INHERITANCE
1. Passage of information from parent to offspring
1.1 Haploidy and diploidy
Inheritance is the transmission of genetic information from generation to generation
Chromosome is a thread-like structure of DNA, carrying genetic information in the form of genes
Gene is a sequence of DNA nucleotides that codes for a specific polypeptide
Locus (loci) is the position of gene on a chromosome
Allele is alternative nucleotide sequences at a single gene locus
A diploid cell contains two complete sets of chromosomes (2n) – 23 pairs/46 chromosomes in the
nucleus – all body cells
Haploid cells contain one complete set of chromosomes (n) – 23 chromosomes in the nucleus -
gametes
Homologous Chromosomes
Homologous chromosomes are chromosomes that carry the same genes in the
same positions. Same position centromere. Same shape and length.
These are present in diploid cells – two sets of chromosomes
During fertilization a diploid zygote is formed – contains a homologous
chromosome, one from female gamete and one from male gamete
1.2 Meiosis in animal and plant cells
Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells.
Meiosis involves one cycle of DNA replication and two cycle of nuclear and cell division.
Need for reduction division – number of chromosomes reduced by half so when haploid gamete fertilizes,
diploid zygote formed. Increases genetic variation – crossing over and independent assortment occurs.
First stage of meiosis: meiosis I (reduction division)
Prophase I
o DNA replication has already occurred - each chromosome consists of two sister
chromatids (bivalent) joined together by a centromere
o Chromatin condenses and becomes visible as chromosomes – homologous pair (tetrad)
, o Crossing over of non-sister chromatids occurs at recombination nodule– exchange of genetic
material – chiasmata is X shaped structure during crossing over
o The nuclear envelope breaks down and the nucleolus disintegrates
Metaphase I
o The chromosomes line up along the equator of the spindle, with the spindle fibres attached to
the centromeres
o independent assortment of homologous (pairs of) chromosomes
o 4n chromosome
Anaphase I
o The microtubules pull chromosomes to opposite ends of the spindle - homologous pairs of
chromosomes are separated.
o The centromeres do not divide
o Reduction division
Telophase I
o chromosomes reach the opposite poles and decondense
o Spindle fibres start to break down
o Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
o Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
Cytokinesis
o division of the cytoplasm - Cell organelles get distributed between the two developing cells
o The product of cytokinesis in meiosis I: two diploid cells
1. Passage of information from parent to offspring
1.1 Haploidy and diploidy
Inheritance is the transmission of genetic information from generation to generation
Chromosome is a thread-like structure of DNA, carrying genetic information in the form of genes
Gene is a sequence of DNA nucleotides that codes for a specific polypeptide
Locus (loci) is the position of gene on a chromosome
Allele is alternative nucleotide sequences at a single gene locus
A diploid cell contains two complete sets of chromosomes (2n) – 23 pairs/46 chromosomes in the
nucleus – all body cells
Haploid cells contain one complete set of chromosomes (n) – 23 chromosomes in the nucleus -
gametes
Homologous Chromosomes
Homologous chromosomes are chromosomes that carry the same genes in the
same positions. Same position centromere. Same shape and length.
These are present in diploid cells – two sets of chromosomes
During fertilization a diploid zygote is formed – contains a homologous
chromosome, one from female gamete and one from male gamete
1.2 Meiosis in animal and plant cells
Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells.
Meiosis involves one cycle of DNA replication and two cycle of nuclear and cell division.
Need for reduction division – number of chromosomes reduced by half so when haploid gamete fertilizes,
diploid zygote formed. Increases genetic variation – crossing over and independent assortment occurs.
First stage of meiosis: meiosis I (reduction division)
Prophase I
o DNA replication has already occurred - each chromosome consists of two sister
chromatids (bivalent) joined together by a centromere
o Chromatin condenses and becomes visible as chromosomes – homologous pair (tetrad)
, o Crossing over of non-sister chromatids occurs at recombination nodule– exchange of genetic
material – chiasmata is X shaped structure during crossing over
o The nuclear envelope breaks down and the nucleolus disintegrates
Metaphase I
o The chromosomes line up along the equator of the spindle, with the spindle fibres attached to
the centromeres
o independent assortment of homologous (pairs of) chromosomes
o 4n chromosome
Anaphase I
o The microtubules pull chromosomes to opposite ends of the spindle - homologous pairs of
chromosomes are separated.
o The centromeres do not divide
o Reduction division
Telophase I
o chromosomes reach the opposite poles and decondense
o Spindle fibres start to break down
o Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
o Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
Cytokinesis
o division of the cytoplasm - Cell organelles get distributed between the two developing cells
o The product of cytokinesis in meiosis I: two diploid cells
