IB Biology - 3.3 Meiosis

What does meiosis results in?

-Meiosis is a reduction division
-after Meiosis 1, 2 haploid cells
-after Meiosis 2, 4 haploid cells

Why is the halving of chromosomes important in sexual reproduction?

-otherwise, total chromosome no. would double each generation
-in order to reproduce the two fusing gametes have to be haploid to start dividing to form life

Sister chromatid

-during interphase of meiosis DNA is replicated
-one chromosome consists of two sister chromatids
-In meiosis 2, the DNA isnt replicated again, the sister chromatids are separated

Bivalent

Homologous chromosomes that each consist of sister chromatids

Synapsis

-The pairing of a bivalent

Crossing over

-creates genetic distinct gametes
-where a junction is created in which one chromatid in each of the homologous chromosomes breaks and rejoins with the other chromatid
-random
-one crossover occurs in each bivalent
-occurs in prophase 1

Random orientation

-when homologous chromosomes line up in metaphase 1, their orientation towards the opposing pole is random

Attachment between spindles and chromosomes differs between mitosis and meiosis:

-chromosomes in bivalent attached to different poles
-the pole to which each chromosome is attached depends on random orientation
-each chromosome has an equal chance of being attached to each pole

Three main sources of genetic variation:

-Crossing over (prophase 1)
-Random orientation (metaphase 1)
-random fertilisation

Random fertilisation:

-because of crossing over and random orientation, the vast majority of gametes are genetically distinct and random fertilisation by egg and sperm will always generate different zygotes

Non-disjunction

When homologous chromosomes fail to separate during anaphase. Both chromosomes move to one pole and neither to the other

Consequences of non-disjunction

-gametes that either have an extra or deficient chromosome
-if occurs in human fertilisation, will result in individual with either 45 or 47 chromosomes
-causes trisomys and monosomys

Down Syndrome

-trisomy 21
-individuals may suffer hearing loss, heart and vision disorders, and mental and growth impairments

Klinefelter's Syndrome

-Having sex chromosomes XXY

Turner's Syndrome

-caused by having only one sex chromosome, an X

Amniocentesis

The passing of a needle through the women's abdomen wall using ultrasound, to withdraw a sample of amniotic fluid containing fetal cells. Miscarriage risk of 1%

Chorionic Villus Sampling

A sampling tool that enters through the vagina in order to obtain cells from the chorion, one of the membranes from which the placenta develops. Can be done earlier in the pregnancy however has a miscarriage risk of 2%.

Prophase 1

-Cell has 2n
-Homologous Chromosomes pair
-Crossing over

Metaphase 1

-Spindles move chromosomes to equator
-random orientation

Anaphase 1

-Homologous pairs are separated

Telophase 1

-Reduction from 2n to n
-Cytokinesis

Prophase 2

-Chromosomes, which still consist of two chromatids, condense

Metaphase 2

Anaphase 2

-Centromeres separate and chromatids are moved to opposite poles

Telophase 2

-Chromatids reach opposite poles
-nuclear envelope forms
-cytokinesis