Meiosis Flashcards
What is a diploid cell?
A nucleus with two full sets of chromasomes (2n)
Found in most cells
What is a haploid cell?
A nucleus with one full set of chromasomes (n)
Found in gametes
Are gametes diploid or haploid?
Haploid
What is a zygote?
A cell formed when two haploid gametes fuse at fertalisation
What is meosis?
Cell division leading to 4 haploid cells
What are the stages of meiosis?
Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Prophase 2 Metaphase 2 Anaphase 2 Telophase 2 Cytokenesis
What happens at prophase 1?
Each chromasome appears in a condensed form with two chromatids
Homolougous chromasomes associate with each other
Crossing over occurs
What is crossing over?
Enzymes cut out parts of chromasomes and join parts of the maternal and paternal chromasomes together.
What happens during metaphase 1?
Spindle fibres form and the chromasoems line up along the equator in two lines
What happens during anaphase 1?
The centromeres do not break and one pair of chromatids (one chromasome) is pulled to each side of the cell
Each new cell has half the number of chromasomes as the original as it doesn’t have any copies
What happens during telophase 1?
The nuclear membrane reforms and the cells may divide
What happens in prophase 2?
New spindles are formed
What happens in metaphase 2?
The chromasomes line up along the equator
What happens in anaphase 2?
The centromeres break and chromatids are pulled to opposite sides of the cell
What happens in telophase 2?
The nuclear envelop reforms and the cell divides which gives rise to 4 haploid daughter cells
How is variation introduced in meiosis?
Independent assortment
Crossing over
How does independent assortment lead to genetic variation?
Chromasomes from the person’s parents are distributed randomally so that when they line up along the cell centre, chromasomes from any parent could be pull to either side of the cell
How does crossing over lead to genetic variation?
Enzymes cut and join different parts of enzymes together so the chromasomes each contain a mix of both maternal and paternal DNA
What is a translocation mutation?
When one part of the chromasome breaks off and joins to a competely different part of another chomasome that codes for something different
Like a bit of chromasome 4 joining to chromasome 13
What mutation results in leukaemia?
Translocations between chromasome 8 and 21
What mutation results in Burkitt’s lymphoma?
Translocations between chromasome 8 and 14
What is a non-disjunction mutation?
Members of a pair of chromasomes fail to seperate during meiosis so one cell has two copies of that chromasome and the other has no copies
What is monosomy?
Where only one member of a pair has chromasomes present in the cell
What is polysomy?
When a cell contains three or more rather than two chromasomes of a particular type
What is aneuploidy?
When a cell contains too few or too many chromasomes
What mutation results in Down’s syndrome?
Trisomy (3 copies) of chromasome 21
What mutation results in Turner’s syndrome?
Where there is just one X chromasome
What is gametogenesis?
The formation of gametes by meiosis in the sex organs
What is spermatogenesis?
The formation of spermatoza
What happens during spermatogenesis?
The diploid primordial germ cell divides by mitosis to form diploid spermatogonia
They grow until they’re big enough to be called primary spermatocytes
They undergo meiosis to form spermatids which differentiate to form spermatoza
What is oogenesis?
The formation of ova
What happens during oogenesis?
The diploid primordial germ cell divides by mitosis diploid oogonia
It undergos meiosis with the first division resulting in a polar body
The second division happens at fertalisation which results in another polar body and a haploid ovum
What are pollen sacks?
Part of the anther where pollen grains develop
What are the two typeso of fertalisation in animals?
External
Internal
What is pollination?
The transfer of pollen from the anther to the stigma, often from one flower to another
How does external fertalisation in animals take place?
Female and male gametes are shed into the environment where they fuse, most common in underwater species
How does internal fertalisation in animals take place?
Transfer of male gametes directly to the female
What is the acrosome region in sperm?
Part of the head that contains enzymes to break down the protective layers around the ovum
What is the zona pellucida in the ovum?
The layer of protective jelly around the unfertalised ovum
What happens when a sperm breaks through the zona pellucida?
The oocyte completes it’s second meiotic division and fuses with the male nucleus
The charge of the ovum changes so no other sperm can enter
Where is the male gametes in plants?
Pollen grain
Where does the pollen grain land on the flower during pollenation?
Stigma
What happens after the pollen grain lands on the stigma?
A pollen tube grows through the stigma and into the style
How does the pollen tube grow through the stigma and style?
It has hydrolytic enzymes
What is a totipotent cell?
A cell that can develop into all types of cells and can become an entirely new organism
What is the first stage of embryo development?
Cleavage
What is the cleavage stage of embryo development?
Cells divide repeatedly by mitosis without normal interphase for growth which creates a blastocyst (small group of identical undifferentiated cells)
What is a blastocyst?
small group of identical undifferentiated cells
What is a pluripotent cell?
A cell that can develop into almost all kinds of cells but not become a new organism
