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
