Passage of Information from Parents to Offspring (Cambridge (CIE) A Level Biology): Flashcards
Exam code: 9700
1/320Still learning
Know0
- FrontHaploidy & Diploidy
Define haploid (n).
Was this flashcard helpful?
Cards in this collection (32)
Define haploid (n).
A haploid cell contains a single set of chromosomes — one of each type, with no homologous pairs.
Gametes are haploid.
Define diploid (2n).
A diploid cell contains two complete sets of chromosomes (2n) — one set inherited from each parent, arranged in homologous pairs.
In humans, what are the diploid and haploid chromosome numbers?
Diploid (2n) = 46 — 23 homologous pairs in body cells.
Haploid (n) = 23 — found in gametes.
Why is a reduction division needed during meiosis in the production of gametes?
Meiosis halves the chromosome number from diploid (2n) to haploid (n).
This means that when two gametes fuse at fertilisation, the diploid number is restored in the zygote.
What would happen to the chromosome number over generations if gametes were diploid?
The chromosome number would double every generation.
The reduction division in meiosis prevents this by keeping gametes haploid.
A cell that contains two complete sets of chromosomes is described as .
A cell that contains two complete sets of chromosomes is described as diploid.
True or False?
A haploid cell contains homologous pairs of chromosomes.
False.
A haploid cell has only one of each type of chromosome, so there are no homologous pairs.
What is a homologous pair of chromosomes?
A pair of chromosomes — one maternal and one paternal — that:
carry the same genes at the same loci
are the same length with the centromere in the same position
The alleles they carry may differ.
Do homologous chromosomes carry identical alleles?
No.
They carry the same genes at the same loci, but the alleles of those genes may be different.
When and where do homologous chromosomes pair up?
During prophase I of meiosis.
Each homologous pair comes together to form a bivalent.
How many homologous pairs are there in a human diploid cell?
23 pairs (2n = 46).
Chromosomes that carry the same genes at the same loci form a pair.
Chromosomes that carry the same genes at the same loci form a homologous pair.
What are the two divisions of meiosis, and what separates in each?
Meiosis I (reduction division): homologous chromosomes are separated.
Meiosis II: sister chromatids are separated.
The result is four haploid cells.
Name the main stages of meiosis in order.
Meiosis I: prophase I → metaphase I → anaphase I → telophase I
Meiosis II: prophase II → metaphase II → anaphase II → telophase II
What happens during prophase I of meiosis?
Chromosomes condense and become visible
The nuclear envelope breaks down
Homologous chromosomes pair up to form bivalents
Crossing over occurs
The spindle begins to form
What happens during metaphase I and anaphase I?
Metaphase I: bivalents line up at the equator, attached to spindle fibres, with random orientation.
Anaphase I: homologous chromosomes are pulled to opposite poles (sister chromatids stay joined).
What is separated during anaphase II, and how does this differ from anaphase I?
Anaphase II: sister chromatids are separated to opposite poles.
Anaphase I: whole homologous chromosomes are separated.
How does cytokinesis at the end of meiosis differ between animal and plant cells?
Animal cells: the cell surface membrane pinches inwards (cleavage furrow) to divide the cell.
Plant cells: a cell plate forms and a new cell wall is laid down.
What is produced at the end of meiosis?
Four genetically different haploid cells.
Homologous chromosomes are separated during I of meiosis.
Homologous chromosomes are separated during anaphase I of meiosis.
In a diagram, how can you identify metaphase I?
Bivalents (pairs of homologous chromosomes) are lined up together at the equator of the cell.
How do you tell anaphase I from anaphase II in a photomicrograph?
Anaphase I: whole chromosomes (each still two chromatids) move to the poles.
Anaphase II: single chromatids move to the poles.
How can you distinguish metaphase I of meiosis from metaphase of mitosis?
In meiosis I, chromosomes line up as homologous pairs (bivalents) at the equator.
In mitosis, chromosomes line up singly.
What features identify a cell in prophase I?
Chromosomes are visible as bivalents, often showing chiasmata (points of crossing over), and the nuclear envelope is breaking down.
In metaphase I, homologous chromosomes line up at the equator as .
In metaphase I, homologous chromosomes line up at the equator as bivalents.
Define crossing over.
During prophase I, homologous chromosomes exchange sections of chromatid, swapping alleles between them.
This produces new combinations of alleles.
What is random orientation (independent assortment) in meiosis?
In metaphase I, each homologous pair lines up at the equator independently of the others.
So maternal and paternal chromosomes are distributed to gametes in random combinations.
How does crossing over produce genetic variation?
It swaps alleles between homologous chromosomes, creating new allele combinations on each chromosome and therefore genetically different gametes.
How does random orientation of chromosomes produce variation?
Homologous pairs (and sister chromatids in meiosis II) can line up in many different combinations.
This mixes maternal and paternal chromosomes, giving a huge variety of possible gametes.
How does the random fusion of gametes at fertilisation produce variation?
Any male gamete can fuse with any female gamete.
Because the gametes are already genetically different, this random fusion produces genetically different individuals.
The exchange of alleles between homologous chromosomes during prophase I is called over.
The exchange of alleles between homologous chromosomes during prophase I is called crossing over.
True or False?
Crossing over occurs during meiosis II.
False.
Crossing over occurs during prophase I of meiosis I, when homologous chromosomes pair up as bivalents.
Sign up to unlock flashcards
or