Meiosis Flashcards
The two key processes are…
-Formation of haploid sex cells called gametes(sperm and egg)
-Fertilization, when the sec cells join to form a zygote(cells made by fusion of gametes)
In animals, the reproductive organs that sex cells are
Testes and Ovaries
Ovum = 1 egg
Ova = more than 1 egg
Sexual reproduction relies on Meiosis
what is meiosis
It is a two stage cell division in which the resulting daughter cells have half the number of chromosomes (haploid)
Meiosis results in the formation of gametes
The number of chromosomes in a gamete is called the
haploid number(n)
The number of chromosomes in body cells is twice the haploid number called…
diploid
We get 23 chromosomes from mom and 23 from dad
Fertilization occurs when..
a haploid sperm cell (n=23) unites with a haploid egg cell (n=23) to form a diploid zygote(2n=46)
Haploid
the number of chromosomes in a sec cell
Diploid
the number of chromosomes in somatic cells
Somatic cells:
every other cell except sex cells
Homologous chromosomes
- carry information for the same genes and we recieve one from each parent
-similar in size and shape - exception the sex genes- X and Y chromosomes
Stages of Meiosis
- Involves 2 stages that make 4 haploid cells
- Meiosis 1: interphase, prophase 1, metaphase 1, anaphase 1, telophase 1
- Meiosis 2: prophase 2, metaphase 2, anaphase 2, telophase 2
Meiosis 1
Interphase: During interphase of sex cells, DNA is replicated to produce a texrad(4 copies) = a pair of homologous chromosomes consisting of sister chromatids
Prophase 1:
- Chromosomes shorten and thicken
- During prophase 1 “crossing over” takes place while the tetrad are together which mixed generic material
- each sister chromatid intertwines with a sister chromatid from its matching homologous chromosome in a process called synapse
- nuclear membrane starts to dissolve
- spindle fibres start to form and centrioles move to ends of the cell
Metaphase 1:
-Spindle fibres attach to centromeres of the chromosomes
- Homologous chromosomes (tetrads) line up along the equator of the cell
Anaphase 1:
- Spindle fibres begin to pull the tetrads apart
- Homologous pairs separate and move to opposite poles of the cell
Telophase 1:
- Homologous pairs are now at opposite poles of the cell
- The cell starts to divide into 2 (pinches in)
- Spindle fibres disappear
- Nuclear membrane forms around the sister chromatids
- Each daughter nucleus only receives 1 member of the original chromosome pair
Meiosis 2
Prophase 2:
-Note there is no DNA duplication between meiosis 1 and 2
- Each new cell now has a total of 46 chromosomes
- Spindle fibres begin to form again and the nuclear membrane breaks down
Metaphase 2:
-Sister chromatids line up along the equator of the cell
Anaphase 2:
-Sister chromatids begin to separate by moving to opposite poles
Telophase 2:
- Cells begin to pinch in
- Spindle fibres disappear
- Chromosomes finish moving to opposite poles and the nuclear membrane reforms
- The chromosomes turn into chromatin
Cytokinesis:
- Produced 4 cells, each containing 23 chromosomes(haploids)
Random assortment of homologous chromosomes
The chromosomes in each pair om are assorted independently, a sex cell can receive either chromosome of each homologous pair
Variability
- The number of possible combinations of chromosomes depends on the chromosome pairs, for diploid(2n) organisms the number of possible combinations is 2”
- Tgis variation does not include added variation from crossing over - this is why we are so unique
Gametogenesis
-Production of gametes(sex cells) in animals
-Spermatogenesis(produces sperm) and oogenesis(produces ova)
- In spermatogensis, cytoplasm is divided evenly resulting in 4 equal sized sperm cells
- Each sperm is small in size and streamlined
- In oogenesis, note that cytoplasm does not divide evenly
- Cells that don’t receive enough cytoplasm are called polar bodied and they die
- One cell(ovum) is produced
Sex chromosomes and sex determination
Sex chromosomes are chromosomes that differ in males and females of the same species
Female: XX
Make: XY
Non-disjunction
-Non-disjunction: homologous chromosomes fail to separate during meiosis
-Result: One cell has an extra chromosome, one cell is kissing a chromosome (gametes have either 22 or 24)
-If a gamete with 24 chromosomes, combined with one with 23 chromosomes, the zygote will have 47 chromosomes
Trisomy: a case where there are 3 homologous chromosomes instead of a pair ( Ex: Down syndrome)
Monosomy: a case where there is a single chromosome in place of a homologous pair ( Ex: Turner syndrome)
Diagnosing Non-disjunction
• Karyotypes: A picture of all chromosomes arranged by size, number and shape
- Humans have 22 pairs of autosomes(all chromosomes except sex) and 1 pair of sex chromosomes(2 in total)
• Prenatal testing: Sampling fetal cells
Example of non-disjunction disorders:
Turner syndrome: Only 1 X chromosome (only effects females)
Klinefelter syndrome: Two X chromosomes and one Y chromosomes (only effects male)
Patau syndrome: Trisomy of chromosome 13
Edward’s syndrome: Trisomy of chromosome 18
