Title: Stay alert....feast on this lecture because it is all about you
1Stay alert....feast on this lecture because it is
all about you!
2Chapter 13 Meiosis Sexual Life Cycles
- Introduction to Heredity
- Genes, DNA Chromosomes
- Sexual Asexual Reproduction
- Sexual Life Cycle Humans
- Meiosis Role in Sexual Life Cycles
- Variety of Sexual Life Cycles
3Chapter 13 Meiosis Sexual Life Cycles
- Meiosis A Detailed Look
- Chromosome number reduced
- Comparison Mitosis Meiosis
4Chapter 13 Meiosis Sexual Life Cycles
- Origins of Genetic Variation.
- Sexual Sources of Genetic Variation
- Independent assortment of chromosomes
- Crossing over
- Random variation
5Like Begats Like
- Offspring resemble their parents more than they
do less closely related individuals. - This continuity of traits from one generation to
the next is called HEREDITY (heir).
6Like Begats Like
7Like Begats Like
- Along with inherited similarity there is also
variation. - So how are these similarities and variations
selected at a fundamental level?
8Like Begats Like
- Mechanisms of heredity and variation eluded
biologists until the development of genetics in
the last century (1900s). - GENETICS is the scientific study of heredity
variation.
9Genes, DNA Chromosomes
- Parents endow their offspring with coded
information in the form of hereditary units
called GENES (not Levis 501 JEANS) Genes vs.
Jeans
10Genes, DNA Chromosomes
- Our genes program the emergence of specific
traits as we develop from fertilized eggs into
adults.
11Genes, DNA Chromosomes
- Genes are made of DNA, which is a polymer of
four different kinds of monomers called
NUCLEOTIDES. - Review Chapters 1 5.
12Genes, DNA Chromosomes
- Inherited information is passed along in the
form of each genes specific nucleotide sequence.
13Genes, DNA Chromosomes
- The language is abstract...the sequence (word or
sentence) does not resemble what is actually made.
14Genes, DNA Chromosomes
- Cells translate genetic sentences into big
noses and other features that bear no
resemblence to genes.
15Genes, DNA Chromosomes
- Inheritance has its chemical basis in the precise
replication of DNA, which produces copies of
genes that can be passed along from parents to
offspring.
16Genes, DNA Chromosomes
- The cellular vehicles for these genes are
sperm (males) ova (females)
17Genes, DNA Chromosomes
- Biological order based on heritable programs in
the form of DNA is one of the uniflying themes in
biology (e.g. biology megatheme)
18Genes, DNA Chromosomes
- DNA in eukaryotes is subdivided into chromosomes
located within the nucleus....each species has a
characteristic chromosome number (e.g. you know
we have 46 except in reproductive cells).
19Genes, DNA Chromosomes
- Each chromosome is composed of a single DNA
molecule that is very long. - Along with proteins, the DNA is folded and coiled.
20Genes, DNA Chromosomes
- Each chromosome has hundreds or thousands of
genes each occupying a specific region on the
chromosome (e.g. locus, not locust).
21Chromosomal Behavior
- Our genetic endowment consists of whatever genes
happened to be part of the chromosomes we
inherited from our parents.
22Chromosomal Behavior
- How does sexual reproduction pass chromosomes
from parent to offspring??????? - This is what your parents couldnt tell
you.........
23Sexual Asexual Reproduction A Comparison
- Strictly speaking, like begats like only to
organisms that reproduce asexually.
24Asexual Reproduction
- A single individual is the sole parent and
passes on all its genes to its offspring exact
copy clone - Remember Dolly was a clone...was it asexual
25Asexual Reproduction
- Differences in asexual reproduction are due to
mutations, which are rare changes in DNA. - Sexual reproduction usually results in much
greater variation than for asexual reproduction.
26Sexual Reproduction
- Two parents give rise to offspring..they have a
unique combination of genes from both parents.
27Sexual Reproduction
- What mechanisms generate this genetic variation?
- The key is the activity of chromosomes during
the sexual cycle (e.g. megatheme)
28Sexual Life Cycles Humans
- Life cycle refers to generation-to-generation
sequence of stages in the reproductive history of
an organism, from conception to production of its
own offspring.
29Sexual Life Cycles Humans
- Each somatic cell (other than sperm or egg) has
46 chromosomes.
30Sexual Life Cycles Humans
- Appearance of each chromosome is used to
distinguish them (banding, centromere
position/size)
31Sexual Life Cycles Humans
- Karyotypes are displays of an individuals
somatic-cell metaphase chromosomes arranged in a
standard sequence to determine if there are
abnormalities (e.g. lymphocytes examined)
32Karyotyping
X Y Chromosomes
33Sexual Life Cycles Humans
- Homologous Chromosomes a pair of chromosomes
that have the same size, centromere position and
staining pattern.
34Sexual Life Cycles Humans
- Homologous AUTOSOMES carry the same genetic
loci however, human sex chromosomes carry
different loci even though they pair during
prophase of Meiosis I........more later!
35Sexual Life Cycles Humans
- Human females have a pair of homologous X
chromosomes (XX), but males have one X and one Y
chromosome (XY).
36Sexual Life Cycles Humans
- We have 22 pairs of autosomes and 1 pair of sex
chromosomes.
37Karyotype Human Lymphocytes22 Autosomal Pairs,
1 Sex Pair (XY)
38Sexual Life Cycles Humans
- Somatic cells in humans and most other animals
are diploid (2 sets of 23 chromsomes, one
maternal (mommy) one paternal (daddy) set - (2n46, humans)
39Sexual Life Cycles Humans
- A cell with a single chromosome set is a haploid
cell - n 23 in humans
40Sexual Life Cycles Humans
- The union of haploid sperm with haploid ovum is
called fertilization or syngamy. Syntogether
gamygamete
41Sexual Life Cycles Humans
- Via mitosis, the zygotes genes are passed along
to all somatic cells.... - Thus, all somatic cells are diploid.
42- The only cells in the body not produced by
mitosis are gametes.
43Sexual Life Cycles Humans
- Sexually reproducing cells carry out a process
that halves the chromosome number in the gametes,
compensating for the doubling that occurs at
fertilization...this cell division is MEIOSIS
(Fig. 13.3)
44Human Life Cycle Alternates Meiosis with
Fertilization
n
n
Meiosis
2n
Mitosis
45Sexual Life Cycles
- Although the alternation of meiosis
fertilization is common to all sexually
reproducing organisms, the timing of the events
in the life cycle may vary. Review Fig. 13.4
46Meiosis Reduces Chromosome Number Diploid to
Haploid
- Meiosis, like mitosis, is preceded by the
replication of chromsomes. - This is followed by two consecutive cell
divisionsmeiosis I meiosis II
47Meiosis Reduces Chromosome Number Diploid to
Haploid
- Cell divisions produce 4 daughter cells instead
of 2 as in mitosis. - Resulting daughter cells have half the number of
chromosomes are different from their parent
cells.
48How Meiosis Reduces Chromosome Number (Fig. 13.5)
Homologues Separate Meiosis I
Sister Chromatids Separate Meiosis II
Chromosomes Replicate Only One Time
Two Haploid Daughter Cells
Four Haploid Daughter Cells Half of Parental DNA
49Stages of Meiotic Cell Division
Meiosis I
Interphase I
Prophase I
Metaphase I
Anaphase I
Separates Homologues
50Meiosis I
Meiosis II Chromatids Separated
HAPLOID
Telophase I Cytokinesis
Anaphase II
Telophase II Cytokinesis
Prophase II
Metaphase II
51MEIOSIS MITOSIS COMPARISON (Fig. 13.7, p. 234)
- Chromosome number is reduced by half in meiosis,
but not mitosis. - Mitosis produces identical daughter cells
meiosis produces cells different from parents and
each other.
52MEIOSIS MITOSIS COMPARISON
- Meiosis Synapsis occurs, pairing homologous
chromosomes (tetrads) - Cross-over region is called chiasmata, and
nonsister chromatids exchange genetic material.
53MEIOSIS MITOSIS COMPARISON
- Homologous pairs (meiosis) align on plate, rather
than as individuals (mitosis).
54MEIOSIS MITOSIS COMPARISON
- Anaphase I of meiosis, sister chromatids remain
together (Bowtie intact), rather than separating
(mitosis).
55MEIOSIS MITOSIS COMPARISON
- Meiosis II similar to mitosis but without
replication, thus the number of chromosomes is
halved. Bowties broken separate chromatids,
haploid cells.
56MEIOSIS
MITOSIS
Parent cell
Chiasma
Crossing over
Chromosomes
Tetrads
Daughter cells of Meiosis I
2n
2n
n
n
n
n
Daughter cells of Meiosis II
57SEXUAL SOURCES OF VARIATION Three Major Sources
(pp. 235-37)
- Independent Assortment of Chromosomes
Orientation of homologous chromosome pair
relative to the two poles is random. See Fig.
13.8(see Meiosis I Metaphase I).
58Metaphase of Meiosis IITwo equally probable
arrangements of chromosomes (Diploid, 2n 4)
Blue Chromosome from DAD Yellow is from MOM
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
59SEXUAL SOURCES OF VARIATION
- The number of combinations possible for gametes
formed by meiosis starting with two homologous
pairs of chromosomes is four or 2n. - n haploid number
60SEXUAL SOURCES OF VARIATIONIndependent
Assortment
- Haploid number (n) for humans is 23, thus the
possible number of maternal and paternal
chromosomes is 223 or about 8 million. Or, 8
million possible assortments of combinations of
heads or tails for simultaneous tossing of 23
coins.
61SEXUAL SOURCES OF VARIATION Source Number 2
- Crossing Over (Fig. 13.9)Occurs during prophase
I of meiosis I. SYNAPTONEMAL COMPLEX (protein)
brings nonsister chromosomes into close proximity
for precise pairing and alignment of genes.
62Crossing Over Fig. 13.9
Tetrad
Chiasma
63SEXUAL SOURCES OF VARIATION
- Homologous portions of nonsister chromotids trade
places. For us, this occurs two or three times
per chromosome pair. This is an important
source of genetic variation.
64SEXUAL SOURCES OF VARIATION Source Number 3
- Random fertilization A human ovum, representing
8 million combinations, combines with a sperm,
representing 8 million different possibilities to
produce a zygote with any of 64 TRILLION diploid
combinations (8 X 8 million).
65Sexual Sources of VariationSummary
- Independent Assortment of Chromosomes
- Crossing Over
- Random Fertilization
66Chapter 13 OverviewMeiosis Sexual Life Cycles
- Genes, DNA Chromosomes
- Sexual Asexual Reproduction (see next slide)
- Sexual Life Cycle Humans
- Variety of Sexual Life Cycles
67Asexual vs. Sexual Reproduction
- Asexual Reproduction
- Single individual is sole parent
- Single parent passes all genes to offspring
- Offspring genetically identical to parent
- Result is a clone. Mutation is only source of
variation (rare).
- Sexual Reproduction
- Two parents give rise to offspring
- Each parent passes on half its genes to offspring
- Offspring have unique combination of genes from
both parents - Results in greater genetic variation offspring
vary genetically from parents and siblings.
68Chapter 13 Meiosis Sexual Life Cycles
- A Detailed Look at Meiosis
- Comparison of Mitosis Meiosis
- Sexual Sources of Genetic Variation
69Chapter 13 Summary
- Review Study Outline
- Take Self-Study Quiz
- Review BOLDFACE TERMS
- Read Chapter 14
- Ask questions of Recitation TA
- Use Biology Tutor (no charge!)