Eukaryotic cell cycle and division Flashcards
cell cycle
interphase - the time taken between divisions
mitosis - separation of the chromatids of each chromosome to form two new nuclei
cytokinesis - division of the cytoplasm
Interphase
G1 - first growth phase
- organelles replicate protein synthesis cell size grows - respiration
S - synthesis
- DNA replication
- each chromosome becomes two chromatids attached at the centromere
G2 - second growth phase
- G1 + error checking
State the cell divisions in mitosis
prophase, metaphase, anaphase and telophase
Prophase
- replicated chromosomes super coil ( shorten and thicken )
- nuclear envelope breaks down and disappears
- centrioles divide into two and each daughter centriole moves to form a spindle
Metaphase
- chromosomes line up and move to the central region of the spindle known as the equator
- each chromosome becomes attached to the spindle thread by its centrosome
Anaphase
- replicated sister chromatids that make up the chromosome separate from each other when the centromere holding them splits
- each of the sisters becomes an individual chromosome
- spindle fibre shorten
Telophase
- as the separated sister chromatids reach the poles of the cell, a new nuclear envelope forms around each cell
- the spindle breaks down and disappears
- chromosomes uncoil, so you can no longer see them under a light microscope
Cytokinesis - after mitosis
- the whole cell now splits to form two new cells, each one containing a full set of chromosomes identical to that found in original parent cell
where is mitosis commonly observed ?
mammals: tissues - epithelial tissues such as the skin and the lining of the intestine
plants: meristems - the actively growing tips of shoots and roots
In prokaryotic cells, cell division ..
occurs by a process known as binary fission
In eukaryotic cells, cell division ..
part of a regulated process called the cell cycle
what is asexual reproduction ?
the formation of new organisms that does involve the fusion of gametes
gene
a base sequence of DNA that codes of the amino acid sequence of a polypeptide or a functional RNA molecule
allele
versions of genes
chromosome
structure in a cell that contains genetic material also known as DNA
chromatid
one of the two identical halves of a chromosome that has been replicated in preparation for cell division
centromere
a constricted region of a chromosome to which the microtubules of the spindle attach during cell division
chromatin
a mixture of DNA and proteins that form the chromosomes found in the cells of humans and other higher organisms
DNA + histone = chromatin
how many daughter cells does meiosis produce ?
4 daughter cells
differences between mitosis and meiosis
meiosis:
- have half the number of chromosomes as the parent cell
- genetically different
mitosis:
- same chromosomes complement as the parent cell
- genetically identical to each other - clones
where does meiosis occur in humans ?
occurs in the gonads - testes and ovaries - and results in the formation of haploid gametes that fuse at fertilisation to form a diploid zygote
what is a haploid ?
contains only one chromosome from each of its homologous series
overview of meiosis
in meiosis 1, homologous chromosomes are separated
in meiosis 2, the chromatids of each chromosome are separated
Meiosis 1
- prophase 1 (early )
- prophase 1 ( mid )
- prophase 1 ( late )
- metaphase 1
- anaphase 1
- telophase 1
prophase 1 ( early )
during interphase the chromosomes replicate into chromatids held together by a centromere ( the chromatids are not visible ). NOW, the chromosomes condense ( shorten and thicken ) and become visible
prophase 1 ( mid )
homologous chromosomes pair up
( becoming bivalents ) as they continue to shorten and thicken. Centrioles duplicate.
prophase 1 ( late )
Homologous chromosomes repel each other. Chromosomes can now be seen to consist of chromatids. Sites where chromatids have broken and rejoined, causing crossing over, are visible as chiasmata
metaphase 1
nuclear membrane breaks down. Spindle forms. Bivalents line up at the equator, attached by centromeres
anaphase 1
homologous chromosomes separate. Whole chromosomes are pulled towards opposite poles of the spindle, centromere first ( dragging along the chromatids )
telophase 1
nuclear membrane re-forms around the daughter nuclei. The chromosome number has been halved. The chromosomes start to decondense
Meiosis 2
prophase 2
metaphase 2
anaphase 2
telophase 2
prophase 2
the chromosomes condense and the centrioles duplicate
metaphase 2
the nuclear membrane breaks down and spindle forms. The chromosomes attach by their centromere to spindle fibres at the equator of the spindle
anaphase 2
the chromatids separate at their centromeres and are pulled to opposite poles of the spindle
telophase 2
the chromatids ( now called chromosomes ) decondense. The nuclear membrane re-forms . The cells divide
what is meiosis a major source of ?
genetic variation in organisms
the cells produced by meiosis are genetically different because of two different reasons :
independent assortment and crossing over
Independent assortment
bivalents line up at the equator of the spindle in meiosis 1 entirely at random
Crossing over
exchange in DNA between chromatids of homologous chromosomes
w/hat is a chromosome mutation ?
a change in the number or the sequence of genes can be brought about in a number of different ways
chromosome translocation
part of a chromosome breaks and re-joins a completely different chromosome
chromosome non disjunction
the members of a homologous pair fail to be separated during meiosis
polysomy
a zygote with more copies of this chromosome than usual
monosomy
a zygote with only one copy of this chromosome
Turner’s syndrome - monosomy
- only one case of full monosomy in humans
- Women have only one X chromosome rather than the normal two (represented XX)
Down’s syndrome - polysomy
non disjunction of chromosome 21 during gamete production - total of 47 chromosomes
