Download Chromosome Lecture

Chromosomes & Cell Cycle
SDK
October 1, 2013
Road Map
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Chromosomes
Chromosome Anatomy
Defining Chromosomal Location
Cell Cycle
Cell Division
Mitosis
 Phases of Mitosis
Cell Cycle Checkpoints
Functions of Cell Division
Uncontrolled Cell division(Cancer)
Sexual Reproduction
Sex Cells – Gametes
Meosis
 Significance of Meiosis
Mitosis vs Meiosis
KARYOTYPE
Making a Karyotype
Individual chromosomal importance
Books
Chromosomes
 Linear units of DNA
 Thread-like structures located inside the nucleus,
 Each chromosome is made of protein and a molecule of
deoxyribonucleic acid (DNA).
 Come in pairs (homologous pairs)
 Each species has characteristic number
 Human has 23 pairs of the chromosome
 22 are the autosomes
 One is Sex Chromosome
CHROMOSOME
It is a combination of two words, i.e.,
“Chroma”-means ‘colour’
and “Somes”-means ‘body’.
So the coloured thread like bodies present in the nucleus
of the living cells, which helps in the inheritance
(transmission) of characters
generation
to
CHROMOSOMES.
in form of Genes from
generation
are
known
as
Chromosome Anatomy
 Chromosomes come in two forms:
Unduplicated(Monad state)
Duplicated(Diads state)
 Duplicated chromosomes are made up of two chromatids
 Referred to as sister chromatids
 Chromatids are joined at a region called the Centromere
 Chromatids are identical
 Each chromatid is a double helix of DNA
 The diad state is what is typically seen in pictures of chromosomes, but it only occurs briefly
in the life of the cell. During most of interphase
Chromosome Anatomy
• Each chromosome has two components
1.
2.
Telomere (Upper & Lower end of chromosomes)
Constriction Point/Centromere (Position is variable)
• The constriction point called the centromere, which divides the
chromosome into two sections, or “arms” A short arm and A
long arm.
• The short arm of the chromosome is labeled the “p arm” The
long arm of the chromosome is labeled the “q arm” .
Centromere
 Sometimes called the “primary constriction” on a chromosome
 This is the attachment point for the spindle
 This is the many repeats of about 170 bp element.
 The location of the centromere on each chromosome gives the
chromosome its characteristic shape, and can be used to help
describe the location of specific genes and further classify the
chromosomes.
Types of Chromosomes
There are four types of chromosomes based upon the position
of the centromere.
1) Metacentric : Centromere occurs in the centre and all the four
chromatids are of equal length.
2) Submetacentric : Centromere is a little away from the centre
and therefore chromatids of one side are slightly longer than
the other side.
3) Acrocentric : Centromere is located closer to one end of
chromatid therefore the chromatids on opposite side are very
long. A small round structure, attached by a very thin thread is
observed on the side of shorter chromatid. The small round
structure that is a part of the chromatid is termed as satellite.
4) Telocentric : Centromere is placed at one end of the chromatid
and hence only one arm. Such telocentric chromosomes are not
seen in human cells.
Defining Chromosomal Location
• Each chromosome is further divided into
regions, labeled p1,p2,p3…. and q1,q2,q3….
counting outwards from the centromere.
• Regions are divided into bands and sub-bands
labeled p11.1, 012.3, p13.5 ….)
Centromeres/Telomeres
What are entromeres?

The constricted region of
linear
chromosomes
is
known as the centromere.
 Although this constriction is
called the centromere, it
usually is not located
exactly in the center of the
chromosome and, in some
cases, is located almost at
the chromosome's end.
What are telomeres?
 Telomeres are repetitive
stretches of DNA located at
the
ends
of
linear
chromosomes.
• Teleomers consist of as
many as 2000 repeats of the
sequence: 5' TTAGGG 3'.
 They protect the ends of
chromosomes in a manner
similar to the way the tips of
shoelaces keep them from
unraveling.
The normal human chromosome diploid number
is:
A. 23
B. 24
C. 46
D. 48
46
The chromosome which picture is shown below is
1. Metacentric
2) Submetacentric
3) Acrocentric
4) Telocentric
Submetacentric
The mark point on the
Chromosome is
•
•
•
•
P arm
q arm
Centromere
Sub band
Centromere
Cell Cycle
 The continuity of life is based upon the reproduction of cells, or
cell division
 A typical human cell undergoes a division about every 24 hours
(there are many exceptions!)
 The cell cycle is basically an alternation of 2 major phases –
Mitosis and Interphase
 Interphase is the phase in which the cell spends 23 of the 24
hours
 The cell grows and preform its normal cell activity
 Mitosis takes about 1 hour & cell division phase
The Cell Cycle
 Interphase can be broken down into 3 distinct subphases:
» G1 (Known as gap 1)
» S (for synthesis of DNA)
» G2 (gap 2)
 Cells that do not divide are considered to be in a G0
phase – where they carry on normal housekeeping and
do not prepare to divide.
 Mitosis. Is comprised of Prophase, Anaphase,
Metaphase and Telophase.
Cell Division
Cell division is of two types
 Mitosis
 Meosis
 Mitosis is a process of cell duplication, or reproduction,
during which one cell gives rise to two genetically identical
daughter cells. Take place in all somatic cells
 Meiosis, is a division of a germ cell( sperm/ova) involving two
fissions of the nucleus and giving rise to four gametes, or sex
cells, each possessing half the number of chromosomes of the
original cell. Take place only in germ cells.
Mitosis
Mitosis is ordinary cell division among the cells of the body. During
mitosis the chromosomes are divided evenly, so that each of the two
daughter cells ends up with 1 copy of each chromosome.
In mitosis the homologs chromosomedo not pair up.
Rather they behave independently.
Each resultant cell receives one copy of each homolog.
Phases of Mitosis
1. Prophase:
 Chromosomes condense
 Nuclear envelope disappears
 Chromosome is visible
 Centrioles move to opposite ends of the cell
 Spindle forms
2. Metaphase:
Chromosomes are lined up on cell equator, attached to the spindle
at the centromeres
Mitosis
3. Anaphase:
 Centromeres divide. Now chromosomes are monads
 The monad chromosomes are pulled to opposite poles
by the spindle.
4. Telophase:
 Cytokinesis: cytoplasm divided into 2 separate cells
 Chromosomes de-condense
 Nuclear envelope re-forms
 Spindle vanishes
Phases of Mitosis
G2 of Interphase
Prophase
Anaphase
Prometaphase
Telophase & beginning
of cytokinesis
Metaphase
Completion of cytokinesis
DNA replication takes place during
A.
B.
C.
D.
S - phase
G2 - phase
G1 - phase
Prophase
A.
S - phase
The stage in which daughter chromosomes move
toward the poles of the spindle is
A. Anaphase
B. Metaphase
C. Prophase
D. Telophase
A.Anaphase
During which stage the chromosomes first
become visible
A. Anaphase
B. Metaphase
C. Prophase
D. Telophase
C.
Prophase
Cell Cycle Checkpoints
 If cell size inadequate
G1 or G2 arrest
 If nutrient supply inadequate
G1 arrest
 If an essential external stimulus is lacking
G1 arrest (at R)
 If the DNA is not replicated
S arrest
 If DNA damage is detected
G1 or G2 arrest
 If the spindle formation is improper,
chromosome misalignment
 M-phase arrest
Functions of Cell Division
(a) Reproduction. An amoeba,
a single-celled eukaryote, is
dividing into two cells. Each
new cell will be an individual
organism (LM).
(b) Growth and development.
This micrograph shows a
sand dollar embryo shortly after
the fertilized egg divided, forming
two cells (LM).
(c) Tissue renewal. These dividing
bone marrow cells (arrow) will
give rise to new blood cells (LM).
Uncontrolled Cell division
leads to Cancer?
Uncontrolled Cell division
Loss of cell cycle control and checkpoints
What does a cancerous cell look like?
•Large or multiple nuclei
•Irregular shape
•Cells overlapping neighboring cells – loss of density-dependent or contact inhibition
•Loss of anchorage dependence
Sexual Reproduction
• Fertilization and meiosis alternate in sexual life cycles
• A life cycle is the generation-to-generation sequence of stages in
the reproductive history of an organism
Key
Haploid
Diploid
n
MEIOSIS
2n
Diploid
multicellular
organism
n
Gametes
n
FERTILIZATION
Zygote
2n
Mitosis
(a) Animals
Sex Cells - Gametes
• Unlike somatic cells, sperm and egg cells are
haploid cells, containing only one set of
chromosomes
• At sexual maturity the ovaries and testes produce
haploid gametes by meiosis
• During fertilization, sperm and ovum fuse
forming a diploid zygote
• The zygote develops into an adult organism
Meiosis
5.1.Prophase
ofthe
M1special
is very
with
numberdiploid
of subMeiosis is
celllong,
division
thata converts
body
stages.
cells into the haploid gametes. Only occurs in specialized
6.Main event in prophase of M1 is “crossing over”, also
cells.
called “recombination”.
7.2.In Takes
crossing
homologous
chromosomes
up,
2 cellover,
divisions,
M1 and M2,
with no DNApair
synthesis
and exchange segments by breaking and rejoining at
between.
identical locations.
8.3.Several
crossovers
per
with
random
In humans,
start with
46chromosome,
chromosomes (23
pairs)
in diad
positions.This is the basis for linkage mapping.
state. After M1,
there
are 2recombine
cells with 23seem
diad chromosomes
9. Chromosomes
that
don’t
to have a
high rate of non-disjunction (chromosome goes to
each.
the wrong pole).
4. After M2 there are 4 cells with 23 monad chromosomes each.
In meiosis the products are haploid gametes so two divisions are necessary.
Prior to the first division, the homologs pair up (synapse) and segregate from each other.
In the second meiotic division sister chromatids segregate.
Each cell receives a single chromatid from only one of the two homologs.
Meiosis
 Only diploid cells can divide by meiosis.
 Prior to meiosis I, DNA replication occurs.
 During meiosis, there will be two nuclear divisions, and the result will be four
haploid nuclei.
 No replication of DNA occurs between meiosis I and meiosis II.
Significance of Meiosis
• Sexual Reproduction
• If meiosis did not occur fusion of gametes would result in
doubling of chromosomes for each successively reproduced
generation
• Genetic Variation
• Meiosis provides opportunities for new combinations of genes
to occurs in gametes via independent assortment of
chromosomes (metaphase) and crossing over (prophase I).
• Pairing up of homologous chromosomes- Synapsis (each pair
is a bivalent, consist of 4 chromatids)
Mitosis
vs
Meiosis
 One Division
 Two Divisions
 Homologues do not pair
 Homologues Pair up
 Centromeres divide
 In meiosis I, centromeres do
not divide
 Each cell inherits both
homologues
 Homologues segregate from
each other.
 Mitosis is conservative
producing daughter cells that
are like parental cell.
 Meiosis is not conservative,
rather it promotes variation
through segregation of
chromosomes and recombination
Meiosis occurs for the human female in
A. Pancreas
B. Liver
C. Ovary
D. Kidney
A.Ovary
In humans, at the end of the first meiotic division,
the male germ cells differentiate into the
A. Spermatogonia
B. Primary spermatocytes
C. Secondary spermatocytes
D. Spermatids
C. Secondary spermatocytes
Given below is a schematic break-up of the phases/stages of cell
cycle Which one of the following is the correct indication of the
stage/phase in the cell cycle?
A. Cytokinesis
B. Metaphase
C. Karyokinesis
D. Synthetic Phase
D.
Synthetic Phase
KARYOTYPE
What is a Karyotype?
 A picture (actual photograph) , or chromosome map, of
all 46 chromosomes is called a Karyotype.
 The karyotype can help identify chromosome
abnormalities that are evident in either the structure or
the number of chromosomes.
Homologous Pairs
• Every individual has two copies of every chromosome
– One from your mother
– One from your father
• Homologous pairs have the same genes but not always the
same form of the gene
• 23 pairs 23 x 2 = 46
Making a Karyotype
 First the chromosomes are stained
 Once stained, the chromosomes look like strings with
light and dark "bands"
Making a Karyotype
• Then they are organized by height and centromere
location
Autosomes vs sex chromosomes
• Chromosome pairs # 1- # 22 are called Autosomes
• Chromosome pairs # 23 are called the X and Y chromosomes
– Referred to as the Sex Chromosomes because they contain
genes which determine gender
• Two X’s means you are genetically female
• One X and a Y means you are genetically male
Presence of a Y chromosome means the person is male
Male or Female?
Chromosome 1
Contains over 3000 genes
Contains over 240 million base pairs, of which ~90% have been determined
Chromosome 2
• Contains over
2500 genes
• Contains over
240 million base
pairs, of which
~95% have been
determined
Chromosome 4
• Contains approximately 1600 genes
• Contains approximately 190 million base pairs,
of which ~95% have been determined
Chromosome 7
• Contains approximately 1800 genes
• Contains over 150 million base pairs, of which
over 95% have been determined
Chromosome 11
• Contains approximately 2000 genes
• Contains over 130 million base pairs, of which
over 95% have been determined
Chromosome 13
• Contains approximately 800 genes
• Contains over 110 million base pairs, of which
over 80% have been determined
Chromosome 17
• Contains over 1600 genes
• Contains approximately 80 million base pairs,
of which over 95% have been