The Cell Cycle, Mitosis, Meiosis Flashcards
What are the 2 Cell Types based on Chromosome Content?
- Gametes
- Somatic Cells
Gametes
- 23 different chromosomes = haploid (n)
- Includes ova (eggs) and sperm
Somatic Cells
- Includes all cells of body except cells undergoing/resulting from meiosis
- Muscle cell, nerve cell, etc
- 46 chromosomes = diploid (2n)
- = 23 Homologous pairs
What is Homologous Chromosomes?
- A chromosome pair (1 from each parent) that are highly similar in length, centromere position, and have genes for the same trait (e.g. eye colour) in the same location (locus)
- But the homologous chromosome may have different versions of that gene (alleles) that code for proteins that produce alternate versions of that trait
- e.g blue eyes (chromosome from one parent), brown eyes (chromosome from the second parent)
What are the 23 chromosome pairs that each somatic cell has?
- 23 chromosomes from the ovum paired with 23 chromosomes from a sperm = 46 chromosomes (diploid/2n somatic cell)
- Chromosomes and chromosome pairs are numbered 1-23:
- 1 to 22 = autosomal chromosomes (autosomes)
- 23 = sex chromosome
1 to 22 = autosomal chromosomes (autosomes)
- contain genes for somatic characteristics
- e.g. hair and eye colour, height
- Each autosomal chromosome pair is not identical, but equivalent = homologous chromosomes
23 = sex chromosome
- Contain genes that determine biological sex, XX = female & XY = Male
- X or Y from Sperm
- X from ovum
Cell Cycle
Diploid somatic cell (2n) → 2 genetically identical somatic diploid cells (2n)
What are the two stages of Cell Cycle?
- Interphase
- Mitotic (M)/ Cell Division phase
Interphase
- Chromosomes are present as long thread-like chromatin strands
- Normal cellular metabolic activities occuring e.g. protein synthesis
- Phases of Interphase:
- G1 Phase
- S Phase
- G2 Phase
What happens in G1 Phase?
- Growth, metabolism
- Centrosome replication begins
- NOTE: cells that do not divide again once mature, (i.e. remain in G1) are said to be in the G0 Phase
- e.g. nerve and some muscle cells
What happens in S Phase?
- Growth & DNA synthesis
- Chromosomes replicate, but are still present as long thread-like chromatin strands (not individually visible)
- ALWAYS occurs before cell division (mitosis and meiosis)
- Replicates are called sister chromatids (Identical chromatids)
- Attached to each other at an area of DNA called the centromere
- Kinetochores (proteins) form on each centromere (1/chromatid)
What Happens in G2 Phase?
- Growth, metabolism
- Production of enzymes & other proteins needed for cell division
- Centrosome replication is completed
What is Involved in the Mitotic Phase (M Phase)?
- Mitosis
- Cytokinesis
What is Mitosis?
- Division of nuclear material (chromosomes)
- 4 phases, but continuous cycle
- Prophase
- Metaphase
- Anaphase
- Telophase
- 4 phases, but continuous cycle
What is Cytokinesis?
Division of the cytoplasm
When does Mitotic Phase ends?
- Mitosis & cytokinesis complete
- Resulting diploid cells go into interphase (G1)
- Cycle starts over
What happens in Prophase?
(All over the place)
- Chromosomes condense (thicken, coil) - become individually visible
- Nucleoli disappear and nuclear envelope breaks up
- Centrosomes move to opposites sides (poles) of the cell
- Spindle microtubules grow out of centrosomes and attach to kinetochoreproteins of each sister chromatid, forming the spindle apparatus
- Spindle begins to move chromosomes toward cell equator
What happens in Metaphase?
46 replicated chromosomes line up on cell equator
What happens in Anaphase?
- Spindle microtubules shorten and pull the kinetochores away from each other - causes separation of centromeres, which results in separation of the sister chromatids into 92 individual chromosomes
- 46 chromosomes migrate to each pole
- Cytokinesis (division of cytoplasm) begins
What happens in Telophase?
(Prophase in Reverse)
- Chromosomes uncoil to long thread-like strands of chromatin
- nucleoli, nuclear envelope reappear
- Spindle disassembles
- Cytokinesis completes (or may end after telophase ends)
Meiosis
- Produces gametes (ovum and sperm)
- 1 cell (diploid, 2n) produces 4 genetically unique gametes (each haploid, n)
What is involved in Gamete Formation?
- Interphase (G1, S, G2)
↓ - 2 cell division
- Meiosis I
- Meiosis II
What happens in Meiosis I?
- Reduction division in which chromosome number changes from 2n to n
- events same as in mitosis except:
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
Prophase I
Homologous chromosomes attach together to form tetrads (4 chromatids in a row)
Metaphase I
- 23 tetrads line up along cell equator
Anaphase I
- Tetrads (homologous chromosome pairs0 separate and migrate to opposite poles i.e. 1 homologous chromosome (with 2 sister chromatids) goes to each pole
- Sister chromatids DON’T separate
Telophase I
Identical steps to Telophase of mitosis but with 23 replicated chromosomes instead of 46
What happens after Meiosis and cytokinesis?
- Each new cell has 23 different chromosomes (half the number of chromosomes) = haploid (n)
- These cells DO NOT have homologous chromosomes
- Each cell has a single copy of each autosomal chromosome and a single sex chromosome
Meiosis II
Is the same as mitosis except it starts with 23 chromosomes instead of 46
What are the events in Meiosis II?
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
- Cytokinesis
Metaphase II
23 replicated chromosomes line up at equator
Anaphase II
Sister chromatids split and 23 chromosomes migrate to each pole
Cytokinesis
Is the same as for mitosis/meiosis I (begins in anaphase/anaphase I/ anaphase II, and continues through telophase/ telophase I/ telophase II)
Metaphase (Mitosis)
46 chromosomes line up at equator
Anaphase (Mitosis)
Kinetochores split & 46 chromosomes migrate to each pole
After Cytokinesis (Mitosis)
- = interphase
- 2 identical diploid cells
- 23 pairs of chromosomes in each = 46 in total
Prophase I (Meiosis I)
Tetrads form
Metaphase I (Meiosis I)
23 tetrads line up at equator
Anaphase I (Meiosis I)
Tetrads split & 23 replicated chromosomes migrate to each pole
After Cytokinesis (Meiosis I)
- 2 haploid cells
- 2 nearly identical copies of each of 23 chromosomes with sister chromatids
Metaphase II (Meiosis II)
23 chromosomes line up at equator
Anaphase II (Meiosis II)
Kinetochores split, sister chromatids separate and 23 chromosomes migrate
After Cytokinesis (Meiosis II)
- = gametes
- 4 haploid cells
- 1 copy of each of 23 chromosomes
Why Meiosis?
- Need to turn diploid germ cells into haploid gametes (ova/sperm)
- 23 chromosomes from haploid (n) sperm cell combine with 23 chromosomes from haploid ovum (n) to form a single 46 chromosome cell called a zygote (2n)
