Unit 1. 6 mitosis and meiosis Flashcards
what are chromosomes?
long sections of DNA, proteins and small amount of RNA
chromatids?
the single thread of DNA that becomes 2 identical threads
how many chromosomes do humans have?
46 chromosomes = 23 pairs of homologous chromosomes
what is the diploid number?
total number of chromosomes
what is the haploid number?
half the diploid number - human gametes = 23 chromosomes
Mitosis?
nuclear division that produces 2 genetially identical daughter cells that are genetically identical to the parent cell
What are the 4stages of mitosis?
Interphase (not part but very important part of cell cycle)
prophase
metaphase
anaphase
telophase
Interphase?
Replication of DNA
Replication of organelles
synthesis of ATP
synthesis of proteins
increase in cell size
Prophase? - longest stage of mitosis
The chromosomes condense and
become visible as two sister
chromatids joined by a centromere.
Centrioles migrate to the opposite sides
of the cell (the poles).
The spindle begins to form in animal cells
only, made by the centrioles.
The nuclear membrane disintegrates and
the nucleolus disappears.
Metaphase
The spindle completes
The chromosomes migrate to the
equator of the cell
Spindle fibres attach to the chromosome
at the centromere
Each chromosome is attached to one
spindle fibre
Anaphase? fastest stage of mitosis
The centromere divides.
The spindle fibres contract and shorten pulling
the chromatids to opposite poles, centromere
first.
cytokinesis?
The cytoplasm divides
In animal cells, the cell constricts around the
equator
In plant cells, a cell plate forms across the
equator to form a new cell wall
2 genetically identical cells are formed
what is the purpose of mitosis?
essential for growth, the repair of tissues and the replacement of dead or worn out cells.
what happens by mitosis?
asexual reproduction - offspring = genetically identical to the parent
advantage = ability to increase in numbers quickly to take advantage of an ideal environment
disadvantage = lack of genetic variation , leading to an inability to adapt if the environment changes.
mitotic index?
number of cells
cancer?
result of uncontrolled mitosis . cancerous cells divide repeatedly out of control with the formation of a tumour.
tumour = irregular mass of cells that prevent the normal function of body organs. cancers thought to be initiated when mutations occur in the genes that control cell division.
what is meiosis?
type of nuclear division that gives rise to cells that are genetically different. used to produce gametes.
what happens to the number of chromosomes?
must be halved when gametes = formed, otherwise there would be double the number of chromsomes after they join at fertilisation in the zygote
what are the phases?
meiosis starts exactly the same as in mitosis but there are 2 divisions . meiosis 1 and meiosis 2
interphase?
Synthesis of ATP
Synthesis of proteins
Replication of organelles
Replication of DNA.
Increase in size
Prophase 1?
DNA condenses and becomes visible as chromosomes
The chromosomes are arranged side by side in homologous pairs
A pair of homologous chromosomes is called a bivalent
As the homologous chromosomes are very close together the crossing over of
non-sister chromatids may occur. The point at which the crossing over occurs is
called the chiasma (chiasmata; plural)
In this stage centrioles migrate to opposite poles and the spindle is formed
The nuclear envelope breaks down and the nucleolus disintegrates
metaphase 1 ?
The bivalents line up along the equator of the spindle, with the spindle fibres
attached to the centromeres
The maternal and paternal chromosomes in each pair position themselves
independently of the others; this is known as independent assortment
This means that the proportion of paternal or maternal chromosomes that end up on
each side of the equator is due to chance
Anaphase 1?
The homologous pairs of chromosomes are separated as the spindle fibres pull
whole chromosomes to opposite poles of the cell
The centromeres do not divide
Telophase 1?
The chromosomes arrive at opposite poles
Spindle fibres start to break down
Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
Some plant cells go straight into meiosis II without reformation of the nucleus in
telophase I
Prophase 2?
There is no interphase between meiosis I and meiosis II so the DNA is
not replicated. The second division of meiosis is almost identical to
the stages of mitosis.
In prophase II:
The nuclear envelope breaks down and chromosomes
condense
Centrioles migrate to opposite poles but this time,
A spindle forms at a right angle to the old one
Metaphase 2?
Chromosomes line up in a single file along the equator of the spindle
Anaphase 2?
Centromeres divide and individual chromatids are pulled to
opposite poles
This creates four groups of chromosomes that have half the
number of chromosomes compared to the original parent
cell
Telophase 2?
Nuclear membranes form around each group of
chromosomes
In cytokinesis:
Cytoplasm divides as new cell surface membranes are formed creating four haploid
cells – they have half the number of chromosomes as the parent cell.
The 4 cells are genetically different from each other
what happens at the end of meiosis 2?
4 haploid genetically different daughter cells.
Differences in mitosis and meiosis?
mitosis
One division resulting in 2 daughter
cells
Number of chromosomes is unchanged
Homologous chromosomes do not
associate in pairs
