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Flashcards in Meiosis Deck (21)
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1

Meiosis intro

• Parents endow offspring with encoded information called genes
• Genes program the specific traits that emerge as we develop from fertilized eggs into adults
• In animals and plants, reproductive cells called gametes are the vehicles that transmit genes from one generation to
the next
• A gene’s specific location along the length of a chromosome is called the gene’s locus (loci)

2

Asexual Reproduction

• Only organisms that reproduce asexually have
offspring that are exact genetic copies of
themselves
• Asexual reproduction involves:
o One parent
o Transfer of all its genes to offspring
o No fusion of gametes
o Gives rise to clones
• Genetic differences only arise from mutations

3

Sexual Reproduction

• Two parents give rise to offspring that have unique
combinations of genes inherited from two parents
• In contrast to clones, offspring of sexual
reproduction vary genetically from siblings and
parents
• Involves:
o Two parents
o Transfer of genes from both parents
o Fusion of gametes

4

Human karyotype

• A set of chromosomes in a cell is called a karyotype
• It shows the number, size and shape of the chromosomes during metaphase of mitosis
• In human karyotype, non-sex chromosomes (autosomes) are grouped together in pairs and placed in groups A-G
• Each chromosome of a pair have the same length, centrosome position and staining pattern à homologous
chromosomes or homologs
• Sex chromosomes, gonosomes, (X-female, Y-male) are placed separately
• Karyotypes are useful as they show:
o Whether a cell comes from a male or female
o Abnormal chromosomes

5

Meiosis

• Meiosis is cell division that takes place in the reproductive organs of
both plants and animals and produces:
o Gametes – animals
o Spores – plants
• In meiosis the number of chromosomes is reduced from two sets (2n)
in the parent cell to one set (n) each of the daughter cells formed
• The gametes/spores formed are called haploid as they only have one
set of chromosomes
• In sexual reproduction a male haploid gamete fuses with female
haploid gamete during fertilization to form a diploid zygote
• Animals – testes & ovaries
o Formation of sperm – spermatogenesis
o Formation of eggs – oogenesis
• Plants – sporangia in seed-bearing plants
o Microsporangia – pollen sacs in anthers
o Mega sporangia – ovules on female ovaries

6

How does meiosis take place?

• DNA of parent cell is replicated in interphase
• This precedes both meiosis and mitosis
• However, meiosis replication is followed by two divisions
o Meiosis 1 – reduction division which results in two cells forming with half number of chromosomes – haploid
o Meiosis 2 – copying division where two haploid cells are divided again by mitosis to form 4 haploid cells

7

Prophase I

• Chromatin network condenses
• Separate chromatids (although replication has occurred)
cannot be distinguished
• Homologous chromosomes arrange themselves in pairs
• Two chromosomes of each homologous pair lie along
side each other to form a bivalent
• Each chromosome of the bivalent splits longitudinally into
two identical chromatids called sister chromatids
• Crossing over of the chromatids takes place
• Nuclear membrane and nucleolus disappear
• Centrosomes duplicates and centrioles move to opposite
poles
• Spindle fibres develop between the centrioles

8

Metaphase I

• Bivalents position themselves on equator
• Chromosomes lie in a double row on either side of
equator
• Some spindle fibres attach to centromeres

9

Anaphase I

• Spindle fibres contract and shorten pulling chromosomes
towards poles
• Homologous chromosomes are separated
• One chromosome of bivalent moves to one pole and the
other chromosome to the other pole
• Cytokinesis begins
NB: no division of centromeres occurs. Therefore, chromosomes
move to poles and not chromatids. Separation of homologous
pairs results in chromosome number halving

10

Telophase I

• Chromosomes group at poles
• Spindle fibers disappear
• Nuclear membrane and nucleolus form
• Invagination occurs to form two daughter cells
• Each daughter cell has one chromosome from each
homologous pair – haploid number
• Chromosome is double stranded with recombinant
chromatids

11

Crossing Over

The mutual exchange of pieces of chromosomes.
Groups of genes are swapped between maternal
and paternal chromosomes
• When homologous pairs come together
(synapsis) to form bivalents, they swap
pieces of their inner chromatids – thus
breaking and reforming their DNA
• Points of crossing over where chromatids
break are called chiasmata
• In this way some genes of paternal
chromatid change place with that of
maternal chromatid
• Forms a recombinant chromatid
• Outer unchanged chromatids are called
parentals
Importance
• Exchange produces unique combination of
genes and increases variation
• During exchange process mistakes may
occur leading to mutation – sometimes
beneficial in influencing evolution

12

Prophase II

• Each chromosome consists of two chromatids and
centromere
• The nucleolus a nuclear membrane disappears
• Spindle forms consisting of centrioles at opposite poles

13

Metaphase II

• The chromosomes move to the equator and align in a single
row
• Some spindle fibres attach to the centromeres

14

Anaphase II

• The centromere of each chromosome splits into two
• The two chromatids separate and move to opposite poles
• Chromatids at poles are known as daughter chromosomes
• Cytokinesis begins

15

Telophase II

• Daughter chromosomes group at poles
• Chromosomes in each daughter cell are single stranded with
recombinant chromatids
• Spindle fibres disappear
• Nuclear membrane and nucleolus forms
• Invagination occurs
• 4 daughter cells have formed each with haploid number of
chromosomes
• Due to crossing over genetic information in each cell is not
identical
• Daughter cells in male = sperm
• Female = eggs

16

Mitosis and Meiosis: similarities

• Both are cell division processes
• Both require DNA replication during Interphase before division begins
• Both go through: Prophase, Metaphase, Anaphase and Telophase
• In both cases the chromatin network forms chromosomes in early prophase

17

Mitosis and Meiosis: Differences
Site where process takes place


Mitosis:
• Takes place in plants and animals when zygote
develops into fully grown organism
• Meristematic tissue – plants (stem, roots &
cambium)
• Damaged tissue in animals and plants

Meiosis:
• Takes place in reproductive organs of animals
• Takes place in sporangia of sporophyte
generation in plants to produce spores

18

Mitosis and Meiosis: Differences
Purpose of the process

Mitosis:
• Multiplication of cells with no change in
chromosome number – daughter cells have
same number of chromosomes as mother cell
• Production of identical daughter cells – no
variation
• Mitosis takes place during:
• Growth: zygote – embryo – adult
• Repair of damaged tissue o
• Asexual reproduction

Meiosis:
• The halving of chromosome number from the
diploid (2n) to haploid (n) number during
production of gametes – prevents doubling in
each generation
• Introduces variation through the crossing over
process and random assortment

19

Mitosis and Meiosis: Differences
Differences in the division process

Mitosis:
• Only one division takes place
• Chromosome number in each nucleus remains
the same after division
• Division is preceded by interphase – DNA
replication
• No pairing of homologous chromosomes
• No bivalent formation
• No crossing over and exchange of genetic
material
• Centromeres divide during Anaphase
• Two daughter cells each with same number of
chromosomes as mother cell are formed
• Daughter cells form somatic cells and are
identical to each other and mother cell

Meiosis:
• Two divisions take place – meiosis I & II
• Chromosome number in each nucleus is halved
from diploid (2n) to haploid (n) after division
• Only meiosis I is preceded by Interphase
• Homologous chromosomes arrange themselves
in pairs and bivalents form during Prophase I
• Crossing over occurs between homologous
pairs
• Centromeres do not divide during Anaphase I
but divide during Anaphase II
• Four daughter cell are formed each with haploid
number of chromosomes
• Daughter cells formed are not identical to each
other or to the mother cell

20

Genetic variation

Variation is the differences that occur between
organisms of the same species In sexual
reproduction there is always variation
Causes:
• Crossing over of pieces of chromatids
causes reshuffling of genes in gametes
formed
• Random movement of maternal and
paternal chromosomes to each pole
during anaphase (independent
assortment)
• Mutations

21

Mutations

Unpredictable change in genetic make-up when meiosis does not
occur normally
Causes:
• Genetic mutation
• Chromosome mutation – occurs during meiosis, change in
structure and distribution of chromosomes
• Also change in chromosome number
Examples:
• Polyploidy – having more than one set of chromosomes (4n
& 6n etc.)
o As a result of abnormal meiosis, chromosomes do
not separate during anaphase I – gametes are
diploid
o Aneuploidy – having extra or missing chromosomes