Download chromosomes and meiosis

Genetics: What is inherited?
Chromosomes and how they are
passed down
Heredity
 Heredity is the action of inheritance:
gaining material or characteristics
from our parents
 We gain material from our parents’
DNA
 The DNA we gain is packaged into
cells in the gonads, called sex cells
 Half of your DNA is from your mother,
and the other half from your father
Chromosomes
 “gene carriers”
 Structures
where DNA is
organised into
easily accessibly
sections, called
GENES
Organising DNA
 The DNA in just ONE of your cells
would be many kilometres long if you
were to stretch it out. It must be
organised to resist tangling and
breakage.
Representing chromosomes
Stylised,
showing
gene
banding
Stylised, showing
shape and overall
structure
Highly stylised
representation
within cells
Photograph
taken from
microscope,
showing gene
banding (from
stain)
How many chromosomes?
 Normal human cell = 23 pairs (46 in
total). 22 AUTOSOME pairs, and 1 sex
chromosome pair
 When chromosomes are paired, the
cell is DIPLOID (also written as 2n)
 In sex cells (eg. Sperm, egg), there is
only one copy of each chromosome,
so there are 23 in total (in humans).
This is a HAPLOID cell.
http://www.personal.psu.edu/staff/d/r/drs18/bisciImages/haploidDiploid.png
Karyotypes
 A picture made, which puts
chromosomes present in a cell in
order, from largest pair (1) to
smallest pair (22 in humans), and
finally the sex chromosomes.
 Used to quickly diagnose
chromosomal disorders
 Usually include gene banding
Each matching
pair is called a
HOMOLOGOUS
pair
http://sciencegirlsrock.files.wordpress.com/2011/05/karyotype.jpg
Errors in the karyotype
 If there are any chromosomes
missing or extra, the problem
probably occurred during the creation
of the sex cells, in a process called
MEIOSIS
Meiosis
 The formation of new sex cells
 Only have ONE of each chromosome
(haploid)
 Two sex cells meet in fertilisation and
give rise to a GAMETE (a new diploid
cell)
 Can include a phenomenon called
CROSSING OVER, where the arms of
two chromosome swap information
Who remembers MITOSIS?
 The reproduction of somatic cells (ie.
Not sex cells)
 Occurs in growth regions
 Requires duplication of chromosomes
 Gives rise to TWO IDENTICAL diploid
cells
The phases of
Mitosis can be
remembered using
the following:
Interphase
Prophase
Metaphase
Anaphase
Telophase
IPMAT
MEIOSIS
 Gives rise to FOUR daughter cells, all
haploid
 Daughter cells are NOT genetically
identical, as there has been random
assortment of chromosomes that have been
placed in each
 Sometimes tangles occur, and genetic
information CROSSES OVER from one
chromosome to another, producing yet
another possible point of difference
MEIOSIS
The great gene
shuffling
machine
© 2007 Paul Billiet ODWS
Copyright Dr. Michael Knee
Source: http://hcs.osu.edu/hcs300/
Meiosis performs two functions
 It halves the number of chromosomes
to make haploid sets
 It shuffles the genes to produce new
combinations (recombinations)
© 2007 Paul Billiet ODWS
Meiosis and sexual reproduction
 Meiosis is needed to produce sex cells
(gametes) with unpaired sets of
chromosomes (haploid)
 Sex cells are used in fertilisation
 At fertilisation two sets of genes come
together to form a hybrid with a set
of paired chromosomes (diploid)
 The hybrid, whilst similar to the
parents, is unique
© 2007 Paul Billiet ODWS
Haploid and Diploid
 Karyotypes of somatic cells show
paired sets of chromosomes
 The origin of the pairs are the
maternal and paternal chromosomes
of the egg and the sperm
 The number of types of chromosomes
of a species is constant = n
 So the diploid (paired set) = 2n
© 2007 Paul Billiet ODWS
The sexual reproduction life cycle
Diploid (2n)
Fertilisation
Meiosis
Haploid (n)
© 2007 Paul Billiet ODWS
Meiosis a two step process
 Meiosis 1 is the reduction division
 Meiosis 2 resembles mitosis
© 2007 Paul Billiet ODWS
Meiosis 1: Early Prophase 1
 Chromosomes
condense
 Homologous pairs
linked by
chiasmata
(chiasma sing.)
© 2007 Paul Billiet ODWS
Meiosis 1: Late Prophase 1
 Spindle fibres
form and spread
out between the
centrioles
© 2007 Paul Billiet ODWS
Meiosis 1: Metaphase 1
 The pairs of
chromosomes line
up on the equator
 The orientation of
the maternal and
the paternal
chromosomes is
random
© 2007 Paul Billiet ODWS
Meiosis 1: Anaphase 1
 Maternal and
paternal
chromosomes
segregate
(pulled separate
on the spindle)
 They move to
opposite poles
© 2007 Paul Billiet ODWS
Meiosis 1: Metaphase 1 revisited
 The pairs of
chromosomes
could orientate in
different ways
© 2007 Paul Billiet ODWS
Meiosis 1: Anaphase 1 revisited
 Resulting in
different
combinations of
chromosomes
 This means there
are 2n
combinations
 In humans this
means 223 or
over 8 million
combinations
© 2007 Paul Billiet ODWS
Meiosis 2: Prophase 2
 Now the cells are
haploid
 The chromosomes
do not
decondense at
the end of meiosis
1
 Each chromosome
has still two
chromatids
© 2007 Paul Billiet ODWS
Meiosis 2: Prophase 2
 Spindles form
again
© 2007 Paul Billiet ODWS
Meiosis 2: Metaphase 2
 The
chromosomes
line up on the
spindle equator
independently
© 2007 Paul Billiet ODWS
Meiosis 2: Anaphase 2
 The sister chromatids
separate on the
spindle
 Each cell will receive
a copy of each
chromosome type
(i.e. it will receive n
chromatids all
different)
 The genes on the
different
chromosomes are
recombined
© 2007 Paul Billiet ODWS
(shuffled)
Meiosis 2: Telophase 2
 Four haploid sex
cells are
produced
© 2007 Paul Billiet ODWS
Crossing over
 During prophase 1 not only
do the homologous pairs link
 They exchange genetic
material
 The genes on each
chromosome are not
identical they may be alleles
 Alleles are different versions
of a gene
 e.g. Ear shape gene has two
alleles the ear lobe allele and
the no ear lobe allele
© 2007 Paul Billiet ODWS
Meiosis: Anaphase 1
 So when the pairs
are separated, the
alleles of the genes
on the same
chromosome are
recombined
(reshuffled)
 Genes on the same
chromosome are
called linked
genes
© 2007 Paul Billiet ODWS
Meiosis: Prophase 2
 Each cell is haploid
(n)
 The sister
chromatids are no
longer identical
copies
© 2007 Paul Billiet ODWS
Meiosis: Anaphase 2
 At anaphase 2 the
chromatids
segregate
(separate)
randomly
 Even greater
variation is
achieved in the
sex cells
© 2007 Paul Billiet ODWS
Meiosis: Telophase 2
 Thus an infinite
variety of sex cells
is possible
 Combined with
random mating
between males and
females an infinite
variety of
individuals is
conceived at
fertilisation
© 2007 Paul Billiet ODWS