Cambridge A Levels A2 Biology Chapter 16 Inheritance
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Cambridge A Levels A2 Biology
Institution
Chapter 16 Inheritance: 92 pages
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Chapter 16 Inheritance
16.1 Passage of Information from Parents to Offspring
16.2 The Roles of Genes in Determining the Phenotype
16.3 Gene Control
16.1 Passage of Information from Parents to Offspring
Haploid and Diploid Cells
Haploid Cell Diploid Cell
A cell that contains one complete set of A cell that contains two complete sets of
chromosomes (n). chromosomes (2n).
A cell that contains one of each pair of A cell that contains pairs of homologous
homologous chromosomes. chromosomes.
A cell that contains half the chromosome A cell that contains the chromosome number
number of a body/somatic cell. of a body/somatic cell.
• Haploid and diploid describe the number of sets of chromosomes, not the total number
of chromosomes.
• In humans, there are 23 chromosomes in one complete set.
Diploid cell: 46 chromosomes
Haploid cell: 23 chromosomes
• Humans have haploid cells that contain 23 chromosomes in their nucleus.
These haploid cells are called gametes and they are involved in sexual reproduction.
For humans they are the female egg and the male sperm
,The Need for Reduction Division during Meiosis
• During sexual reproduction, each gamete contributes one set of chromosomes to form
the zygote.
Gametes need to be haploid so that, when their nuclei fuse, a zygote is formed with the
diploid number of chromosomes.
• Meiosis produces haploid gametes during sexual reproduction.
• The first cell division of meiosis is a reduction division.
This is a nuclear division that reduces the chromosome number of a cell.
In humans the chromosome number is reduced from 46 (diploid) to 23 (haploid)
• The reduction in chromosome number during meiosis ensures the gametes formed are
haploid.
Red blood cells are an exception when it comes to chromosome number as they don’t have a nucleus.
,Homologous Chromosome
Homologous chromosomes are a pair of chromosomes in a diploid cell that have the same
structure as each other, with the same genes (but not necessarily the same alleles of those
genes) at the same loci, and that pair together to form a bivalent during the first division of
meiosis.
Pair of Homologous Chromosome:
• Has the same shape, length and centromere position.
• Has the same linear sequence of genes that code for the same characteristics at the
same loci.
• May have different alleles of a particular gene at the same time.
• One is a paternal chromosome and the other is a maternal chromosome.
,Karyogram of Humans
There are 22 matching pairs of chromosomes. These are called homologous chromosomes.
There is also a non-matching pair labelled X and Y (sex chromosomes).
There are two sets of 23 chromosomes–one set of 23 from the father and one set of 23 from
the mother.
The non-matching X and Y chromosomes are the sex chromosomes, which determine the sex
of the individual. All the other chromosomes are called autosomes.
,Meiosis
Involves two stages:
• Meiosis I
• Meiosis II
Meiosis results in the formation of four haploid daughter cells that are genetically different.
Meiosis is important for the formation of gametes.
Meiosis occurs in sexual organs, testes and ovaries.
Meiosis I
Prophase I
• DNA condenses to form visible chromosomes.
• Each chromosome consists of two sister chromatids joined together by a centromere.
• Homologous chromosomes pair up in a process called synapsis,
o A pair of homologous chromosomes is called a bivalent.
• A chromatid of one of these chromosomes intertwines with a chromatid of the other.
Each crossing point is called a chiasma (plural: chiasmata).
There is almost always at least one, and often several, chiasmata in each pair.
The chiasmata help to hold the chromosomes together in their pair, as they move
through the next stages.
• As the homologous chromosomes are very close together the crossing over of non-sister
chromatids may occur.
The point at which the crossing over occurs at the chiasma.
• Centrioles move to opposite poles of the cell and the spindle is formed.
• The nuclear envelope breaks down and the nucleolus disintegrates.
,Metaphase I
• The bivalents line up along the equator of the spindle forming two rows of
chromosomes at the metaphase plate.
• Spindle fibres are attached to the centromeres of each chromosome.
• Independent assortment of homologous chromosomes will occur.
Anaphase I
• The homologous pairs of chromosomes are separated as the spindle fibres contract and
whole chromosomes are pulled to the opposite poles of the cell.
• Centromeres do not split.
Telophase I
• The chromosomes arrive at opposite poles.
• Spindle fibres start to break down.
• Nuclear envelopes form around the two groups of chromosomes and nucleolus reforms.
Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I.
Cytokinesis
• This is when the division of the cytoplasm occurs.
• Cell organelles also get distributed between the two developing cells.
• In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in
the middle of the cell which contracts, dividing the cytoplasm in half
• In plant cells: vesicles from the Golgi apparatus gather along the equator of the spindle
(the cell plate). The vesicles merge with each other to form the new cell surface
membrane and also secrete a layer of calcium pectate which becomes the middle
lamella. Layers of cellulose are laid upon the middle lamella to form the primary and
secondary walls of the cell.
• The end product of cytokinesis in meiosis I: two haploid cells
o These cells are haploid as they contain half the number of centromeres.
, There is no interphase between meiosis I and meiosis II, so the DNA is not replicated.
The second division of meiosis is almost identical to the stages of mitosis.
Prophase II
Nuclear envelope and nucleolus break down and disappear.
Chromosomes condense.
Spindle formation begins.
Metaphase II
Chromosomes line up in a single file along the equator of the spindle and are held in place
when spindles are attached to the centromeres.
Anaphase II
Spindle fibres contract.
Centromeres divide and individual sister chromatids are pulled to opposite poles.
This creates four groups of chromosomes that have half the number of chromosomes
compared to the original parent cell.
Telophase II
Chromosomes decondense forming chromatin.
Nuclear membranes form around each group of chromosomes.
Nucleolus reforms.
Spindle fibres break down and disappear.
Cytokinesis
Cytoplasm divides as new cell surface membranes are formed creating four haploid cells.
The cells still contain the same number of centromeres as they did at the start of meiosis I but
they now only have half the number of chromosomes (previously chromatids).
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