Flashcards in Meiosis and Gametogenesis (Chapter 16) Deck (57)
What are 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 form a bivalent during the first division of meiosis
Where are the chromosomes from in the original zygote?
One chromosome of each pair came from the mother and one from the father
How many sets of how many chromosomes are there in a human?
Two sets of 23 chromosomes - one set of 23 from the mother and one set of 23 from the father
What are the sex chromosomes?
The non-matching x and y chromosomes, which determine the sex of the individual
What are the sex chromosomes in female humans?
2 x chromosomes
What are the sex chromosomes in male humans?
1 x chromosome and 1 y chromosome
What are autosomes?
All chromosomes besides the sex chromosomes
How can pairs of chromosomes be distinguished?
By size, shape and the distinctive bonding pattern when stained with certain stains
Each chromosomes has a characteristic set of...?
Where are genes found?
The gene for a particular characteristic is always found at the same locus on a chromosome
Describe the genes in homologous chromosomes
- Each member of a homologous pair possesses genes controlling the same characteristics
- A gene for a given characteristic may exist in different forms (alleles) which are expressed differently
- Homologous chromosomes carry the same genes on the same loci
What is a gene?
A length of DNA that codes for a particular protein or polypeptide
What is an allele?
A particular variety of gene
What is a locus?
The position at which a particular gene is found on a particular chromosome
- the same gene is always found on the same locus
What is a diploid cell?
A cell that contains two complete set of chromosomes
- represented as 2n, where n = the number of chromosomes in one set of chromosomes
What is a haploid cell?
A cell that contains one complete set of chromosomes (n)
What are human body cells?
What are human gametes?
Explain the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction
- For the life cycle to contain sexual reproduction, there must be a point before fertilisation takes place where the number of chromosomes is halved
- This results in the gametes containing only one set of chromosomes, rather than two sets
- If there were no point in the life cycle where the number of chromosomes halved, then the number of chromosomes would double with every generation
What is meiosis?
- A type of nuclear division in which daughter cells are produced that have half the normal amount of DNA and each one is genetically different
- The process of nuclear division in which the number of chromosomes in a cell halved
- Gametes are always haploid as a result of meiosis
What is meiosis sometimes described as and why?
A reduction division - because the number of chromosomes is reduced
How can genetic variation arise in gametes and therefore zygotes?
Through mitosis and mutation
What is meiosis 1?
A reduction division, resulting in two parent daughter nuclei with half the number of chromosomes of the parent nucleus - members of homologous pairs of chromosomes are separated
What happens during meiosis 2?
The chromosomes behave as in mitosis, so that each of the two haploid daughter nuclei divide again - the twin chromatids are separated
What does meiosis result in?
A total of 4 haploid nuclei
What two events that take place during meiosis help to produce genetic variation in the daughter cells?
1) Independent assortment of the homologous chromosomes
2) Crossing over, which happens between the chromatids of homologous chromosomes
- These form new combinations of alleles
What else produces more variation amongst offspring?
When these genetically different gametes fuse randomly at fertilisation
What happens during early prophase 1 of meiosis?
- Chromosomes start to appear as chromatin condenses (same as mitosis prophase 1)
What happens during middle prophase 1 of meiosis?
- Centrosomes move to opposite ends of the nucleus
- Synapsis - homologous chromosomes pair up, each pair is called a bivalent