomes pair up – now called bivalents (also referred to as a tetrad)
Crossing over (known as recombination) occurs: The exchange of genes between bivalents as chromatids entangle.
What occurs during ‘Metaphase 1’?
the homologous pairs of chromosomes align on either side of the equatorial plate.
What occurs during ‘Anaphase 1’?
the spindle fibers contract and pull the homologous pairs, each with two chromatids, away from each other and toward each pole of the cell.
What occurs during ‘Telophase 1’?
the chromosomes arrive at the cell poles, the mitotic spindle disassembles, and the vesicles that contain fragments of the original nuclear membrane assemble around the two sets of chromosomes.
What occurs during ‘Prophase 2’?
no crossing over occurs as there are no homologous chromosomes in pairs (bivalents), instead the spindle reforms, nucleolus disappears, and the nuclear membrane begins to disintegrate.
What occurs during ‘Metaphase 2’?
the centromeres of the paired chromatids align along the equatorial plate in both cells.
What occurs during ‘Anaphase 2’?
the chromosomes separate at the centromeres.
What occurs during ‘Telophase 2’?
nuclear membranes form around each set of chromosomes, and the chromosomes decondense.
