Covers the entire section on Chromosomes, Mitosis and Meiosis. Includes notes from the textbook, as well as additional class, video and research information, diagrams and practice questions.
Applicable to all IEB Grade 12s.
Written by a 90% < student.
Strand 3 - life at molecular, cellular and tissue level (chromosomes, mitosis and meiosis)
December 10, 2020
13
2020/2021
Summary
Subjects
bio
biology
grade 12
ieb
life sciences
matric
matrix
chromosomes
mitosis and meiosis
strand 3
life at molecular
cellular and tissue level
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Chromosomes
= long, thread-like structures that form part of the chromatin network in the nuclei of cells.
(Made up of a strand of DNA wound around proteins called histones)
Chromosomes absorb dye easily and where therefore named coloured bodies.
They are however only visible when they are dividing.
In somatic cells:
- The number of chromosomes in each cell is the same
- Chromosomes are made up of two sets, one is a maternal chromosome from the mother and
the other is a paternal chromosome from the father (Therefore Diploid – 2n)
- Each paternal and maternal chromosome form a pair called a homologous pair
(these pairs only pair up in meiosis, not mitosis hence the somatic cells)
- The paternal and maternal chromosomes are equal in size and shape, have the same gene in
the same place, but may have a varying alleles for each
- The DNA of each chromosome replicates to form two identical threats called chromatids joined
by a centromere
During interphase
Maternal
Paternal Maternal
Paternal
Same gene on same chromosome
e.g. for red blood cell shape is gene
21 on chromosome 9 for everyone
Sequence of nucleotides that codes for
one specific characteristic
Crossing over
(leads to genetic diversity)
When homologous pairs are formed, the chromosomes can touch.
A gene segment from the maternal and paternal chromosome then breaks off and is exchanged
with the other.
Bivalent
the breaking point
Dominance and recession
Physical trait = phenotype
e.g. eye colour
B = brown
b = blue
B B B b b b
B B = homozygous dominant
B b = homozygous recessive Genotype
b b = heterozygous
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