Genetics
the study of heredity
Heredity
transfer of traits from one generation to the next
Parents pass genes to offspring consisting of a specific sequence of DNA in a specific locus
Passed genes provide code for enzymes or proteins whose cumulative action produces a trait
DNA passed via gametes
Meiosis (definition)
type of cell division that results in cells with half the number of chromosomes as the original cell
Asexual Reproduction
A single parent passes on its genes; Daughter cells have identical genes as parent cells called clones
Only exception is when mutations occur
Examples: Hydra (budding); Bacteria (binary fission)
Advantages
Faster; doesn’t require wasting energy on seeking out to find a sexual partner
Reproduce when conditions are favorable
Disadvantages
No genetic diversity; if it’s an environment that has changed, it does not have the diversity needed to survive
Genome
all the genes necessary to make an organism
somatic cells
all body cells except sex cells; contain 46 chromosomes in humans
gametes
sex cells which contain 23 chromosomes in humans
homologous chromosomes
pair of chromosomes that contain the same genes in the same loci; pairs of those same genes which code for same trait are called alleles
exception are the sex chromosomes in males (XY)
autosomes
all chromosomes other than sex chromosomes
sister chromatids
two identical copies formed by the DNA replication (S phase) of a chromosome
nonsister chromatids
chromosome couples having the same length, patterns and position of the centromere
karyotype
an individual's complete set of chromosomes; usually refers to the image
diploid
2n; has two sets of chromosomes
haploid
n; has one set of chromosomes
Sexual Reproduction
When 2 individuals contribute genetic material
Male contributes sperm made in the testis; female with egg made in ovaries
Each gamete contains half the number of chromosomes of a somatic cell (haploid; n)
When the 2 gametes unite, fertilization occurs; sperm dumps its nucleus into the egg (diploid; 2n)
Zygote now contains 2 sets of chromosomes
Zygote grows into multicellular organism through mitosis
Advantages
More genetic variation, chose their mate
the species can adapt to new environments due to variation, which gives them a survival advantage
Disadvantages
has to be under specific conditions
takes longer
Variation in Sexual Lifecycles
Animals
Two gametes fertilize each other forming a zygote (n + n = 2n)
The zygote does mitosis becoming a larger organism
The zygote then does meiosis forming gametes
Cycle continues
Plants and Some Algae
Two gametes fertilize each other forming a zygote (n + n = 2n)
The zygote does mitosis, grows into a multicellular organism (diploid organism called sporophyte)
The sporophyte does meiosis and produces spores
Spores grow into gametophytes and do mitosis to produce gametes
Cycle continues
Most Fungi and some protist
Two gametes fertilize each other forming a zygote (n + n = 2n)
Zygote does meiosis immediately (no mitosis)
Produces spores that do mitosis and become haploid organism
Gametes produced from mitosis
Cycle continues
Meiosis (basic overview)
Cells start off with two sets of chromosomes
Diploid: cells that have two sets of chromosomes
DNA replicated; creates two sister chromatids
Meiosis I: Homologous chromosomes separated and each goes into daughter
Each cell has 1 set of chromosomes
Haploid
Meiosis II: sister chromatids are pulled apart and each goes into daughter
Cells are STILL haploid!!
Mitosis
S & G2 of Interphase
DNA replication (S)
Centrosome replication (G2)
Prophase
Nuclear envelope breaks
Centrosomes start to move apart
Mitotic spindle starts to form
Nucleoli disappear
Prometaphase
Nuclear envelope disappears
microtubules attach to chromosomes and each other
Metaphase
Chromosomes line up on the metaphase plate
Centrosomes are at opposite ends
Anaphase
Chromosomes are pulled apart
cohesion bonds break
kinetochore microtubules shorten and pull them apart
Cell elongates as nonkinetochore microtubules elongate
Telophase
2 nuclear envelopes form creating 2 nuclei
nucleolus reappears
Remaining spindle is depolymerized
chromosomes begin to uncoil
contractile ring (actin+myosin) shrinks and pulls membrane inward
Cells separate
Prophase I
Chromatin condenses into chromosomes
Homologous chromosomes some into contact and match up
Synaptomeal complex holds them together
Synaptonemal complex is protein structure that pairs chromosomes up alone their entire length
Crossing over
Synaptonemal complex is broken down, but homologous chromosomes still attached because of cohesions with sister chromatids
Chiasmata: location where crossing over occurs
Other Events
Spindle starts to form
Centrosomes move apart
Nuclear envelope breaks down
Spindle attaches to chromosomes
Metaphase I
Homologous chromosomes (tetrads) line up on plate, one pair one each side
Tetrads: 4 chromatids
Anaphase I
Cell elongates from spindle
Homologous Pairs are pulled apart
Telophase I
Cytokinesis occurs
Cleavage furrow
2 haploid cells are formed; chromosomes are still composed of two sister chromatids
Centrosomes need to double
Prophase II
Centrosomes duplicated and pulled apart
Spindle starts to form
Metaphase II
Chromosomes line up on metaphase plate
Microtubules attach
Mitotic spindle formed
Anaphase II
Sister chromatids are pulled apart
Telophase II
Two cells formed by cytokinesis
Chromosomes consist of 1 sister chromatid
alleles
Different versions of the same gene
Genetic Variation
In species that reproduce sexually, meiosis and fertilization are responsible for most of the variation
In Meiosis…
Random/Independent Assortment
Crossing over
In Sexual Reproduction…
Random Fertilization
Independent Assortment
During Metaphases I and II
The position of each chromosome pair is by chance and independent of any pair
2^n = number of possible combinations of maternal and paternal combinations in an individuals gametes
In a human cell, there are 2^23 (8.4 million) possible combinations
Crossing Over
Occurs during Prophase I between homolgous chromosome pairs
Corresponding regions of sister chromatids are exchanged
Results in recombinant chromosomes
Increases genetic variation by making “new” chromosomes that are different combinations of maternal and paternal genes.
Each gene on one homologous chromosome is aligned with a corresponding gene on another homologous chromosome
DNA of 2 nonsister chromatids are broken by specific proteins at points and the 2 segments beyond the crossover point are each joined to the other chromatid
In detail
During early prophase I the homologous pairs held together along length by protein structure.
Protein is synaptonemal complex
Process is synapsis
During late prophase synaptonemal complex broken down so homologs pull apart slightly but are connect at the chiasmata.
Chiasmata are the result of cohesins btw the segments of sister chromatids that have been separated by crossing over.
Random Fertilization
The fact that a given sperm will randomly fertilize a given egg
2^23 x 2^23 = 70 trillion possible chromosomal combinations in the zygote.
Oogenesis
female gamete mitosis
Meiosis produces 1 ovum and 3 small polar bodies
Polar bodies are normally discarded
Spermogenesis
Produces 4 haploid cells of similar size
Non-disjunction
Occurs during meiosis I or meiosis II when sister chromatids/homologous chromosomes do not separate properly during anaphase
Chromosomes not properly aligned or pulsed apart resulting in extra chromosomes in one cell
Examples: Down Syndrome and Klinefelter Syndrome
Down Syndrome
Chromosome pair 21 has 3 chromosomes
Results in characteristic facial features, short stature, intellectual challenges, and increased risk of heart problems
Klinefelter Syndrome
In male, Sex chromosomes have additional chromosome (XXY)
Secondary sexual characteristics from more estrogen
Characterized with long arms and legs, wider hips, narrower shoulders, breast tissue growth