16.1 Passage of information from parent to offspring Flashcards
Inheritance
the transmission of genetic information from generation to (the next) generation.
Chromosome
a thread of DNA, made up of a string of genes.
Allele
any of two or more alternative forms of a gene.
Homologous Chromosomes
A pair of chromosomes in a diploid cell that have the same structure as each other, with the same genes (but not necessarily the same alleles of those genes) at the same loci, and that pair together to for a bivalent during the first division of meiosis
Types of nuclear division
1) Mitosis - growth
2) Meiosis (reduction division) - sexual reproduction
Meiosis
A type of cell division in which a diploid cell divides to form four haploid cells, in which different combinations of alleles are present. It involves 2 divisions, so 4 daughter cells are formed.
In Meiosis what do the cells have?
- Only half the chromosomes as the parent cell
- Different combinations of alleles from each other and from the parent cell
What does meiosis produce in animals and plants?
Gametes
How many pairs chromosomes are there in a cell?
- 23 pairs
- so 2 sets of chromosomes
- 22 matching pairs
- 1 non-matching pair labelled X and Y
Sex Chromosomes
X and Y non-matching chromosomes
Y chromosome has portions missing so is shorter than the X chromosome
Autosomes
All other chromosomes besides the sex chromosomes
Gene
a length of DNA that is the unit of heredity and codes for a specific protein. A gene may be copied and passed on to the next generation
Locus
The position at which a particular gene is found on a particular chromosome; the same gene is always found at the same locus
Each chromosome has a characteristic set of
genes that code for different features
abbreviation n =
the number of chromosomes in one set
of chromosomes.
Diploid
A cell that possesses two complete sets of chromosomes; the abbreviation for diploid is 2n
Haploid
A cell that possesses one complete set of chromosomes; the abbreviation for haploid is n
Centrosome
- produce the spindle fibers that are required during metaphase of mitosis.
- Each centrosome consists of two centrioles that are orientated at right-angles to each other.
- Each centriole is a cylindrical array of 9 microtubules
Importance of Meiosis
- Sexual Reproduction - introduces genetic variation into the gametes and therefore the zygotes that are produced.
Meiosis Divisions
1) Interphase
2) Meiosis I is a reduction division, resulting in two daughter nuclei with half the number of chromosomes of the parent nucleus.
3) In Meiosis II, the chromosomes behave as in mitosis, so that each of the two haploid daughter nuclei divides again. Meiosis therefore results in a total of four haploid nuclei.
Meiosis Steps
1) Prophase I
2) Metaphase I
3) Anaphase I
4) Telophase I and Cytokinesis
6) Prophase II
7) Metaphase II
8) Anaphase II
9) Telophase II and Cytokinesis
Prophase I
- Centrosomes divide and are involved in spindle formation
- Chromosomes begin to condense
- The spindle continues to form
- Homologous chromosomes pair up. This process is called synapsis.
- Each pair is called a bivalent
- Nuclear envelope breaks up as in mitosis
- Nucleolus disappears
- Crossing over of Chromatids may occur
Metaphase I
- Bivalents line up across equator of spindle, attached by centromeres
- spindle formed, as in mitosis
Anaphase I
- Centromeres do not divide, unlike in mitosis.
- Whole chromosomes move towards opposite ends of spindle, centromeres first, pulled by microtubules
Telophase I and Cytokinesis
- Animal cells usually divide before entering meiosis II.
- Chromosomes partly unwind
- Spindle disappears and the nuclear envelopes form
- Many plant cells go straight into meiosis II with no reformation of nuclear envelopes or nucleoli.
Prophase II
- nuclear envelope and nucleolus disperse
- centrosomes and centrioles replicate and move to opposite poles of the cell
Metaphase II
chromosomes line up separately across equator of spindle
Anaphase II
centromeres divide and spindle microtubules pull the chromatids to opposite poles
Telophase and Cytokinesis
- This is like telophase of mitosis, but in meiosis
telophase II four haploid daughter cells are formed - Spindles Disappear
- Nuclear envelopes reappear
- Chromosomes unwind
3 sources of Genetic Varation
- Crossing Over (Meiosis)
- Independent Assortment (Meiosis)
- Fusion between random gametes at fertilisation
Crossing Over
- Occurs at Prophase I
- The two homologous chromosomes lie side by side and their chromatids form links called chiasmata (singular chiasma) with each other
- When they move apart, a piece of chromatid from one chromosome may swap with a piece from the other chromosome. This is called Crossing Over
- It results in each chromosome having different combinations of alleles than it did before
Independent Assortment
- Shuffles the alleles like Crossing Over
- Occurs at Metaphase I
- The pairs of homologous chromosomes line up on the equator before being pulled to opposite poles of cell
- Each pair behaves independently from every other pair, so there are many different combinations that can end up together
- In a human there are 23 pairs, so there is a huge number of different possibilities
