Download DNA, Mitosis and Meiosis Theory

•Watson and Crick model  2 long strands of sub-units
called NUCLEOTIDES wound around each other to form a
DOUBLE HELIX
•Each nucleotide is made up of
SUGAR – PHOSPHATE – BASE subunits with the sugar
being deoxyribose
•The spine of the DNA is made up of 4 nitrogenous bases:
Adenine (A)
Thymine (T)
 Guanine (G)
Cytosine (C)
•The Base-Pairing Rule states:
A bonds with T
C bonds with G
•The code is organised in to triplets called CODONS
•Each codon codes for an amino acid which will in turn
form a protein
•Over a metre of DNA in each cell, twisted and coiled into
46 CHROMOSOMES
•Not all the length of DNA codes for proteins
•Parts of DNA that do code for proteins are called GENES
and the set of total genes for an organism is called its
GENOME
•In eukaryotic organisms the DNA molecules
and associated molecules are contained in
the nucleus as tangled fibres of CHROMATIN
•They are coiled around proteins closely
associated with DNA called HISTONES
•When cells are about to divide the
chromosomes thicken and double in genetic
material becoming joined at the
CENTROMERE (see right)
•They form two sister CHROMATIDS
•Usually only visible at this stage
•Eukaryotic chromosomes exist in pairs
known as HOMOLOGOUS PAIRS
•Along the length of each DNA molecule,
particular regions (GENES) code for different
proteins that determine particular
characteristics or TRAITS
•The location of a particular gene on a
chromosome is referred to as its LOCUS and
on homologous chromosomes the
corresponding gene is found at the same
locus
•These alternative forms of the gene are
called ALLELES
Packaging of DNA:
2 meters of DNA is
packaged into a cell
no bigger than a full
stop
DNA is wrapped
around protein units,
like string of beads
This structure is called
CHROMATIN
During cell replication,
the chromatin folds
many times so that is
is very condensed
Codons –
Is this DNA?
KARYOTYPES
•Karyotypes are photographic images of chromosomes
arranged and ordered into matched pairs from largest to
smallest.
•Each organism is characterised by the number of
chromosomes in each cell (eg. Humans= 46)
•Genetic disorders such as Trisomy 21 and whole
chromosome disorders can be determined via a karyotype
•In the nucleus of a somatic cell there will be 23
homologous pairs. 22 AUTOSOMES and 1 pair of SEX
CHROMOSOMES (XX=female/XY=male)
•The DIPLOID number of chromosomes are all
chromosomes with both pair members present (Human
diploid number = 46)
•The HAPLOID number of chromosomes are
chromosomes with only one pair member present (Human
haploid number = 23)
•The only haploid cells in humans is the gametes (sperm
& ova)

4 Phases of Mitosis
1. PROPHASE
2. METAPHASE
3. ANAPHASE
4. TELOPHASE

Mitosis is the process used by cells to make new,
genetically identical, cells. This is predominantly used in
growth and repair

INTERPHASE is a phase in a non-dividing cell where all the
chromosomes exist in a single chromatid form. Cells spend
the majority of their lives in this form
 P.M.A.T.I OR
I.P.M.A.T

PROPHASE
 chromosomes become shorter and fatter by coiling.
Repeated coiling is called supercoiling
 microtubules grow from the poles of the cell and the
centrioles
 these form a spindle called the MITOTIC SPINDLE

METAPHASE
 spindle microtubules attach to the chromosome
centromeres
 chromosomes are moved to the equator. Each
chromatid moves to the opposite pole

ANAPHASE
 the pairs of sister chromatids separate, centromeres
split and the microtubules pull them to opposite poles

TELOPHASE
 chromatids at each pole gather together and become
enclosed in a nuclear membrane
 the whole cell splits at the equator and forms 2 new
cells
 each cell has the SAME number of chromosomes as
the parent cell and contains the same genetic
information
 cell enters INTERPHASE again
UNDERSTAND THE PROCESS DON’T MEMORISE THE STEPS
 VIDEO: CRASHCOURSE MITOSIS
 VIDEO: THE EUKARYOTIC CELL CYCLE

Meiosis is a type of cell division that produces
gametes (sex cells)

Cells produced are haploid (contain HALF the original
chromosome number)

Process accounts for great genetic variation due to
 RANDOM ASSORTMENT
 CROSSING OVER

2 cell divisions:
1st:
PROPHASE I
METAPHASE I
ANAPHASE I
2nd: PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I
TELOPHASE II
 PROPHASE I
 cell has 2n (diploid) number of chromosomes
 chromosomes become visible
 homologous pairs line-up
 crossing over occurs and exchange of genetic material
(see right)
 METAPHASE I
 spindle microtubules move homologous pairs to the
equator of the cell
 orientation of paternal and maternal chromosomes at either
side of the equator is random (RANDOM ASSORTMENT)
 ANAPHASE I
 Homologous pairs are separated. One chromosome of
each pair moves to each pole
 TELOPHASE I
 Nuclear membranes form around chromosomes
 cell membranes form around each nucleus
 reduction of chromosome number from diploid to haploid
is completed (2 haploid cells now formed)
 cytokinesis occurs
 PROPHASE II
 nuclear envelope begins to disintegrate
 spindle microtubules form
 chromosomes (with 2 chromatids) condense and
become visible
 METAPHASE II
 chromosomes line up along the equator (in a
straight line)
 ANAPHASE II
 centromeres separate and chromatids are moved
to opposite poles
 TELOPHASE II
 chromatids reach opposite poles
 nuclear envelope forms
 cell membrane re-forms
 cytokinesis occurs

At this stage there are 4 GENETICALLY DIFFERENT
daughter cells

They contain HALF the original chromosome number
(haploid)

Phase II is very similar to mitosis