Meiosis-Introduction
Structure of the chromosomes: Chromosomes:
Revision- FUNCTION- CONTROL THE FUNCTIONA OF THE
o Chromosomes occur in the nucleus of every CELLS AND THEREFORE THE ORGANISMS.
living cell CONTROL ALL THE CHEMICAL REACTIONS
o A chromosome consists of a DNA molecule (METABOLISM)
coiled around proteins known as histones § There exist as single stranded structure
o There are various segments and each intertwined to form a chromatin network
chromosome strand known as genes each § They replicate during interphase of a cell cycle-
gene controls a particular inherited between cell division to form 2 identical
characteristic chromatids (called Sister chromatids) they are
o When the cells not dividing the chromosomes joined by a centromere this takes place just before
the cell divides by either mitosis or meiosis
appear as a thread-like network the chromatin
§ They only become visible when the chromosomes
network
shorten and thicken as the cell divides
o When the cell divides the chromatin
Chromosome consists of DNA:
network condenses, the threads shorten and • A chromosome consists of a DNA molecule coiled
thicken and become visible as single around proteins known as histones
chromosomes • There are various segments and each chromosome
o Before a cell divides all the genetic material in strand known as genes
a nucleus in the DNA must be duplicated this • Each gene controls a particular inherited
process where DNA makes an exact copy of characteristic
itself is known as DNA replication The number of chromosomes in a cell:
o Replication is necessary to ensure that both • In an organism that reproduces sexually the
daughter cells formed during mitosis have number of chromosomes is always an even
identical genetic information to the original number because it consists of two sets of
chromosomes
mother cell • The double set of chromosome is known as known
o After replication occurs the single stranded as the diploid chromosome number (2n) in each
chromosome DNA and histones consists of chromosome of one set normally has a partner
two identical units of chromatids that are similar in size shape and genetic
o The two chromatids of the new double- composition
stranded chromosomes are joined by • 1 chromosome of the pair comes from the mother
a centromere of the organism and the other comes from the
father- as homologous chromosome pairs. This
ensures that the organism has characteristics of
both parents
Process of how the number of chromosomes
occur in a cell- for HUMANS
• Gametes (sex cells) i.e. egg, and sperm have only one
set of chromosomes which is known as the haploid
chromosome number (n)
• During fertilization, the egg and sperm each with one
set of chromosomes fuse to form a zygote (2n)
• Therefore the zygote contains two sets of
chromosome i.e. the diploid chromosome number
(2n)
• The zygote divides by mitosis and forms an embryo
consisting of many cells each with a diploid
chromosome number
• Each organism has a unique number of
chromosomes in each somatic cell
, Meiosis
Structure of the cell:
Animal Cell:
Human Cell:
Structure of the chromosomes: Chromosomes:
Revision- FUNCTION- CONTROL THE FUNCTIONA OF THE
o Chromosomes occur in the nucleus of every CELLS AND THEREFORE THE ORGANISMS.
living cell CONTROL ALL THE CHEMICAL REACTIONS
o A chromosome consists of a DNA molecule (METABOLISM)
coiled around proteins known as histones § There exist as single stranded structure
o There are various segments and each intertwined to form a chromatin network
chromosome strand known as genes each § They replicate during interphase of a cell cycle-
gene controls a particular inherited between cell division to form 2 identical
characteristic chromatids (called Sister chromatids) they are
o When the cells not dividing the chromosomes joined by a centromere this takes place just before
the cell divides by either mitosis or meiosis
appear as a thread-like network the chromatin
§ They only become visible when the chromosomes
network
shorten and thicken as the cell divides
o When the cell divides the chromatin
Chromosome consists of DNA:
network condenses, the threads shorten and • A chromosome consists of a DNA molecule coiled
thicken and become visible as single around proteins known as histones
chromosomes • There are various segments and each chromosome
o Before a cell divides all the genetic material in strand known as genes
a nucleus in the DNA must be duplicated this • Each gene controls a particular inherited
process where DNA makes an exact copy of characteristic
itself is known as DNA replication The number of chromosomes in a cell:
o Replication is necessary to ensure that both • In an organism that reproduces sexually the
daughter cells formed during mitosis have number of chromosomes is always an even
identical genetic information to the original number because it consists of two sets of
chromosomes
mother cell • The double set of chromosome is known as known
o After replication occurs the single stranded as the diploid chromosome number (2n) in each
chromosome DNA and histones consists of chromosome of one set normally has a partner
two identical units of chromatids that are similar in size shape and genetic
o The two chromatids of the new double- composition
stranded chromosomes are joined by • 1 chromosome of the pair comes from the mother
a centromere of the organism and the other comes from the
father- as homologous chromosome pairs. This
ensures that the organism has characteristics of
both parents
Process of how the number of chromosomes
occur in a cell- for HUMANS
• Gametes (sex cells) i.e. egg, and sperm have only one
set of chromosomes which is known as the haploid
chromosome number (n)
• During fertilization, the egg and sperm each with one
set of chromosomes fuse to form a zygote (2n)
• Therefore the zygote contains two sets of
chromosome i.e. the diploid chromosome number
(2n)
• The zygote divides by mitosis and forms an embryo
consisting of many cells each with a diploid
chromosome number
• Each organism has a unique number of
chromosomes in each somatic cell
, Meiosis
Structure of the cell:
Animal Cell:
Human Cell:
