Meiosis + Sexual Reproduction Flashcards
Heredity
The transmission of traits from one generation to the next
–> Also known as “inheritance”
Impacts of inherited variation:
Permits flexibility and survival of a population in a changing environment
Genetics
The study of heredity and inherited variation
–> Attempting to answer: “What accounts for similarities + variation?”
Transmission (of traits)
The passage of DNA, on chromosome, from parents to offspring
–> What makes inheritance possible
Genes
DNA segments that encodes for specific traits that emerge as we develop
Genes are the _________ _________ to our parents and account for family ____________
1) Genetic link
2) Resemblance
Most genes program cells to synthesize…
specific enzymes and proteins whose cumulative action produces an organism’s inherited traits
Gametes
Reproductive (sex) cells that transmit genes from one generation to the next
Somatic Cells
All cells of the body EXCEPT gametes (and their precursors)
Ploidy
The # of SETS of chromosomes within a given cell
Haploid
= n
–> Single set of chromosomes
Diploid
= 2n
–> Two sets of chromosomes
Human haploids and diploids
Gametes = haploid = 23 chromosomes
Somatic Cells = diploid = 46 chromosomes
We inherit one chromosome of each pair from…
Each parent = one maternal chromosome set + one paternal chromosome set
Even after DNA is replicated to form duplicated chromosomes, we still refer to the cells as being…
Haploid or diploid
–> BECAUSE the cell still only has one or two sets of info REGARDLESS of how many chromatid copies there are of that info
Homologous Chromosomes
The maternal and paternal copies of a chromosome found in diploid cells
Similarities Between Homologous Chromosomes
1) Length
2) Centromere Position
3) Type of genes/Order of genes found/gene loci
Differences Between Homologous Chromosomes
Can possibly differ in alleles
–> Same genes, different alleles
Homologous chromosomes carry genes that…. (1)
BUT the _______________________(2) of these genes may differ between them
1) Control the same inherited characteristics
2) Variants/versions of these genes
Homologous chromosomes ARE NOT…
Sister Chromatids (which are IDENTICAL copies of each other)
Locus
Location/Position of a gene in a chromosome
Allele
A variant or form of a gene
How many alleles of each genes do diploids have?
2 alleles of each gene
(One from mother, one from father)
Sex Chromosomes
Chromosome that determine sex (X + Y)
Sex Chromosomes ARE NOT…
Homologs
Our 46 chromosomes =
44 (22 pairs) autosomes + 2 sex chromosomes
Autosomes
All chromosomes EXCEPT sex chromosomes
–> Essentially, the homologous chromosomes
Karyotype
An ordered display of all human chromosomes arranged in pairs –> How we can study chromosomes
How does karyotype work?
Homologs stain similarly making it easy to pair up homologs by shared size/length and staining
2 different types of cell division:
1) Asexual Reproduction
2) Sexual Reproduction
Asexual Cell Division
A single individual (cell) is the sole parent
–> = Genetically identical offspring (clones)
1) Mitosis
2) Binary Fission
Sexual Cell Division
TWO parents contribute DNA to one offspring
–> = Genetically different (unique) offspring
The # of chromosomes/genetic content must…
remain constant from cell to cell and generation to generation
Meiosis
A specialized cell division in which gametes are produced whose chromosome # is reduced to HALF
How is diploid restored after meiosis?
The fusion of gametes (haploid cells) restore diploid state in daughter cell
What must occur in all sexual life cycles?
An alternation of fertilization and meiosis
Fertilization
The fusing of gametes resulting in the fusion of their nuclei, producing a zygote
Zygote
Cell produced by gametes fusing (fertilized egg)
Higher Eukaryote Sexual Life Cycle
Diploid Organism —-> (Meiosis) —-> Haploid gametes
—-> (Fertilization) —-> Diploid Zygote —-> (Mitosis) —> Diploid Organism
Lower Eukaryote Sexual Life Cycle
Haploid Organisms —> (Mating) —> Diploid Zygote —> (Meiosis) —> Haploid Cells —> (Mitosis) —> Haploid Organisms
Sexual life cycles differ between organisms BUT…
it always involves an alternation between haploid and diploid cells
Ploidy of cells that can undergo mitosis and meiosis
Mitosis = Haploid or Diploid can undergo
Meiosis = Only diploid can undero
Meiosis is this type of division:
Reductional Division –> Reduces # of chromosome sets by half
Daughter Cells of Meiosis
1/2 # chromosomes BUT genetic info is equivalent (maintains same genes just in one copy)
Germ Cells
The cells that undergo meiosis to produce gametes
Components of Meiosis
1) Interphase (DNA rep. occurs)
2) Meiosis I –> Cell Division I
3) Meiosis II –> Cell Division II
What does meiosis produce?
4 daughter cells with ONE unreplicated copy of each chromosome
Meiosis has ONE _________ but TWO ______________
1) Round of DNA replication
2) Cell Divisions
Meiosis I
Homologous chromosomes pair and then are separated
–> The reductional step: Reduces chromosomes by 1/2 in each cell
Meiosis II
Sister chromatids are separated (like regular mitosis)
= 4 daughter cells with 1/2 # of chromosomes
What is the critical difference between meiosis and mitosis?
Meiosis I –> The pairing and separation of homologs
Prophase I (5)
1) Nuclear envelope breaks down
2) Chromosomes progressively condense
3) Homologs pair (synapsis)
4) Crossing over occurs –> DNA exchanges
5) Mitotic spindle formation + attachment of MT to kinetochores
Synapsis
Homologs pair up with each other by physically attaching (synapse)
Chiasmata
Physical bridge between homologs: The point where crossing over occurs
Synaptonemal Complex
A protein structure/complex that forms between homologous chromosomes (between non-sister chromatids) that attached them along their length
Synaptonemal Complex Function
Functions primarily as a scaffold to allow interacting chromatids to complete crossing over
Synaptonemal Complex Disassembles when…
when chiasmata fully form
Non-Sister Chromatids
Chromatids from homologous chromosomes
–> Have the same genes but may differ in alleles
Crossing Over
DNA molecules of non-sister chromatids are BROKEN and exchange equivalent bits of DNA with each other
–> Causes exchange of genetic material
Purposes of Crossing Over:
1) Establishes physical connections between homologs
2) Exchange genetic info which causes INCREASE of genetic variation
Crossing over causes
Recombination
Recombination
The production of different combinations of alleles
–> Each gamete produced has a different combo of alleles
Metaphase I (2)
Chromosomes line up by homologous pairs
1) Microtubules from spindles on opposite poles attach to each homologous chromosome
2) Pairs of the homologous chromosomes are now arranged at the metaphase plate with one chromosome of each pair facing each pole
Kinetochore Orientation in Metaphase I
Same-Side Kinetochore Orientation (Kinetochores of sister chromatids align side-to-side
–> BOTH kinetochores in ONE homolog gets attached by MT of the same pole of the spindle
Spindle Attachment to the Homologs
Each homolog is connected to microtubules from opposite poles (to allow for separation of homologs)
Anaphase I (2)
Separation of homologs
1) Cleavage/breakdown of proteins (cohesins) that are responsible for holding homologs together
2) Microtubules pull each homolog toward opposite poles
What persists in anaphase I?
Sister chromatid cohesins persist
–> Sister chromatids remain attached after anaphase I
Telophase I (2)
+ Cytokinesis
Formation of 2 haploid cells
1) The separated homologs cluster at each pole of the cell
2) Nuclear membrane reforms around each daughter cell nucleus
–> Followed by cytokinesis
What is different about the sister chromatids found in cells produced by Meiosis I?
They are NOT IDENTICAL –> Due to earlier crossing over
Uniqueness of Meiosis to Mitosis (3):
1) Synapsis and Crossing Over –> In prophase I, homologs physically connect and exchange genetic info
2) Metaphase I –> Homologs pair up at the metaphase plate
3) Anaphase I –> Homologs separate and get pulled to opposite poles
The greater the variation…
The greater the chance of survival of species in an unpredictable event
Origins of genetic variation:
Comes down to the behavior of chromosomes during meiosis and fertilization
1) Independent Assortment
2) Crossing Over (Recombination)
3) Random Fertilization
Independent Assortment
The different possible arrangements of homologs at Metaphase I –> Causes different variants of a chromosome ending up in the final daughter cells
Independent Assortment: What causes it?
Each homolog pair can arrange in 1 of TWO ways independently of other pairs
–> How they orient themselves at metaphase determines which variants go to which pole (and which cell) they end up in
What does independent assortment produce?
Different combinations of alleles of genes
How many unique gametes are possible?
2^n
n = haploid # chromosomes
Recombinant Chromosomes
Carry genes from 2 different parents in ONE chromosome
How many cross overs occur per homologous pair?
At least 2-3
Random Fertilization
During sexual reproduction, the male and female gametes that end up fusing are selected randomly from the large pool of male + female gametes
–> Which gametes fuse = matter of chance
The chance of any one sperm fusing with any one egg (for humans):
(2^23) x (2^23) = ~70 trillion
–> Every person is a 1 in 70 trillion chance
–> Makes it virtually impossible for humans to produce identical offspring through separate pregnancies
