Meiosis And Sexual Reproduction Flashcards
What are gametes?
Specialised cells containing haploid nuclei (half of the full chromosome number)
What are the sex organs in flowering plants?
Male = anthers (gametes are formed in the pollen which is produced in the anthers)
Female = ovaries (gametes are formed in the ovules which are contained in the ovaries)
What are the gonads in animals?
Male = testes which produce spermatozoa
Female = ovaries which produce ova
What is meiosis?
A reduction division which occurs only in the sex organs and in animals forms gametes
What does meiosis produce in plants?
Males = microspores
Females = megaspores
These then produce gametes
Steps of meiosis
Prophase 1 - Each chromosome appears in a condensed form with two chromatids. Homologous pairs of chromosomes associate with each other. Crossing over occurs.
Metaphase 1 - Spindle forms and pairs of chromosomes line up on the metaphase plate.
Anaphase 1 - Centromeres do not divide. One chromosome from each homologous pair moved to each end of the cell.
Telophase 1 - Nuclear membrane reforms and the cells begin to divide.
Prophase 2 - New spindles are formed.
Metaphase 2 - Chromosomes line up on the metaphase plate.
Anaphase 2 - Centromeres divide and the chromatids move to opposite ends of the cell
Telophase 2 - Nuclear envelopes reform, chromosomes return to their interphase state and cytokinesis occurs giving 4 daughter cells with half the chromosome number of the original diploid cell.
What is independent assortment (random assortment)?
The chromosomes that came from the individuals two parents are distributed into the gametes completely at random. (There are more than 8 million potential genetic combinations within the sperm or egg guaranteeing great variety in the gametes)
What is crossing over (recombination)?
Large multi enzyme complexes cut and join bits of the maternal and paternal chromatids together. The points where the two chromatids break are called chiasmata. This leads to genetic variation and also mutations which further results in new combinations of a species’ genetic makeup.
What are chromosome mutations?
During the process of meiosis when parts of the chromosome break off and become reattached in the wrong place. One of the most common chromosome mutation is called translocation.
What is non disjunction?
A mutation of whole chromosomes.
When one of the homologous pairs of chromosomes fail to separate in anaphase 2.
If one of these abnormal gametes is fertilised it can result in:
Monosomy (only 1 member of the homologous pair present from the normal gamete)
OR
Polysomy (3 or more rather than two chromosomes)
This is called aneuploidy.
Example of polysomy
Down syndrome
Can result in problems such as heart abnormalities, learning difficulties, lack of muscle tone and visual problems
Example of monosomy
Turner’s syndrome
Less life threatening but usually causes fertility problems
What is gametogenesis?
The process that forms gametes in plants
Primordial germ cells differentiate into these gametes
Spermatogenesis process
-Primordial germ cell divides by mitosis to form spermatogonia
-Spermatogonia grow to primary spermatocytes
-Primary spermatocytes undergo meiosis
First division = secondary spermatocytes
Second division = spermatids
-Spermatids differentiate into the tubules of the testes to form spermatozoa
Oogenesis process
-Primordial germ cells differentiate into divided by mitosis to form oogonia. Most oogonia simply degenerate and do not develop further. Only one grows and substantial amount of storage material go into the cell making it very large. This is known as the primary oocyte.
-Oocyte undergoes meiosis
First division = results in 2 cells of very unequal size. The larger is a secondary oocyte. The smaller sticks to the oocyte and is called the first polar body.
(What we call ova in the ovaries are really secondary oocytes)
AFTER FERTILISATION
Second division = secondary oocyte divides to form haploid ovum and another polar body whilst the first polar body divides to form two or more polar bodies. The polar body’s degenerate and die as soon as the ovum develops.
Characteristics of spermatozoa
Many, mini, motile
Acrosome - membrane bound storage site for enzymes that digests layers surrounding the ovum allowing the sperms head to penetrate
Nucleus - contains the highly condensed haploid chromosomes (condensed state of generic material reduces the amount of energy needed to transport it)
Mitochondria - tightly packed in the middle section of the sperm to provide ATP for the lashing of the tail
Microtubukes - produce whip like movements of the tail that keep the mature sperm in suspension and help it swim towards the ovum
Tail - propelled the sperm by its movement in a liquid environment
Length = 50-60 micrometers
Characteristics of ova
Few, fat, fixed
Zone pellucida - clear ‘jelly like’ layer
Haploid polar body - dies
Cell surface membrane of oocyte
Cytoplasm - has many food reserves
Diameter of oocyte = 100 micrometers
What are the two phases of flowering plants’ life cycle?
Sporophyte generation - diploid and produces spores by meiosis
Gametophyte generation - haploid and gives rise to gametes by mitosis
The main body of the plant we see is the diploid sporophyte
The haploid gametophytes are parts of the contents of the anther and the ovary
The formation of a pollen grain
-Pollen mother cell (microspore mother cell)
Meiosis
-4 haploid cells which each develop into a pollen grain
Mitosis
-Pollen grain (microspore)
Pollination
-Pollen grain on stigma
The formation of the egg cell
-Megaspore mother cell
Meiosis
-4 haploid cells (3 degenerate one grows)
-Megaspore (young embryo sac)
Mitosis x3
-Mature embryo sac
What is external fertilisation?
Occurs outside of the body where the female and male gametes shed directly into the environment where they meet and fuse (common only in aquatic animals)
What is internal fertilisation?
Involves the transfer of the male gametes directly into the female
Fertilisation in humans
-As sperm move through the female reproductive tract the acrosome region matures so it is able to release the enzymes and penetrate the ovum
-Many sperm cluster around the ovum and as soon as the heads of the sperm touch the surface of the ovum the acrosome reaction is triggered
-Enzymes are released from the acrosome which digest the follicle cells and the zona pellucida
-One sperm will make its way through the weakened protective barrier and touch the surface membrane of the oocyte
-Oocyte then completes its second meiotic division to produce a haploid egg nucleus to fuse with the haploid male nucleus
-Ion channels in the cell membrane open and close so the inside of the cell goes from negative to positive which blocks the entry of any further sperm
-Tough fertilisation membrane forms around the ovum (as cortical granules released from the oocyte combine with the zona pellucida) so charge returns to normal
-Sperms chromosomes are released forming a diploid zygote
Fertilisation in plants
-Pollen grain lands on the surface of the stigma during pollination
-Molecules on the surface of the pollen grain and the stigma interact
-If they’re the same species the pollen grain begins to germinate (grow)
-Pollen tube begins to grow out from the tube cell of the pollen grain through the stigma into the style
-Tip of the pollen tube produce hydrolytic enzymes to digest tissues of the style so the pollen tube can make its way down between cells (this tissue acts as a nutrient source)
-Generative cell containing generative nucleus travels down the pollen tube (the nucleus of this cell divides by mitosis as it moves down to form two male nuclei)
-Pollen tube grows through an ovary to reach an ovule and eventually the tip of the pollen tube passes through the micropyle of the ovule
-Two male nuclei are then passed into the ovule so fertilisation can occur
-Flowering plants undergo double fertilisation
-One male nucleus fuses with two polar nuclei to form endosperm nucleus (triploid)
-Other male nucleus fuses with egg cell to form diploid zygote
Post fertilisation
Fertilised egg is said to be totipotent (has the potential to form all of the different types of cell for an entire new person
Cleavage - cells divide repeatedly without normal interphase for growth between divisions. This produces a mass of small, identical, undifferentiated cells forming a hollow sphere known as a blastocyst.
-One large zygote cell forms a large number of small cells. These are known as embryonic stem cells.
-Blastocyst cells can form all types of or cells needed in the future but not tissue such as the placenta (they are known as pluripotent embryonic stem cells)
What is cell determination?
The pre destination of cells to become particular types of tissue from early in development of the embryo
