Life Cycles & Meiosis Flashcards
What is Asexual reproduction ?
Based on mitotic divisions
What is sexual reproduction ?
Offspring are not identical to the parents
Requires gametes created by meiosis:parents each contribute to one gamete
Gametes and offspring differ genetically from each other and form parents
Generate genetic diversity
What are somatic cells ?
Body cells not specialised for reproduction
Each somatic cells contain homologous pairs of chromosomes with corresponding genes
Gametes contain only one homologous of each pair the number is haploid
Define fertilization
2 haploid gametes fuse to form a diploid zygote
Early prophase 1
Chromatin coils and compact’s
Mid-prophase 1:
Maternal chromosomes pair with paternal homologs .
The homologous chromosomes pair by adhering along their lengths a process called synapses
DOES NOT OCCUR DURING MITOSIS.
4 chromatids of each homologous pair forms a TETRAD.
Late prophase 1
At some point the homologs seem to repel each other but are held together by cohesions at regions called chiasmata that form between nosister chromatids
Define crossing over ?
Exchange of genetic material occurs between non sister chromatids and chiasmata
Crossing over results in recombinant chromatids and increases genetic variability of the products
Crossing over is one reason for genetic diversity in meiosis 1 products
How long does it take to complete meiosis ?
Human males :prophase 1 lasts about a week and the entire meiotic cycle takes about a month
Human females : prophase 1 begins before birth and ends up to decades later during the monthly ovarian cycle
Metaphase 1?
Homologous pairs line up on the equatorial plate
One of each pair faces each pole
Meiosis 1 guarantees that each daughter nucleus gets one full set of chromosomes
Independent assortment takes place here
Define independent assortment ?
Matter of chance how the homologous chromosomes line up and which one of the pair go to which daughter cell.
Also generates genetic diversity
Allows for new combination of chromosomes
Anaphase1:
Homologous chromosomes (each with 2 chromatids) move to the poles
The greater the number of chromosomes the more combinations that are possible and the greater the potential for genetic diversity
Telophase 1
Chromosomes gather into nuclei and original cell divides
Forms haploids cell (no maternal or paternal copy of chromosome)
Meiosis 2
Not preceded by DNA replication
Prophase 2
The chromosomes condense again following a brief interphase in which DNA DOES NOT replicate
Metaphase 2
Centromere of sister chromatids line up across the equatorial plate
Anaphase 2
Sister chromatids are separated
Chance assortment of the chromatids further to the genetic diversity
Telophase 2
Final products are 4 haploid daughter cells
What is non disjunction ?
Homologous pairs fail to separate at anaphase 1 or sister chromatids fail to separate at anaphase 2
Results in aneuploidy - individual chromosomes are lacking or present in excess
What is aneuploidy ?
Individual chromosomes are lacking or present in excess of
During meiosis 1 may be caused by lack of cohesins that hold the homologous airs together (and that hold the sister chromatids together)
Resulting gametes will have 2 of the same chromosomes or none
Examples of aneuploidy ?
Humans- Down syndrome results from a gamete with 2 copies of chromosome 21 .
After fertilisation there are 3 copies of
(Trisomic)
A fertilised egg that did not receive a copy of chromosome 21 will monoatomic which is lethal
Trisomies & monosomies are common in human zygotes
These for anything apart from chromosome 21 are lethal many miscarriages are due to this
What is polyploid ?
Qoqrganisms with complete extra sets of chromosomes are called polyploid
Triploid (3n),tetraploid (4n) and even higher levels are possible
If there is non-disjunction of all chromosomes during meiosis 1 diploid gametes will form leading to polyploidy after fertilisation
Polyploidy cells tend to be larger and are favoured as crop plants
How do cells die ?
N
1.)Necrosis : cell is damaged or starved of oxygen or nutrients .The cell swells and bursts
Cell contents are released to the extracellular environment and can cause inflammation
Define Apoptosis ?
Genetically programmed cell death
The cell is no longer needed
Old cells are prone to genetic damage that can lead to cancer
Events of. Apoptosis ?
Cell detaches from its neighbours
Chromatin is digested by enzymes that cut DNA between nucleosomes
Forms membrane ours lobes called “blebs’ that break into fragments
Surrounding lining cells I inject remains of the dead cell and recycle content
Signals that initiate apoptosis
Hormones
Growth factors
Viral infections
Toxins
Extensive DNA damage
Caspases hyrdolyze membrane proteins in nucleus and cell membranes and hydrolyze nucleosomes
