Meiosis Flashcards
Why does meiosis happen?
-When gametes fuse at fertilisation to form a zygote, the diploid number is restored which enables a constant chromosome number is maintained from generation to generation
-Introduces variation into the genetic material
What is monosomy?
A sex cell which is missing one chromosome
What is polysomy?
A condition where there are more chromosomes than required
What is aneuploidy?
An abnormal number of chromosomes
What is turners syndrome?
A genetic condition
-Only affects females
-Only have one X chromosome
-They are missing a X chromosome
-Caused by monosomy
What is down syndrome?
A genetic condition
-Caused by having an extra chromosome 21
-Caused by polysomy
What is translocation?
-Take place during meiosis
-The chromosomes can be balanced or unbalanced
-They can change the phenotype or make it so they’re incompatible with life
-Can cause cancers e.g Burkitt’s Lymphoma which is cancer of the white blood cells
What is nondisjunction?
-Can affect the whole chromosomes
-Happens when the homologous pair fails to seperate during meiosis and there may be 2 or 0 gametes produced
-When an abnormal and normal gamete join it forms aneuploidy
What happens in prophase 1?
Each chromosome is condensed into the form of two chromatids, homologous pairs join and crossing over happens
What happens in metaphase 1?
The spindle fibres form and the pairs of chromosomes line up on the metaphase plate
What happens in anaphase 1?
The centromeres don’t divide and one chromosome from each pair moves to the end of the cell, the chromosome number in the cell is now half of the original
What happens in telophase 1?
The nuclear membrane re forms and the cells start to divide
What happens in prophase 2?
New spindle fibres are made
What happens in metaphase 2?
The chromosomes still made up with pairs of chromatids line up on the metaphase plate
What happens in anaphase 2?
The centromeres now divide and the chromatids move to opposite ends of the cells
What happens in telophase 2?
The nuclear envelope re forms and chromosomes return to interphase and cytokinesis starts
How does gametogenesis start?
Primordial germ cells in the gonads divide, grow and divide again and then differentiate to form gametes
What are the steps of sperm production?
-Diploid germ cell divides by mitosis and produces spermatagonia
-The spermatagonia grow until they become primary spermatocytes
-The spermatocytes divide by meosis to give two haploid secondary spermatocytes
-The secondary spermatocytes undergo meiosis again to form four haploid spermatids
-Spermatids then differentiate into spermatozoa which are capable of fertilisation
What is the structure of sperm?
-An acrosome = digestive enzymes to digest outer layer of the ovum
-Haploid nucleus = highly condensed chromosomes
-Mitochondria = release energy for the tail to move
-Tail = contains microtubules which move the tail and keep the sperm in suspension
What are the steps of egg production?
-The diploid germ cell divides by mitosis to form diploid oogonia
-The oogonium grows to form a diploid primary oocyte
-The primary oocyte undergoes meiosis to form a haploid secondary oocyte called ova and a first polar body
-Ovulation occurs
-The second division doesn’t occur until fertilisation
-The secondary oocyte divides by meiosis again to form an ovum and a second polar body
-The polar bodies then disintergrate
What is the structure of the egg?
-Haploid nucleus
-Has cytoplasm with lots of food reserves
-A corona radiata which forms part of the protective barrier of the ovum
-A jelly coating for protection
-Zona pellucida protects the egg by hardening after fertilisation
What is internal fertilisation?
Where the male gametes are transferred directly to the female
What is external fertilisation?
Occurs outside the body and the gametes are released directly to the environment
Where does fertilisation in the female happen?
The oviduct
What are the stages for embryo development?
-Zygote is totipotent and can form all 216 cell types needed
-A type of mitosis (cleavage) happens where the cell divide rapidly with no interphase
-A blastocyst forms which is a mass of undifferentiated cells
-The cells become pluripotent and can differentiate into most cell types
-Outer layer of the blastocyst cells form the placenta
-Blastocyst breaks free and plants into the uterus
