2.4 meiosis and sexual reproduction Flashcards
why is it that many organisms that are able to reproduce asexually, are also able to reproduce sexually
most use sexual reproduction as a back up to be used when living conditions are tough to introduce genetic variation that enable the population or species to survive
what is sexual reproduction
the production of a new individual from the joining of two specialised cells called gametes, and the individuals that are produces are not genetically the same as one parent
disadvantage of sexual reproduction
-more expensive in terms of bodily resources
-relies on two gametes meeting and fusing and it’s not always easy to find a mate
advantage of sexual reproduction
increases genetic variation as a result of the fusion of two gametes, in a changing environment this gives a greater chance that some of the offspring will survive and go on to reproduce
what are gametes
cells that contain haploid nuclei, with one set of chromosomes that fuse to form a diploid cell- a zygote- in the process of fertilisation
where are the male and female gametes formed in flowering plants and what are they
male sex organs - anthers, where pollen is produced and haploid nuclei are inside the pollen grain
female sex organs - ovaries that form ovules which is where the haploid nuclei are
what are gonads
permanent structures in animals that are sex organs
what is meiosis
reduction cell division that only occurs in the sex organs, so form gametes with haploid nuclei
what does meiosis produce in flowering plants
-in females it produces megaspores
-in males it produces microspores
these both go on to form gametes
describe the first half of meiosis
P1 chromosomes appear condensed, homologous pairs associate with each other and crossing over occurs
M1 nuclear membrane breaks down, spindle forms and the pair of chromosomes line up on the metaphase plate
A1 one chromosome from each homologous pair moves to each end of the cell
T1 nuclear membrane re forms, cells begin to divide
describe the second half of meiosis
P2 new spindle is formed
M2 nuclear membrane breaks down and chromosomes line up on the metaphase plate
A2 centromeres divide, 2 chromosomes are now 4 chromatids and move to opposite ends of the cell
T2 cytokinesis nuclear envelope re forms giving 4 daughter cells and chromosomes return to their interphase state
why is meiosis important
-because in induces genetic variation in two ways
1. independent assortment, the parental chromosomes are randomly distributed into the gametes
2. crossing over (recombination), in large multi enzyme complexes, parts pf chromatids are are cut and joined together
describe recombination
its the exchange of genetic information between chromatids, which leads to genetic variation, and errors lead to mutation
describe chromosome mutation
when parts of chromosomes break off and become reattached in the wrong place.
what is translocation in chromosome mutation, and mention the severity
when one piece of a homologous pair of chromosome breaks off and re attached to a completely different pair
non balanced translocations are much more severe than balanced
describe non disjunctions
during reduction division of meiosis, one of the members of the homologous pair fail to separate during anaphase 2, so one gamete has two copies of a chromosome and another has no copies
what is monosomy and what is polysomy
when an abnormal gamete joins with a normal gamete
monosomy is when only one member of the homologous pair of chromosomes is present
polysomy is when three or more members of homologous chromosomes are present
what is aneuploidy
when a cell lacks a whole chromosome or has more than two of a chromosome
when does gametogenesis take place in female and male mammals
- in males it happens constantly from puberty
- in females the mitotic division takes place before birth and the meiotic division takes place in each monthly cycle from puberty to menopause, and is only fully completed when fertilised
describe the process of spermatogenesis
- diploid primordial germ cell divides by mitosis to form spermatogonia
-the spermatogonia grow into primary spermatocytes
-which undergo meiosis, first division results in two haploid secondary spermatocytes
-second division results in 4 haploid spermatids
-these differentiate in the tubules of the testes, mature to become gametes
describe the process of oogenesis
-diploid primordial germ cell divides by mitosis to produce oogonia, primary oocyte
-which undergoes meiosis, first division results in cells of unequal size
-large one is secondary oocyte, smaller is polar body
-second meiotic division is only completed after meiosis occurs to form haploid ovum and polar body
-first polar body divides to form two more polar bodies
describe spermatozoa
-many, mini, motile
they are the male gametes of most mammals
remain in suspension in the semen so they can be transported through the female reproductive tract, and penetrate the ovum to be able to safely transport the male haploid genome
describe the ovum
much larger than sperm and vary in size across different mammals
they do not move on their own and contain food for the developing embryo
what is an acrosome and where is it found
membrane bound storage site for enzymes that digest the layers around the ovum, found at the tip of the head of the spermatozoa
what is a nucleus in a spermatozoa and where is it found
found in the head and contains very condensed, so less energy is needed for them to be transported- haploid chromosomes
what is the function of microtubules in a spermatozoa
produce whip like movements that keep the sperm in suspension and help it swim towards the ova
what is the job of the tail in spermatozoa
propels the sperm, helps it swim in a liquid environment
draw and label an ova
haploid chromosomes in the centre
cytoplasm with food reserves
cell surface membrane polar body
zona pellucida
draw and label a spermatozoa
-acrosome
-nucleus
-mitochondria
-microtubules
-tail/ flagellum
what are the two phases of the formation of gametes in flowering plants
1, sporophyte generation which is diploid and produces haploid spores by meiosis
2, gametophyte generation which is the result of the sporophyte and gives rise to gametes by meiosis
describe how pollen is formed in plants
meiosis occurs in anthers to form pollen grains with male gamete inside - all inside pollen sacs
microspore mother cells divide by meiosis to form haploid microspores which divide by mitosis to produce gametes
what nuclei are in the gametes of flowering plants and state their functions
-haploid tube nuclei, produces a pollen tube that penetrates stigma, style and ovary into the ovule
-haploid generative nucleus which fuses with the ovule nucleus
how are eggs formed in plants
diploid megaspores divide by meiosis to form haploid megaspores
-three degenerate and one continues to develop
-it undergoes mitotic division that results in an embryo sac containing an egg cell and two polar nuclei
define pollination
the male gamete being transferred to the female gamete in plants
describe the process of external fertilisation
happens outside the body, is when gametes shed directly into the environment and meet and fuse
-matter of chance
-some species synchronise their shedding, or shed as a response to environmental cues
describe the process of internal fertilisation
the transfer of the male gamete directly to the female
the way in which the sperm is transferred varies greatly
what happens to the acrosome as the sperm moves through the female reproductive tract
the acrosome region matures and is able to release enzymes that penetrate the ovum
why is fertilisation in humans only a matter if chance
because it requires the gametes to meet and fuse
-sperm survive for 1-2 days in the female reproductive tract and the ovum is only fully viable for a few hours
what is zona-pellucida
protective jelly like layer around the ovum `
what is zona-pellucida
protective jelly like layer around the ovum `
what happens when the sperm reach the ovum
the ovum is really a secondary oocyte because meiosis isn’t fully completed
large number of sperm cluster around ovum, as their heads touch it, acrosome reaction triggered
-enzymes released, digest zona-pellucida
what happens after the sperm have digested the zona pellucida
-one sperm eventually wriggles through weakened protective barriers
-touches oocyte surface membrane
-meiosis completes
-haploid egg fuses with haploid male nucleus
why can only one sperm enter the ovum
when one enters, ion channels in the cell membrane of the ovum cause it to become positive
-temporary measure until fertilisation membrane forms, which repels other sperm
what happens when the sperm enters the sperm enters the oocyte
the head of the sperm enters but the tail is left outside, the sperm head absorbs water and swells which releases its chromosomes to fuse with the ovum
how do seeds germinate
pollen gran lands on the stigma of the flower, they interact and if they recognise that they are of the same species then pollen grain germinates and plant will begin to grow
how does fertilisation happen in plants
tip of the pollen tube produces enzymes that digest the tissue of the style, grows towards ovary
goes through stigma, then style, generative nucleus travels down it
nucleus divides by mitosis
pollen tube grown through ovary, reaches ovule
male nuclei passed through
what is double fertilisation
occurs in flowering plants
one male nucleus fuses with two polar bodies to form endosperm nucleus, which provides plant with food
other nucleus fuses with egg cell to form diploid zygote
what is cleavage
the first stage of what happens after fertilisation
cells divide repeatedly by mitosis without interphase growth between divisions, and form small identical undifferentiated cells which form a hollow sphere - blastocyst
describe the different types of cells in an embryo and blastocyst
-earliest cells are totipotent
-in the blastocyst, the outer layer of cells forms the placenta and the inner ones are pluripotent
describe cell determination and state when it occurs
occurs days after conception, cells are already predetermined to become a certain type of tissue
closely linked with the position of cells in the embryo
