2.4-meiosis and sexual reproduction Flashcards
meiosis- interphase 1
-chromatin not visible
-DNA and proteins are replicated
meiosis- prophase 1
-chromosomes visible
-homologous chromosomes join to form bivalent, chromatids cross over (chiasmata)
meiosis- metaphase 1
bivalents line up along equator
meiosis- anaphase 1
-chromosomes separate
centromere doesn’t split
meiosis- telophase 1
-nuclei form, cell divides, cells have 2 chromosomes, 4 chromatids
aims of meiosis 1
1.to create genetic variation
2.to halve the number of chromosomes
products of meiosis 1
two haploid cells
meiosis- interphase 2
-short
-no DNA replication
-chromosomes remain visible
meiosis- prophase 2
-centrioles replicate and move to new poles
meiosis- metaphase 2
-chromosomes line up along equator
meiosis- anaphase 2
-centromeres split, chromatids separate
meiosis- telophase 2
-4 haploid cells, each with 2 chromatids
products of meiosis 2
4 haploid daughter cells , genetically different
importance of meiosis- crossing over
-takes place when large multi enzyme complexes ‘cut and join’ bits of maternal and paternal chromatids together
-points where chromatids break are called chiasmata, they are important in 2 ways;
1.exchange of genetic material, leading to variation
2.errors in process lead to mutation, introducing new combinations into genetic makeup
importance of meiosis- independent assortment
-chromosomes that came from parents are distributed into gametes and therefore offspring, randomly
-e.g. each gamete receives 23 chromosomes, in each gamete any number from 0-23 could come from maternal or paternal
-this alone guarantees great variety
independent assortment and crossing over diagrams
look at flashacrds
what is a mutation?
- a change in the DNA base sequence which results in a new and different allele
gene mutations- types
- addition or deletion of a base- causes a frame shift
- substitution- causes a change in sequence but only of one area, no frame shift
chromosome mutations- translocations
-take place during meiosis
-some change the phenotype completely or make it incompatible with life
-translocations between certain chromosomes can make you predisposed to cancer e.g. burkitts lymphoma which is a cancer of white blood cells in young people
what happens in translocations
-chromosome from one pair becomes attached to a chromosome from another pair
OR
-parts of 2 chromosomes break off and attach to different chromosomes
chromosome mutations- non-disjunction
-affects whole chromosomes
-homologous pairs fail to separate, so gamete ends up with 2 copies of that chromosome or non at all
-if one of these abnormal gametes join with a normal one, results in monosomy or polysomy
2 types of aneuploidy
1.monosomy- sex cells missing one chromosome e.g. turners syndrome
2.polysomy- more chromosomes than required e.g. down syndrome
early stages of the development of the embryo (6)
1.zygote is totipotent(can form 216 cell types)
2.cleavege-type of mitosis, cells divide rapidly with no interphase- happens as egg travels down oviduct
3.blastocyst (mass of undifferentiated cells) as the zygote moves from oviduct to uterus (5-6days)
4.cells are pluripotent embryonic stem cells (can form most, not placenta)
5.outer layer of blastocyst cells form placenta
6.blastocyst breaks free of outer layer and implants in uterus
5 stages of spermatogenesis
1.diploid(46) germ cell divides by mitosis to give spermatogonia
2.spermatogonia then grow until considered primary spermatocytes
3.spermatocytes divide by meiosis to give 2 haploid (23)secondary spermatocytes
4.secondary spermatocytes undergo meiosis again to form 4 haploid spermatids
5.spermatids differentiate into spermatozoa capable of fertilisation
mini, many, motile
products of spermatogenesis
-4 haploid spermatozoa, capable of fertilisation
7 stages of oogenesis
1.diploid germ cell divides by mitosis to form diploid oogonia
2.oogonium continues to grow to form diploid primary oocyte
3.the primary oocyte undergoes meiosis to form a haploid secondary oocyte (ova) and much smaller cell (first polar body)
4.ovulation occurs
5.second meiotic division does not occur until fertilisation has happened
6.haploid secondary oocyte divides by meiosis again to form ovum and another polar body
7. polar body disintergrates
few, fat, fixed
product of oogenesis
haploid ovum and polar body
fertilisation in humans (mammals)- summary
1.sperm meet oocyte in oviduct, acrosomes have matured since sperm left male reproductive tract
2.sperm touch zona pellucida, triggering acrosome reaction
3.digestive enzymes pour out acrosome and digest zona pellucida, projections of sperm surface shoot forward due to actin like proteins
4.fertillisation occur when sperm touch oocyte and membranes fuse
5.sperm nucleus decondenses, releases chromosomes into oocyte, meiosis completed in oocyte forming ovum
6.m+f chromosomes roup together, corticle granules from oocyte combine with zona pellucida to form fertilisation membrane
formation of pollen grain
-formed in anther
-meiosis of diploid microspore cells in anther form 4 haploid microspores
-haploid microspores mature, into pollen grains via mitosis
formation of embryo sac in plants
-mitosis of diploid megaspore cells in ovule form one ootid and 3 polar bodies
-polar bodies degenerate and reabsorbed
-after 3 mitotic divisions, ootid forms embryo sac
fertilisation in plants
-generative nucleus divides to form two male gametes by mitosis
-pollen tubes fuse with embryo sac
-double fertilisation occurs
-one male nucleus fuses with egg cell nucleus to produce diploid zygote
-other male nucleus fuses with 2 polar nuclei to produce triploid endosperm
