2.2/2.3 DNA, meiosis.

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Module 2.2/2.3
During a mitotic cell cycle genetic information (DNA) is copied and passed genetically identical
In gamete cell formation (meiosis) halving the DNA independent segregation and crossing over
give non identical cells (variety).
Be able to analyse, interpret and evaluate data that shows DNA to be the genetic material.
Appreciate where the terms nucleotides, deoxyribose, phosphate and A,C,T,G, sugar phosphate
backbone, hydrogen bonds between base pairs, base pairing, fit into our understanding of
structure to function for the molecule.
Be able to analyse, interpret and evaluate data that shoes DNA replication to be semi
Appreciate that semi conservative replication leads to exact copying.
Understand this involves breaking hydrogen bonds between polynucleotide strands, attracting
new nucleotides which join through complementary base pairing that enzymes, helicase and
polymerase speed up the process.
Know there is a cell cycle.
Understand chromosomes are long lengths of DNA which is coiled upon itself several times. In
eukaryotes proteins called histones act as "formers" around which the DNA is coiled. In
prokaryotes DNA is circular and without protein.
Know mitosis increases the cell number and so is involved in growth and repair of tissue.
Know replication of DNA occurs during interphase in the S phase.
Name and explain the events in each stage of mitosis, recognising them from drawings and
Relate the cell cycle to treatment of cancer.
Know meiosis is a type of cell division involved in gamete formation for sexual reproduction.
Know meiosis is a type of cell division which produces haploid cells (half the number of
chromosomes/DNA) and which are different from each other in contrast to mitosis.
Recognise stages from meiosis drawings and photographs in sufficient detail to understand
crossing over, independent segregation and haploid cell formation.
Key words:
Mitosis: The process where a single cell divides resulting in generally two identical cells, each
containing the same number of chromosomes and genetic content as that of the original cell.
Prior to this, the genetic material of the original (parent) cell has replicated during the S phase of
the cell cycle so that when the cell enters mitosis it undergoes four major phases which
culminates in the formation of two identical (daughter) cells:
1st phase: Prophase : formation of paired chromosomes , disappearance of membrane,
appearance of the achromatic spindle, formation of polar bodies
2nd phase: Metaphase : arrangement of chromosomes in the equatorial
plane. Chromosomes separate into exactly similar halves.
3rd phase: Anaphase : the two groups of daughter chromosomes separate and move along the
fibres of the central spindle, each toward one of the asters , forming the diaster.
4th phase: Telophase : two daughter nuclei are formed the cytoplasm divides, forming two
complete daughter cells.

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Originally, the term mitosis refers only to nuclear division unaccompanied by cytokinesis (which
is the division of the cytoplasm), as in the case of some cells like certain fungi and in
fertilized egg of many insects. As used now , mitosis used interchangeably with cell division.
Replication: The process of duplicating or producing an exact copy of a polynucleotide strand such as
Interphase: The stage of the cell or nucleus when it is not in mitosis, hence comprising most of
the cell cycle.…read more

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The introns are removed and the exons are joined together to form
the final functional mRNA .
Stage Main Events
Interphase Cell makes a copy of its chromosomes by
replication of DNA cell grows and undergoes its
normal physiological functions.
Prophase Chromosomes coil, becoming shorter and
fatter. We can now see them with an optical
microscope. Nuclear envelope disappears.
Protein fibres form a spindle in the cell.
Metaphase One or more spindle fibres attaches to
centromere of each chromosome.…read more

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Cytoplasm divides to form
2 new cells.
Importance of mitosis:
1. Growth: mitosis ensures that new cells can be formed with identical genetic material to
the parent cell.
2. Repair: if cells are damaged or die they must be replaced with cells that perform the same
functions as the parent cell.
3. Differentation: New cells "differentiate" so that some genes are expressed and the cell is
able to make particular proteins. All cells in a tissue perform the same functions.…read more

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They grew bacteria in growth medium with 15
N then transferred them to 14N for one
generation. DNA extracted and spun in caesium chloride.
Supports semi-conservative replication.
Meiosis ­ seperates homologous chromosomes and produces haploid cells:
Homologous chromosomes
Members of each pair
Controlling the same character in the same order.
One from maternal chromosome and one from paternal.
A cell with pairs of homologous chromosomes is called diploid. (2n)
A cell with only one chromosome from each homologous pair is called haploid.…read more

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Gametes have a mixture.
Each gene can have different alleles in homologous pairs.
Genetic recombination by crossing over.
During the first meoitic division, the members of each homologous pairs lie side by side. If
they break at the same point along their length, the broken segment can join the other
member of the chromosome.
Crossing over increases genetic variation.
They have alleles that were not present in either of the parental chromosomes.…read more

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Notes from revison guide:
Dna is made of nucleotides that contain a sugar, a phosphate and a base.
1. DNA is a polynucleotide ­ it's made up of lots of nucleotides joined together.
2. Each nucleotide is made from a pentose sugar, a phosphate group and a nitrogenous base.
3. The sugar in DNA is a deoxyribose sugar.
4. Each nucleotide has the same sugar and phosophate. The base on each nucleotide can
vary though.
5.…read more

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The nature and development of organisms is determined by genes:
1. Enzymes speed up most of our metabolic pathways ­ the chemical reactions that occur in
the body. These pathways determine how we grow and develop.
2. Because enzymes control the metabolic pathways, they contibute to our development,
and ultimately what we look like (our phenotype).
3. All enzymes are proteins, which are built using the instructions contained within gens.…read more

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