GAMETE PRODUCTION IN SEXUAL REPRODUCTION MEIOSIS Slide 2 What
are chromosomes? Human cell = >6 billion nucleotide base pairs
(~2 meters) Wrapped around protein = chromatin DNA/protein =
chromosome Slide 3 How many chromosomes do humans have? Humans are
diploid (2n) Two of each chromosome, one from each parent. n = 23
unique chromosomes (haploid #) 2(n) = 46 total chromosomes The two
copies of each chromosome in human cells are homologous Curly hair
allele Strait hair allele Slide 4 Families: similar yet different.
Why? Single parent Offspring identical to each other and parent
Sexual Reproduction Two parents Offspring are unique Offspring are
similar to each other and parents Combine DNA from two individuals
Combines characteristics of both individuals Asexual Reproduction
Slide 5 MEIOSIS While mitosis conserves chromosome number, meiosis
reduces the chromosome number by half Meiosis has a few stages:
Interphase, Meiosis I, and Prophase I Metaphase I Anaphase I
TelophaseI Meiosis II Prophase II Metaphase II Anaphase II
Telophase II Slide 6 MEIOSIS INTERPHASE Interphase is similar to
the process in mitosis The process results in two genetically
identical sister chromatids The chromatids are connected at their
centromeres Centrosomes also replicate Slide 7 PROPHASE I
Chromosomes begin to condense Homologues (consisting of two sister
chromatids) pair up When the chromosome pairs become visible, they
show up as a tetrad, a cluster of four chromatids Homologous
chromosomes can criss-cross, allowing the chromatids to switch
segments, called crossing- over Slide 8 PROPHASE I continued
Centrosomes move away from each other Microtubules form between the
centrosomes Nuclear envelope and nucleoli disperse Prophase I can
last for days or longer, and takes up 90% of the time required for
meiosis Slide 9 METAPHASE I Chromosomes arrange on the metaphase
plate (equator), still in homologous pairs Microtubules from one
pole attach to one chromosome from each pair The other chromosome
(homologous pair) is attached to a microtubule from the other pole
Slide 10 ANAPHASE I The spindle apparatus guides the chromosomes
towards the poles Sister chromatids remain attached at their
centromeres The sister chromatids move together towards the same
pole Slide 11 TELOPHASE I The homologous chromosome pairs continue
to move apart to each pole Each pole contains a haploid (n) number
of a chromosome set Cleavage furrows form in animal cells, and cell
plates appear in plant cells Slide 12 TELOPHASE I AND CYTOKINESIS
Cytokinesis usually occurs simultaneously with telophase I, forming
two daughter cells There is no further replication of genetic
material Depending on the species, the chromosomes de- condense and
the nuclear membranes and nucleoli re-form. Slide 13 PROPHASE II A
spindle apparatus forms The chromosomes progress towards the
metaphase II plate (equator of the cell) Slide 14 METAPHASE II The
chromosomes are positioned on the metaphase plate in mitosis-like
fashion The microtubules attach to the centromeres of the sister
chromatids Slide 15 ANAPHASE II The centromeres of the sister
chromatids finally separate The sister chromatids of each pair, now
individual chromosomes, move toward opposite poles of the cell
Slide 16 TELOPHASE II AND CYTOKINESIS Nuclei form at opposite poles
of the cell Cytokinesis occurs In the end, there are four daughter
cells, each with the haploid number of un-replicated
chromosomes
