Unformatted text preview: Lecture 14
Chapter 2 Chromosome and Cell Division
How genetic material pass from cell to cell, from
generation to generation? 1. Somatic cell division
2. Gametic cell division mitosis
Structure of organized DNA in theeukaryotic .
In the eukaryotic nucleus, one of the threadlike structures
consisting of a single DNA molecule packaged in a protein
Morphological parts: Centromere, 2 arms and 2 telemeres Chromosome Morphology
Centromere: constricted region on chromosome
where the two chromatids are connected
p and q arms: p top, q bottom
Telomere: tip of the
chromosome Chromosome Structure Replicated for
cell division Essential for
segregation Somatic Cell Cycle
G1 = Gap 1: prepare for DNA replication, it is the longest
phase of the cell cycle
S = Synthesis: DNA replication
G2 = Gap 2: prepare for cell division
G1 + S + G2 = Interphase
M = Mitosis: cell division
Chromosomes must be exactly replicated and
accurately divided into 2 daughter cells
Daughter cells are identical to each other and to the
Note the DNA amount changes: G1:normal S:double G2:
double M:dynamic Mitosis
Telophase http://bcs.whfreeman.com/WebPub/Biology/pierce4e/Animations%20%20Podcasts/ch02/0201_cell_cycle.html Mitosis - Prophase
>1/2 of mitosis is spent
- Threadlike chromosomes
- Double structure with 2
- Chromatids are held
together by the centromere Formation of mitotic
- Pull the chromatids apart
to the opposite poles.
- Poles - axis for
chromosomal segregation Mitosis - Prometaphase
Microtubules move and
make contact with
toward the equatorial
plane of the cell
(middle of the cell Mitosis - Metaphase
Chromosomes are fully
Centromeres align on
the metaphase plate
Sister chromatids remain
intact, but are ready for
migration to the opposite
Occurs via the spindle
fibers attached to the
kinetochore Mitosis - Anaphase
Shortest stage of mitosis
Sister chromatids disjoin
and migrate to the
opposite ends of the cell.
Spindle fibers pull the
chromatids apart at the
Each chromatid is now At the end of anaphase
each pole has an
identical set of
chromosomes. Mitosis - Telophase
Final stage of mitosis
A complete set of
chromosomes arrive to
each pole of cell.
Chromosomes relax and
uncoil into chromatin
Nuclear membrane reforms.
Division of the cytoplasm
into two cells. Two daughter cells
contain identical copies
of the genetic material. Gamete cell division
Mitosis results in daughter cells with the same amount of
genetic material in the somatic cells
How does the genetic material pass to gamete cells: egg and
sperm (then to next generation)?
- Egg and sperm combine their genetic material as for baby
- Why does a child have 46 instead of 92 chromosomes
after the genetic material combination?
Meiosis results in daughter cells with half the genetic material
as the paternal cell during gamete cell division. Homologous Chromosomes
In most eukaryotic cells, there are two identical sets of
Identical with respect to their gene content, structure, and
One set is inherited from male and
the other from female parents.
Total Chromosome Number (2n)
Number of chromosomes in an
Human 2n=46 Haploid Number (n) ½ the
total chromosome number.
Number of chromosomes in a
gamete. One set of
Human n=23 Meiosis
Meiosis: from the Greek
Two successive nuclear
divisions produce gametes,
in which chromosome
number is reduced by half.
Two divisions produce four
cells with each containing
only one member of each
pair of homologous
chromosomes. Meiosis Phases
Interphase occurs prior to meiosis just as it did before mitosis
DNA or chromosomes replicates
Stages of Meiosis:
Meiosis I (First meiotic division or reduction division)
Prophase I, Metaphase I, Anaphase I, and Telophase I Meiosis II (Second meiotic division or equational division)
Prophase II, Metaphase II, Anaphase II, and Telophase II http://bcs.whfreeman.com/WebPub/Biology/pierce4e/Animations
%20-%20Podcasts/ch02/0202_meiosis.html Meiosis - Prophase I
Several days 5 sub-stages:
Chromosomes condense and become more and more visible.
Homologous Chromosomes pair (synapsis).
Paired homologs Crossing over occurs between non-sister chromotids
Chiasma result of breakage and rejoining between nonsister chromatids. Crossing over
Crossing over: Physical exchange of genetic material between
chromatids of homologous chromosomes.
Chromosomes become a mosaic of the maternal and paternal
I n c r e a ses ge n e t i c v a r i a t i o n .
P a i r e d homologs C r ossi ng ov e r C h i asm a - Poi n t of
c r oss i ng ov e r R esu l t i ng
c h r om a t i ds G e n e t i c r e com b i n a t ion Chromosome Crossing Over Causes Genetic Variation
Locus: gene site on homologous chromosomes (pl. loci).
Each diploid organism contains two copies of each gene Alleles: alternative forms of a gene at certain locus
Each locus may have different or identical alleles.
The alleles specifically control expression of the related trait. Meiosis I
Metaphase I : Tetrads move and align on the mataphase plate.
Anaphase I: Homologs disjoin - Homologous chromosomes are
drawn to opposite poles; centromeres do not divide
Telophase I: haploid set chromosomes are at each poles;
Interkinesis period between meiosis I & II
Nuclear membrane forms; chromosomes relax and each
one still consists of two sister chromatids. Meiosis II
Meiosis II resembles normal mitosis- separation of sister
The chromosome number remains the same in each cell
before and after the second division (equational division).
No DNA replication occurs before meiosis II.
Resulting only one member of each chromosome pair in the
nucleus of cell and each cell contains one single (haploid) set
of chromosomes and becoming gametes. Meiosis II
Prophase II: Chromosomes condense; Nuclear envelopes break down
Metaphase II: Spindle fibers form attaching to the centromeres
(kinetochores); the centromeres line up on the metaphase plate.
Anaphase II: Centromeres separate and each sister chromatids move
to opposite poles = chromatids disjoin
Telophase II: Chromosomes reach the poles; nuclear membranes
form around each set of chromosomes; cytokinesis occurs (cytoplasm
divides) resulting in 4 daughter cells Mitosis vs Meiosis Postmeiotic Events Gamete
Formation and Fertilization
Meiosis produces four haploid cells. These cells must mature
to become functional gametes:
eggs and sperm.
occurs with the union of egg and sperm. The sperm nuclei
unites with the egg producing a zygote (embryo) Meiosis Creates Genetic Variation
Two processes in meiosis creates genetic variation
Chromosome crossing over at prophase I (genetic
Refer to exchange of genes between non-sister
Basis for intra-chromosomal recombination
Random distribution of chromosomes in anaphase I
Refer to independent assortment and random
separation of homologous chromosomes
http://bcs.whfreeman.com/WebPub/Biology/pierce4e/Animations%20%20Podcasts/ch02/0203_genetic_var_meiosis.html Homologous Chromosome Random Separation
Causes Genetic Variation
Determined by the number of chromosomes in the haploid
genome of each organism and is equal to 2n, where n is the
n = 4, then 24 = 16 or 16 different gamete combinations. Example: Human with 23 chromosome pairs (n = 23).
223 = 8,388,608 different gametes
Does not include the additional genetic diversity created through
recombination or crossing over.
Reason we are each distinct in appearance and individuality
except identical twins (monozygotic) Homologous
N=3, 23 = 8 ...
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- Summer '08