chapter 16 Flashcards
Sister chromatid
Single arm of the
chromosome
Somatic Cells
Any body cell in a multicellular organism EXCEPT sex cells or gametes Ex: Neurons - Skin cells - Blood cells - Muscle cells - Internal organ cells - Bone cells - .... any body cell!
The function of cell division for somatic cells is:
Growth
Add new cells to grow the organism
Repair
If damage occurs, the cells must replace
Maintenance
Cells must be replaced as they wear out
In a human somatic cell:
23 pairs of chromosomes
⇒ 22 pairs of autosomes (homologous chromosomes) ⇒ 1 pair of sex chromosomes: X and Y
Homologous chromosomes
Each parent provides the same 22 chromosomes, which code
for the same genes at the same locations (may have different versions of the trait - ALLELES)
Non-homologous chromosomes
Only X and Y (have different genes)
For cell division to occur, genetic material is duplicated and passed from original called to each new cell called
PARENT CELL
DAUGHTER CELLS
Male
XY
Female
XX
Ploidy
Number of sets of each chromosome
Diploid
Chromosomes in pairs (2n)
Human Somatic Cell: 23 pairs = 46 cs
Haploid
Set of unpaired chromosomes (n)
Human gamete cell: 23 cs
Polyploid
More than 2 of each homologous chromosome (3n, 4n, 6n)
Not in human cells
Human Karyotype
- An image of all the chromosomes in one nucleus, arranged from largest pair to smallest.
- Can be used to identify particular genetic conditions
- Geneticists use 3 features to find homologous pairs: • Length of chromosomes
- Pattern of dark bands when stained.
- Position of chromosomes centromere
Stages of the Cell Cycle
Interphase (growth and metabolism) ○ G1 phase ○ S phase (DNA REPLICATION) ○ G2 phase Cell division ○ Mitosis (dividing genetic material) ○ Cytokinesis (dividing cytoplasm)
Interphase
G1
After S phase
Unreplicated chromosomes have a single ‘arm’
Unreplicated chromosomes have a single ‘arm’
Replicated chromosomes have sister chromatids
PROPHASE
Phases of Mitosis
- Chromatin condenses into chromosomes
- Nuclear membrane breaks down
- Centrioles move to the poles of the
cell: spindle apparatus forms between them
METAPHASE
Phases of Mitosis
Spindle fibres attach to centromere of each chromosome and guide them to align along the equator of the cell
ANAPHASE
Phases of Mitosis
Centromere splits and sister chromatids are pulled to opposite poles
TELOPHASE (& CYTOKINESIS)
Phases of Mitosis
- Chromosomes (not chromatids anymore) at poles
- Strands unwind into chromatin
- Nuclear membrane forms at each
pole - Cytoplasm pinches off to form two
daughter cells
Cytokinesis in animal & plant cells
Animal cells form a cleavage furrow (contracted indent) and then separate
Plant cells have a cell well so a cell plate forms between the two daughter cells
Regulation of the cell cycle
n order for organisms to remain healthy, its cells must only divide at certain times.
Cells have START and STOP signals for cell division.
When these regulatory signals are not present (usually due to mutation) we get CANCER
How are sex cells different from somatic cells?
- Sex cells must have 1 set of each chromosome (HAPLOID)
- After FERTILIZATION, a DIPLOID cell will form
- They will need a type of cell division that REDUCES the number of
chromosomes- Purpose of sexual reproduction is to increase VARIATION in a population - Need to make gametes that have different combinations of genes
(RECOMBINATION)
- Purpose of sexual reproduction is to increase VARIATION in a population - Need to make gametes that have different combinations of genes
Recombination of Genes
This happens in gametes in two ways:
- Independent Assortment
- Chromosomes separate independently from one another - Crossing Over
- Homologous chromosomes can exchange genes
Interphase for Germ Cells
Germ cells: cells that will divide to form gametes
- They also have an interphase, and DNA replication still occurs in S phase before meiosis
- Chromosomes will have sister chromatids after replication
PROPHASE I
Phases of Meiosis
- Same as prophase in mitosis EXCEPT:
- Duplicated chromosomes find
homologous pair (synapsis) and form a tetrad - Crossing-over: physical exchange chromosomes parts
METAPHASE I
Phases of Meiosis
Homologous chromosomes are lined up IN PAIRS at equator of cell
- Spindle fibres from one pole attaches to one cs in pair and spindle fibres from the other pole attach to the other cs
- Chromosomes INDEPENDENTLY ASSORT along equator
ANAPHASE I
Phases of Meiosis
Homologous chromosomes separate from one another
- Each pole is now HAPLOID
TELOPHASE I
Phases of Meiosis
Same as telophase in mitosis EXCEPT: - Each new cell is HAPLOID and
genetically different from one another
PROPHASE II
Phases of Meiosis
DNA replication does not occur before second division of meiosis
- Same as prophase in mitosis except two haploid cells
METAPHASE II
Phases of Meiosis
Same as metaphase in meiosis except two haploid cells:
- Spindle fibres attach to centromere of each chromosome and guide them to align along the equator of the cell
ANAPHASE II
Phases of Meiosis
Same as anaphase in mitosis except two haploid cells:
- Centromere splits and sister chromatids are pulled to opposite poles
TELOPHASE II
Phases of Meiosis
Same as telophase in mitosis except results in 4 genetically different HAPLOID cells:
- Chromosomes (not chromatids anymore) at poles
- Strands unwind into chromatin
- Nuclear membrane forms at each
pole
Oogenesis
Before birth, ovaries contain all primary oocytes and she will not make new ones
Secondary oocyte is ovulated and second meiotic division occurs at fertilization
1 large haploid cell results
- Meiosis I and II results in
unequal division of cytoplasm and smaller cells degenerate as polar bodies
Spermatogenesis
Primary spermatocytes (start of meiosis) does not start until puberty 4 haploid cells result of equal size
Monozygotic
zygote splits ->genetically identical -> shared placenta
Dizygotic
Ovum splits-> genetically distinct (1/2) -> separate placenta
What could go wrong with the gamete formation?
Nondisjunction
Nondisjunction
Failure of the homologous chromosomes or sister chromatids to separate properly during cell division
Ex: Trisomy 21 (Down’s syndrome) Monosomy XO (Turner’s syndrome)
Life cycle for sexual reproduction
Meiosis occurs to produce haploid sperm and eggs
-gametes have half the genetic material *Fertilization occurs to make a diploid zygote *Zygote undergoes mitosis to form an adult
Asexual reproduction
Less variety but can be advantageous if environmental conditions are favourable!
- Binary fission: parent cell undergoes mitosis to produce two identical daughter cells
- Fragmentation: parent cell breaks into parts which can all grow into a full sized adult
- Budding: new organism grows from a bud off of the parent -Parthenogenesis: growth of a haploid adult from an unfertilized egg
Parthenogenesis: example with bees!
All male bees are haploid Females are diploid
Males produce sperm cells by mitosis and queen produces eggs through meiosis. If fertilization occurs: female; if no fert: male
Alternation of Generations: Plants and Fungi
- Some organisms can alternate between a sexual reproduction stage and an asexual reproduction stage as part of their life cycle
- They will have a DIPLOID generation (structure that has 2 of each chromosome) and a HAPLOID generation (structure that has 1 of each chromosome)
- Diploid generation is called the SPOROPHYTE
- Haploid generation is called the GAMETOPHYTE
SPOROPHYTE
-Diploid generation
GAMETOPHYTE
Haploid generation
Alternation of generations
plants alternate between a diploid organism and a haploid organism
Sporophytes
always produce SPORES through meiosis (diploid → haploid) -Spores will then grow into gametophyte generation (haploid → haploid)
Gametophytes
always produce GAMETES through mitosis (haploid → haploid) -Fertilization of gametes produces sporophyte (haploid + haploid → diploid)
