Download Reproduction

GeneticsHeredity and Meiosis
IB Biology
Campbell- Chapter 13
Orange Book- Ch. 11
Chromosomes
• Condensed DNA
attached to proteins
• Can only be seen
when a cell is actively
undergoing mitosis.
• Typical humans form
46 chromosomes vs.
other organisms which
varies significantly.
• Our 46 chromosomes
are thought to contain
anywhere from 25,000
to 100,000 genes.
Sister chromatids
Centromere
Terms to Know:
Gene- a heritable factor that controls a specific
characteristic.
Allele- One specific form of a gene.
Genome- the whole genetic information of an
organism.
Heredity and Sexual Reproduction
• Heredity: the transmission of traits from
one generation to the next
• Sexual reproduction = new individual by a
combination of two haploid sex cells
(gametes).
• Fertilization = combination of genetic
information from two separate cells that
have one half the original genetic
information
Sexual Reproduction
• Female- produces an egg
• Male produces sperm
• Both gametes are haploid (n) or a single set of
chromosomes
• new individual called a zygote, with two sets of
chromosomes (diploid or 2n).
• Meiosis is a process to convert a diploid cell
to a haploid gamete, and cause a change in
the genetic information to increase diversity
in the offspring.
Ploidy: Number of sets of
chromosomes in a cell
• Haploid (n)-- one set chromosomes
• Eggs and sperm are haploid (n)
• Diploid (2n)-- two set of chromosomes.
Most plant and animal cells (somatic cells)
are diploid (2n)
Chromosomes in a Diploid Cell
• Summary of chromosome characteristics
• Diploid set for humans; 2n = 46
• Autosomes; homologous chromosomes, one
from each parent (humans = 22 sets of 2), plus:
• Sex chromosomes (humans have 1 set of 2)
– Female-sex chromosomes are homologous (XX)
– Male-sex chromosomes are non-homologous (XY)
The Human Life Cycle
Reduction Division
• Living organisms which reproduce
sexually halve their chromosome number
in preparation for fusion of gametes.
At fertilization, a sperm fuses with an
egg, forming a diploid zygote
– Repeated mitotic divisions lead to the
development of a mature adult
– The adult makes haploid gametes by
meiosis
– All of these processes make up the
sexual life cycle of organisms
Meiosis
• Chromosome replicate
• 2 Cell divisions occur
(Meiosis I & Meiosis II)
• 4 daughter cells are
made all are (n): haploid
• Homologous Chrom’s
separate in meiosis I
• Meiosis II = Mitosis
(chromatids separate)
Homologous chromosomes
carry different versions of genes
• The differences between homologous
chromosomes are based on the fact that
they can carry different versions of a gene
(alleles) at corresponding loci
Meiosis divided into meiosis I and
meiosis II
• Prophase I – pairing
of homologous
chromosomes
• Called synapsis
• Animations (unique
features )
Prophase 1
• Chromosomes pair up
as homologous
chromosomes
• Nuclear membrane
disappears
• Spindle fibers grow
from each pole
Prophase 1
Homologs find their partner, becoming
intimately associated, forming a
synaptonemal complex ,a lattice of
proteins
This is called a tetrad (basically- 2 pairs of
chromosome pairs that are attached)
Prophase 1
• With this relationship of homologs,
crossing over occurs.
• Crossing over is the exchange of
corresponding segments between two
homologous chromosomes
• Genetic recombination results from
crossing over during prophase I of meiosis
• The site of crossing over is called
chiasmata.
Prophase 1
Tetrad
Chiasma
Centromere
Metaphase I
• Terminal chiasmata
hold homologs
together.
• Chromosomes line up
at the center.
• Spindle attaches to
only one side, which
will pull sister
together.
Anaphase I
• Microtubules shorten
and chromosomes
separate.
• Break chiasmata
• At finish each pole
has a full set of
chromosomes which
have been
independently
assorted.
Telophase I
• Chromosomes cluster
• Nuclear membrane
forms
• Cell may or may not
split into 2 (depends
on the species)
Meiosis II*
• Division time variable but typically a short
Interphase
• Cytokinesis may or may not occur
• No synthesis (chromosomes do not
double)
Prophase II
• Breakdown nuclear
membrane
• Centrioles form and
move toward the
poles
• New spindles form
Metaphase II
• Microtubules grow
from the centrioles
and attach to the
centromeres
• The sister chromatids
line up along the cell
equator
Anaphase II
• The centromeres
break and sister
chromatids separate
• Cytokinesis begins
Telophase II
• The chromosomes
may decondense
(depends on species)
• Cytokinesis reaches
completion, creating
four haploid
daughter cells
Origins of Genetic Variation
(1) Independent assortment:
How they line up during metaphase I
Matters!!!
Homologous pairs of chromosomes
position and orient themselves
Randomly. (random positioning)
Different combinations are possible
when gametes are produced.
POSSIBILITY 1
POSSIBILITY 2
Two equally
probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
Origins of Genetic Variation
(2) Crossing over (prophase
I):
-the reciprocal exchange of
genetic material between
nonsister chromatids during
synapsis of meiosis I
(recombinant chromosomes)
(3) Random fertilization:
1 sperm (1 of 8 million possible
chromosome combinations) x
1 ovum (1 of 8 million different
possibilities) = 64 trillion
diploid combinations!
Meiosis vs. Mitosis
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
• Synapsis/tetrad/chiasmata
(prophase I)
• Homologous vs. individual
chromosomes (metaphase
I)
• Sister chromatids do not
separate (anaphase I)
• Meiosis I separates
homologous pairs of
chromosomes, not sister
chromatids of individual
chromosomes.
Mitosis
Meiosis
Results in Identical cells
Cells not identical
Cells are diploid
Cells are haploid
In most all cells
In only the sex cells
No crossing over
Crossing over/chiasmata
Sister chromatids pair up
Tetrads pair up
One set of phases
2 sets of phases
Ends after first set
Meiosis II- Chromosomes
do not double
Let’s watch
• Meiosis
Review
• Meiosis
• meiosis = double cell division
The Human Karyotype
• There are 44 autosomes and 2 sex
chromosomes in the human genome, for a
total of 46.
• Karyotypes are pictures of homologous
chromosomes lined up together during
Metaphase I of meiosis.
• The chromosome micrographs are then
arranged by size and pasted onto a sheet.
Human female karyotype
Human Male Karyotype
Karyotypes
• Used for pre-natal diagnosis of
chromosome abnormalities and to
positively identify gender.
• Performed using cells collected by
amniocentesis or chorionic villi sampling
(the removal of a small piece of placenta
tissue from the uterus during early
pregnancy).
Chorionic Villus sampling
Ethical issues
• Side effects (possible miscarriage)
• Aborting a fetus based on sex or possible
genetic diseases
• Who should make the decisions- parents,
health-care professionals, or both?