2.4 Flashcards
meiosis & sexual reproduction
what are the male sex organs in plants and where are they found
pollen
found in anthers
what are where are the female sex organs in plants
ovules
found in ovaries
what are the name of male and female gametes
spermatozoa
ova
define meiosis
cell division in which the chromosomes number of the original cell is halved leading to the formation of gametes
how are gametes produced in plants
meiosis forms male microspores and female megaspores which then produce gametes
what is crossing over (recombination)
large multi-enzyme complexes cut and rejoin parts of the maternal and paternal chromatids at the end of prophase 1
summarize meiosis (prophase 1 -> telophase 2)
P1 - condensed chromatids, crossing over occurs
M1 - spindle fibers, metaphase plate
A1 - poles of cell, centromeres don’t divide
T1 - nuclear membrane reforms
P2 - spindle fibers form
M2 - metaphase plate
A2 - centromeres now divide
T2 - nuclear membrane reforms
what is a chiasmata
the points where the chromatids break during recombination
what is a polysomy
when a cell or an organism has more than 2 sets of chromosomes
what are gonads
sex organs in animals
define fertilization
the fusing of the haploid nuclei from two gametes to form a diploid zygote in sexual reproduction
what is a zygote
the cell formed when two haploid gametes fuse at fertilization
what is independent assortment
the process by which the chromosomes from the male and female parent are distributed at random to the offspring
what is translocation
a mutation in which part of one chromosome breaks off and rejoins another chromosome, it may be balanced or unbalanced
what is non-disjunction
when members of a pair of chromosomes fail to separate during the reduction division of meiosis resulting in one gamete with two copies of a chromosome and one gamete with no copies of that chromosome
what is a monosomy
when only one member of a pair of chromosomes is present in a cell
what is an aneuploidy
when a cell contains too few or too many chromosomes
what are mutagens
chemicals known to increase the rate of mutation
what disease is caused by trisomy (3 copies of) chromosome 21
down syndrome
what is turners syndrome
when an individual only has 1 X chromosome
what is Kleinfelder’s syndrome
when an individual has 3 sex chromosomes XXY
what is gametogenesis
the formation of gametes by meiosis in the sex organs
what are primordial germ cells
cells which divide by meiosis to form sperm and ova
define spermatogenesis
formation of sperm cells in the testes
define oogenesis
formation of ova in the ovaries
explain the steps in spermatogenesis
diploid primordial germ cell in testes divides by mitosis to form diploid spermatogonia
spermatogonia then grow into primary spermatocytes
primary spermatocytes undergo meiosis to form 2 haploid secondary spermatocytes
these then undergo a second meiosis division to form 4 haploid spermatids
spermatids then differentiate to form spermatozoa
explain the steps in oogenesis
diploid primordial germ cell divides by mitosis to form diploid oogonia (usually many are produced and only 1 survives)
the oogonia then grows into a primary oocyte
the primary oocyte undergoes its first meiosis to form a secondary oocyte and a polar body
the oocyte does not divide until after ovulation now
the second meiotic division occurs after fertilization and the secondary oocyte forms a haploid ovum and a polar body, then the first polar body divides into two more polar bodies
how many sperm are formed in spermatogenesis
many
how many ovum are formed in oogenesis
1
what happens to the polar bodies formed in oogenesis
degenerate and die as the ovum develops
roughly what size are spermatozoa
50micrometers
roughly what size are ovum
0.1mm
what features are present on a spermatozoa and why
acrosome - contains enzymes for penetration of ova
haploid nucleus
mitochondria - for energy
flagellum - for movement
microtubules - produce whip like movements
what features are present on a ovum
cytoplasm - contains nutrients
polar bodies
zona pellucida - jelly coat protects the cell
corona radiata - protects ovum
what are the 2 phases of gametogenesis in plants
sporophyte generation
gametophyte generation
what is sporophyte generation
the diploid generation in plants that produce spores by meiosis
what is gametophyte generation
the haploid generation in plants that gives rise to the gametes by mitosis
where are pollen grains developed
pollen sacs
explain pollen formation
microspore mother cell undergoes 2 meiosis to produce 4 haploid cells
these cells then undergo mitosis to produce a pollen tube nucleus and a generative nucleus both encased in the same cell wall
when pollination occurs 2 male gametes are produced from the generative nucleus
what is the structure of a mature embryo sac
3 antipodal cells
2 polar nuclei
female gamete
2 synergids
explain the formation of a mature embryo sac
diploid megaspore mother cell undergoes 2 meiosis to produce 4 haploid megaspores (of which only 1 survives) the 1 then undergoes 3 mitotic divisions which result in the formation of an embryo sac containing 3 antipodal cells, 2 polar nuclei, a female gamete and 2 synergids
define pollination
the transfer of pollen from the anther to the sigma often from one flower to another
state 2 ways pollination can occur
wind
transfer via other organisms
what is external feralization
the process of fertilization in which the male and female gametes are released outside of the body to meet and fuse in the environment
what is internal fertilisation
the fertilisation of the female gamete by the male gamete which takes place in the body of the mother
what is the acrosome
the region at the head of the sperm that contains enzymes to break down the protective layer around the ovum
what is the zona pellucida
layer of protective jelly around the unfertilized ovum
why do many sperm need to reach the ovum
as one sperms enzymes are not enough to penetrate the protective layer around the ovum
summarise the steps of fertilisation in mammals
sperm reach the ovum and begin to digest the protective barrier
one sperm reaches the surface membrane of the oocyte triggering the second mitotic division of the oocyte to produce a haploid egg and ion channels change so that the cell becomes positively charged stopping the entry of further sperm
then a fertilisation membrane will form when cortical granules are released and take on this role so that the charge can return to normal
the head of the sperm fuses with the oocyte membrane causing the sperm nucleus to be released into the oocyte where it rapidly decondenses and the male and female chromosomes group together
how do plants avoid self-fertilisation
only germinate when the pollen grain is recognized as the same species but a different plant or it begins to germinate however won’t be able to penetrate the carpel
summarise the steps in fertilisation in plants
pollen grain lands on surface of sigma
pollen grain germinates and a pollen tube begins to form through the sigma into the style as the tip produced hydrolytic enzymes which digest the tissue and use it as a nutrient source
as the pollen tube grows the generative nucleus travels down and and divides by mitosis to produce 2 male nuclei
the pollen tube grows through the ovary and tip passes through the micropyle of ovary where the male nuclei are passed on for fertilisation
what is double fertilisation
one male nucleus fuses with 2 polar nuclei to form an endosperm and the other male nucleus fuses with the egg cell to form the diploid zygote
what is an endosperm nucleus needed for
supplies embryo plant with food
define totipotent
a cell is able to develop into all different cell type
define pluripotent
a cell is able to develop into most different cell types
what is a cleavage
a process involving a special type of mitosis with no interphase that results in a mass of small undifferentiated cells
what occurs after fertilization
cleavage occurs forming a morula then a blastocyst as the embryo travels down the oviduct to the uterus
when it reaches the uterus the blastocyst hatches and implants into the lining of the uterus
what is cell determination
the predestination of cells to become particular types of tissue from early in development of the embryo
define differentiation
to develop into specific types of tissues
