meiosis-how we inherit genes from our parents...meiosis –haploid vs. diploid • meiosisis the...
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MEIOSIS-HOW WE INHERIT GENES FROM OUR PARENTS
BIOLOGY– MRS.FLANNERY
• Heredity is the transmission of traits from one generation to the next.
• Genetics is the scientific study of heredity.
• Gregor Mendel
– worked in the 1860s,
– was the first person to analyze patterns of inheritance
– deduced the fundamental principles of genetics, without the knowledge of exactly WHAT is being inherited.
Introduction to Genetics
• Before we talk about Mendel’s work, lets talk about how, through sexual reproduction, we inherit DNA.
• Sexual reproduction
– depends on meiosis and fertilization
– produces offspring that contain a unique combination of genes from the parents.
Meiosis – The Basis of Sexual Reproduction
Meiosis – Haploid vs. Diploid
• Meiosis is the process of cell division that produces haploidgametes in diploid organisms.
• A human somatic cell
– is a typical body cell
– has 46 chromosomes (diploid, 2n=46, n=haploid number).
• A gamete
– is a sex cell, sperm and egg
– made by meiosis in the ovaries and testes
– has 23 chromosomes (haploid, n=23)
– Haploid egg and haploid sperm fuse during fertilization (caused by sex!!!) to form a diploid organism.
Figure 8.12
Multicellular
diploid adults
(2n 46)
MEIOSIS FERTILIZATION
MITOSIS
2n
and development Key
Sperm cell
n
n
Diploid
zygote
(2n 46)
Diploid (2n)
Haploid (n)
Egg cell
Haploid gametes (n 23)
Why do we need meiosis??
• Homologous chromosomes
– remember we (humans = Homo sapiens) have 46 total chromosomes, 23 pairs!!
– are matching pairs of chromosomes that can possess different versions of the same genes.
• A karyotype is an image that reveals an orderly arrangement of chromosomes.
• Humans have
– 22 pairs of matching chromosomes, called autosomes.
– two different sex chromosomes, X and Y,
Homologous Chromosomes
LM
The bands are genes (instructions for making proteins). You get one set from mom and one set from dad. Each chromosome pair
has the same genes, but just different versions called alleles.
Figure 9.29a
Male Female
Sperm Egg
Offspring
Female Male
44
XY
44
XX
22X
22Y
22X
44
XX
44
XY
Somatic
cells
The Process of Meiosis
• In meiosis,
– haploid daughter cells are produced in diploid organisms
– Interphase (chromosome duplication – just like mitosis) is followed by two consecutive divisions, meiosis I and meiosis II
– crossing over occurs.
How Meiosis Halves Chromosome Number
INTERPHASE BEFORE MEIOSIS
Sister
chromatids
Duplicated pair
of homologous
chromosomes
Chromosomes
duplicate.
Pair of
homologous
chromosomes
in diploid
parent cell
1
Figure 8.13-2
MEIOSIS I
Homologous
chromosomes
separate.
INTERPHASE BEFORE MEIOSIS
Sister
chromatids
Duplicated pair
of homologous
chromosomes
Chromosomes
duplicate.
Pair of
homologous
chromosomes
in diploid
parent cell
1 2
Figure 8.13-3
MEIOSIS I
Sister chromatids
separate.
MEIOSIS II
Homologous
chromosomes
separate.
INTERPHASE BEFORE MEIOSIS
Sister
chromatids
Duplicated pair
of homologous
chromosomes
Chromosomes
duplicate.
Pair of
homologous
chromosomes
in diploid
parent cell
1 2 3
Review: Comparing Mitosis and Meiosis
• In mitosis and meiosis, the chromosomes duplicate only once, during interphase.
• The number of cell divisions varies:
– Mitosis uses one division and produces two diploid cells.
– Meiosis uses two divisions and produces four haploid cells.
• All the events unique to meiosis occur during meiosis I
Figure 8.15
Duplicated
chromosome
MITOSIS
Prophase
Chromosomes
align.
Metaphase
Sister chromatidsseparate.
Anaphase
Telophase
2n
Prophase I
Metaphase I
Anaphase I
Telophase I
MEIOSIS
MEIOSIS I
Site of crossing over
Homologous pairs align.
Homologous chromosomes separate.
Sister chromatidsseparate.
Haploid
n 2
MEIOSIS II
Parent cell
n
MEIOSIS I
2n
n n n
The Origins of Genetic Variation
• Offspring of sexual reproduction are genetically different from their parents and one another.
Independent Assortment of Chromosomes
• When aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair of chromosomes is a matter of chance.
• Every chromosome pair orients independently of all of the others at metaphase I.
• For any species, the total number of chromosome combinations that can appear in the gametes due to independent assortment is
2n, where n is the haploid number.
Figure 8.16-1
Two equally probablearrangements ofchromosomes
at metaphase ofmeiosis I
POSSIBILITY 1 POSSIBILITY 2
Figure 8.16-2
Two equally probablearrangements ofchromosomes
at metaphase ofmeiosis I
Metaphase
of
meiosis II
POSSIBILITY 1 POSSIBILITY 2
Figure 8.16-3
Two equally probablearrangements ofchromosomes
at metaphase ofmeiosis I
Metaphase
of
meiosis II
Combination a
POSSIBILITY 1 POSSIBILITY 2
Combination b Combination c Combination d
Gametes
Because possibilities 1 and 2 are equally likely, the four possible types ofgametes will be made in approximately equal numbers.
Random Fertilization
• A human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote.
• If each gamete represents one of 8,388,608 different chromosome combinations, at fertilization, humans would have 8,388,608 ×8,388,608, or more than 70 trillion different possible chromosome combinations.
• So we see that the random nature of fertilization adds a huge amount of potential variability to the offspring of sexual reproduction.
Crossing Over
• In crossing over,
– non-sister chromatids of homologous chromosomes exchange corresponding segments and
– genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs.
Figure 8.18
Prophase I of meiosis Duplicated pair of homologouschromosomes
Chiasma, site ofcrossing over
Spindlemicrotubule
Homologouschromatids exchangecorrespondingsegments.
Metaphase I
Metaphase II
Sister chromatidsremain joined at theircentromeres.
Gametes
Recombinantchromosomes combinegenetic informationfrom different parents.
Recombinant chromosomes
Figure 8.18a
Homologous chromatids exchange correspondingsegments.
Prophase I of meiosis Duplicated pair of homologouschromosomes
Chiasma, site ofcrossing over
SpindlemicrotubuleSister chromatids
remain joined at theircentromeres.
Metaphase I
Figure 8.18b
Metaphase II
Gametes
Recombinant chromosomes
Recombinantchromosomes combinegenetic informationfrom different parents.