Biology SL chapter 4.2 (Meiosis)

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What is meiosis?
Production of sex cells (gametes). Most sexually reproducing animals are diploid (two copes of every chromosome). In order to reproduce, these organisms need to make gametes that are haploid (one copy of each chromosome)
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The two divisions of meitosis
The first division is a reduction division of the diploid nucleus to for haploid nuclei. The second division separates sister chromatids.
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Define homologous chromosomes
Chromosomes that share same structural features (size, banding pattern, centromere position) and the same genes at the loci position (genes are same, alleles may be different).
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Outline meiosis
The process involves two divisions, both of which follow the same basic stages as mitosis (PMAT). Meiosis is preceded by interphase which includes replication of DNA to create chromosomes with identical sister chromatids
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Meiosis 1 pt1
Homologous chromosomes pair up to be separated into separate haploid daughter cells. In prophase 1, the chromosomes undergo synapsis where they pair up to form a bivalent. held together at points called chiasma. Crossing over happens here
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Meiosis 1 pt2
The remainder of meiosis involves separating the homologous chromosomes into daughter cells. Metaphase 1:homologous pairs line up along equator. Anapahse 1: chromosomes split apart and move to opposite poles. Telphase1: cell splits into two haploids
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Meiosis 2
Prophase 2: splindle fibres reform and reconnect to chromosomes. Metaphase2: chromosomes line up along equator. anaphase 2: sister chromatids split apart. Telphase2: cell splits in 2 as cytokinesis happens.
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Result of meiosis
Due to crossing over, sister chromatids might no longer be genetically identical. The process of meiosis results in the formation of four genetically distinct haploid cells.
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Happens when chromosomes fail to separate correctly, resulting in gametes with one extra or one missing chromosome. This happens during anaphase 1 or 2.
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Cause of down syndrome
One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction. The other parental gamete was normal with single copy of chromosome 21. When the 2 gametes fused during fertilisation, the zygote has 3 copies of chromosome21
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Visual profile of all the chromosomes in a cell. chromosomes are arranged into homologous pairs and displayed according to structural characteristics.
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How karyotyping works
harvesting cells (from foetus)then chemically induce cell division, halting it during mitosis when chromosomes are condensed and visible. then chromosomes are stained and photographed and added to structure.
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What is karyotyping used for?
Determine gender of an unborn child by looking at sex chromosomes or test for chromosomal abnormalities.
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How to get chromosomes for karyotyping
1. Aminocentesis: needle inserted through abdominal wall and amniotic fluid sample is taken.(16th week of pregnancy, 0.5% chance of miscarriage).2.Chorionic villus sampling:tube inserted in cervix and sample of chorionic villi from placenta is taken
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Card 2


The two divisions of meitosis


The first division is a reduction division of the diploid nucleus to for haploid nuclei. The second division separates sister chromatids.

Card 3


Define homologous chromosomes


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Card 4


Outline meiosis


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Card 5


Meiosis 1 pt1


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