Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
Biology (9700) notes A-Level - Inheritance (CIE)
Passage of information from parents to offspring
● Haploid(n) and diploid(2n): Haploid is one complete set of
chromosomes and diploid is two complete set of chromosomes.
● Haploid → gametes in sexual reproduction. Diploid → Contain
the DNA necessary for protein synthesis and cell function.
● Chromosomes have fixed length, position of centromere in
A2
particular location.
● Homologous pair of chromosomes: Carry the same genes in the
same position and same shape.
● During fertilization the nuclei of gametes fuse together to
s
form the nucleus of the zygote. Meiosis produces haploid
am
gametes during sexual reproduction. The first cell division of
meiosis is reduction division ( chromosomes from 46 → 23).
Reduction in chromosome number during meiosis ensures the
Ex
gamete formed are haploid.
● Behaviour of chromosomes during meiosis:
● Prophase I → DNA condenses, becomes visible as
ck
chromosomes. DNA replication done → each chromosome
consists of two sister chromatids joined by centromeres.
Homologous pair. Crossing over of non-sister chromatids may
ra
occur(point at which it occurs → chiasma). Centrioles migrate
to opposite poles, spindle formed. The nuclear envelope breaks
C
down and the nucleolus disintegerates.
● Metaphase I → bivalents line up along the equator of the
spindle, spindle fibres attached to centromere.
● Prophase I → Homologous pair of chromosomes as
microtubules pull whole chromosomes to opposite ends.
CRACK EXAMS A2 1
, Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
● Telephase I → chromosomes move to opposite poles, spindle
fibres start to break down, nuclear envelope form around the
two groups of chromosomes and nuclei reform.
● Cytokeinesis → division of cytoplasm
● Prophase II → nuclear envelope break, spindle form 90
degrees to old, chromosomes condense
● Metaphase II → chromosomes line up in a single line along the
equator of the spindle.
A2
● Anaphase II → centromeres divide and individual chromatids
are pulled to opposite poles → 4 group of chromosomes.
● Telophase II → nuclear envelope forms
● Cytokeinesis → cytoplasm divides, 4 haploid cells.
s
● Crossing over: During meiosis I , homologous chromosomes pair
am
up and are in very close proximity to each other. The
non-sister chromatids can cross over and get entangled
(crossing point → chiasmata). Entanglement places stretch on
Ex
DNA molecules → one section of chromatid may break and
rejoin with another. Swapping of alleles → new combination of
alleles on the two chromosomes.
ck
ra
C
CRACK EXAMS A2 2
Biology (9700) notes A-Level - Inheritance (CIE)
Passage of information from parents to offspring
● Haploid(n) and diploid(2n): Haploid is one complete set of
chromosomes and diploid is two complete set of chromosomes.
● Haploid → gametes in sexual reproduction. Diploid → Contain
the DNA necessary for protein synthesis and cell function.
● Chromosomes have fixed length, position of centromere in
A2
particular location.
● Homologous pair of chromosomes: Carry the same genes in the
same position and same shape.
● During fertilization the nuclei of gametes fuse together to
s
form the nucleus of the zygote. Meiosis produces haploid
am
gametes during sexual reproduction. The first cell division of
meiosis is reduction division ( chromosomes from 46 → 23).
Reduction in chromosome number during meiosis ensures the
Ex
gamete formed are haploid.
● Behaviour of chromosomes during meiosis:
● Prophase I → DNA condenses, becomes visible as
ck
chromosomes. DNA replication done → each chromosome
consists of two sister chromatids joined by centromeres.
Homologous pair. Crossing over of non-sister chromatids may
ra
occur(point at which it occurs → chiasma). Centrioles migrate
to opposite poles, spindle formed. The nuclear envelope breaks
C
down and the nucleolus disintegerates.
● Metaphase I → bivalents line up along the equator of the
spindle, spindle fibres attached to centromere.
● Prophase I → Homologous pair of chromosomes as
microtubules pull whole chromosomes to opposite ends.
CRACK EXAMS A2 1
, Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
● Telephase I → chromosomes move to opposite poles, spindle
fibres start to break down, nuclear envelope form around the
two groups of chromosomes and nuclei reform.
● Cytokeinesis → division of cytoplasm
● Prophase II → nuclear envelope break, spindle form 90
degrees to old, chromosomes condense
● Metaphase II → chromosomes line up in a single line along the
equator of the spindle.
A2
● Anaphase II → centromeres divide and individual chromatids
are pulled to opposite poles → 4 group of chromosomes.
● Telophase II → nuclear envelope forms
● Cytokeinesis → cytoplasm divides, 4 haploid cells.
s
● Crossing over: During meiosis I , homologous chromosomes pair
am
up and are in very close proximity to each other. The
non-sister chromatids can cross over and get entangled
(crossing point → chiasmata). Entanglement places stretch on
Ex
DNA molecules → one section of chromatid may break and
rejoin with another. Swapping of alleles → new combination of
alleles on the two chromosomes.
ck
ra
C
CRACK EXAMS A2 2
