MCB 2400 Lecture Notes - Lecture 10: Sister Chromatids, Pseudoautosomal Region, Homologous Chromosome
15 Dec 2016
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Phenomena contributing to the generation of genetic variability during meiosis:
“egegatio①ad①Idepedet①②ssotet①Medel’s①Las
Recombination (crossing-over)
Mixes up possible allele combinations
Each eukaryotic species has a characteristic number of chromosomes
Humans are Diploid, 2N
• Human Genome:
o 23 chromosome pairs
o 22 pairs of autosomes
o 1 pair of sex chromosome
• Karyotype:
o Human karyotype consists of 46 chromosomes
o Male XY
o Female XX
• Homologous Pair:
o Allele A
o Allele a
o One version of each gene
Chromosome Structure:
• Eukaryotic chromosomes exist in four major types based on the position of the centromere
Cell Division Types:
• Mitosis
o Goal: make two genetically identical daughter cells
o Start with diploid parent cell (2N: 2 of each chromosome type in somatic cells)
▪ 22 autosomes
▪ 1 sex chromosome
▪ Chromosomes present in pairs, inherit one copy of each type from sperm and
one from egg
▪ 2 pairs = 4 total chromosomes (2 copies of each chromosome type, 2N=4)
o DNA replicates during S (synthesis) phase of cell cycle
▪ DNA is replicated, each single DNA molecule (double stranded molecule,
antiparallel nucleotide chain) goes through synthesis, each chromosome
consists of dyad: two DNA molecules that are completely identical (each DNA
molecule is one sister chromatid) (two sister chromatids attached at centromere
is a chromosome)
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o 2 daughter cells (2N) formed by mitosis
▪ Genetically identical to one another and parent cell
o One round of DNA replication and one round of cell division
• Meiosis
o Goal: make haploid gametes (N) (no doubling of quantity of DNA present during each
generation)
▪ Germ cells specialized for reproduction
▪ Have only one copy of the genome
o Start with diploid parent cell
o DNA replicates
o Reductional division: 2 daughter cells (N) (meiosis I)
▪ No longer have homologous pairs
▪ 1 member of each homologous pair remaining in each cell
o 4 daughter cells (N) (meiosis II)
▪ Each chromosome consists of one DNA molecule
o One round of DNA replication but two rounds of cell division
o Metaphase 1: replicated dyads, paired with homologous chromosome across
metaphase plate
▪ Cell has to ensure one copy of each chromosome type goes to each pole of cell
to separate into daughter cells
o Anaphase 1: pull apart replicated dyads, homologs separated to opposite poles
o Metaphase II: chromosome types align singly along the plate
o During anaphase II, sister chromatids separate (single chromosomes, single DNA
molecules being pulled to opposite poles)
• Gametogenesis: when cells undergo meiosis generating gametes
o Female: oogenesis in ovary
▪ Generate haploid gametes: eggs
o Male: spermatogenesis in testis
▪ Generate haploid gametes: sperm
o Sperm (N) will locate and fertilize (N) egg generating a diploid zygote (2N)
o Zygote will undergo mitosis
▪ Cell specialization (differentiate) through epigenetic modifications to generate
all different cells/tissue in adult
• Spermatogenesis: continuous process in males
o Each meiosis produces 4 mature gametes (sperm cells)
• Oogenesis: discontinuous, beginning in the fetus (S phase while in utero) but arresting in
prophase I until puberty (cells sitting around stuck in prophase I)
o Meiosis I continues in one or a couple of oocytes each month
o Each meiosis produces one mature gamete (egg)
o Cells only go through meiosis II if fertilization occurs
▪ Secondary oocyte that undergoes ovulation (leave ovary and travel into
fallopian tube)
▪ Sperm find oocyte (mature egg) in fallopian tube
▪ If fertilized, cell will undergo meiosis II
• Crossing over between homologous chromosomes
o During prophase I of meiosis I
o To ensure only one copy of each chromosome type goes to each daughter cell
o Also allows for rearranging/mixing up of alleles on particular chromosome
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o Homologous chromosomes come together
o Crossing over (tetrad: 4 chromatids in contact)
▪ Different allele combinations
▪ 2 non sister chromatids recombine
o Recombinant chromatids
▪ Recessive and dominant alleles mix/change arrangement of alleles (NOT genes)
on particular chromosome
o Increases variation in offspring
o X and Y function as homologous pair during meiosis
▪ Recognize and combine together due to sequence similarities in telomere
regions (pseudoautosomal region) pairing partners
o Y chromosome: SRY region (sex determining region Y)
▪ Region responsible for making males male
▪ Depressing female development (default sex) (initiate development of embryo
as male)
o X and Y sequence is nonrecombining
▪ Can be minimal recombination that occurs in around 5% of X and Y
chromosomes in pseudoautosomal region
• Crossing over produces genetic variation:
o Without crossing over only circled gametes (AB and ab) would be possible
o Initially dominant allele for both genes on one chromosome and homolog has recessive
alleles
▪ Only way to mix up is cross over event (Ab, aB)
• Chromosomes are randomly distributed in meiosis
o Chromosomes align independently of one another during meiosis
o Increased genetic variation
o Paternal copy of chromosome 1 can segregate with maternal copy of chromosome 3
o Important that 1 copy of each homologous pair goes to each daughter cell, many
different arrangement and alignments can occur
o 223 = about 8.4 million different possible combinations of chromosomes in human eggs
or sperm
o 223 x 223 = over 70 trillion combinations
• Meiosis I: Metaphase I
o Homologous pairs line up along the metaphase plate
o Maternally derived centromere faces one pole, paternally derived faces the other pole
• Metaphase II
o Chromosomes (attached sister chromatids) line up individually along the metaphase
plate
• Anaphase II
o Sister chromatids separate to opposite poles for the first and only time in meiosis
o Individual chromosomes move toward opposite poles
• Mitosis and Meiosis
o Successful replication and division of DNA is extremely important
o Transmission of traits and patterns of inheritance
▪ Prevent complications such as cancer
o Aneuploidy in offspring
▪ Chromosomal abnormality
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Document Summary
Phenomena contributing to the generation of genetic variability during meiosis: (cid:3843) eg(cid:396)egatio(cid:374) a(cid:374)d i(cid:374)depe(cid:374)de(cid:374)t sso(cid:396)t(cid:373)e(cid:374)t (cid:894)me(cid:374)del"s la(cid:449)s(cid:895) (cid:3844) recombination (crossing-over) (cid:3843) mixes up possible allele combinations. Each eukaryotic species has a characteristic number of chromosomes. Chromosome structure: eukaryotic chromosomes exist in four major types based on the position of the centromere. Increased genetic variation: paternal copy of chromosome 1 can segregate with maternal copy of chromosome 3. Lacking both members of one particular chromosome type. Lacking one member of a particular chromosome type: result of nondisjunction event. In mitosis only impact daughter cells of particular somatic cells (less harmful, only affect subset of population of cells) In meiosis, additional copy/less copy present in all daughter cells of fertilized zygote: polyploidy (autotriploid): one or more complete sets of chromosomes, 3n = 9, haploid: n. In humans chromosomal abnormalities due to: aneuploidy, chromosome rearrangement, str expansions, snps, nondisjunction, the failure of homologous chromosomes to properly segregate during meiosis 1 or 2.
