inheritance Flashcards
what is a diploid cell?
A cell that contains 2 complete sets of chromosomes (2n)
These chromosomes contain the DNA necessary for protein synthesis and cell function
Nearly all cells in the human body are diploid with 23 pairs (46) of chromosomes in their nucleus
what is a haploid cell?
a cell that contains one complete set of chromosomes (n)
In other words they have half the number of chromosomes compared to diploid cells
Humans have haploid cells that contain 23 chromosomes in their nucleus
These haploid cells are called gametes and they are involved in sexual reproduction
For humans they are the female egg and the male sperm
Haploidy and diploidy are terms that can be applied to cells across different species
They describe the number of sets of chromosomes, not the total number of chromosomes
whate is a gamete?
reproductive (sex) cells that fuse with another during fertilisation
zygote
the diploid cell from which a cell develops
- forms from the fusion of gametes.
What happens during fertilization ?
- the nuclei of gametes fuse together to form the nucleus of the zygote
Both gametes must contain the correct number of chromosomes in order for the zygote to be viable. If a zygote has too many or too few chromosomes it may not survive
For a diploid zygote this means that the gametes must be haploid
n + n = 2n
what does meiosis produce?
haploid gametes during sexual reproduction
What is the first cell division of meiosis called?
a reduction division
This is a nuclear division that reduces the chromosome number of a cell
In humans the chromosome number is reduced from 46 (diploid) to 23 (haploid)
The reduction in chromosome number during meiosis ensures the gametes formed are haploid
chromosomes have a characteristic shape
They have a fixed length
The position of the centromere is in a particular location
These characteristic features allow for each chromosome to be identified in a photomicrograph
In photomicrographs chromosomes are often grouped into their homologous pairs
homologous chromosomes
Carry the same genes in the same positions
Are the same shape
During fertilization what type of zygote forms?
a diploid zygote is formed
In a zygote one chromosome of each homologous pair comes from the female gamete and the other comes from the male gamete
Having the same genes in the same order helps homologous chromosomes line up alongside each other during meiosis
Although homologous pairs of chromosomes contain the same genes in the same order they don’t ….
necessarily carry the same alleles (form) of each gene
meiosis
is a form of nuclear division that results in the production of haploid cells from diploid cells
It produces gametes in plants and animals that are used in sexual reproduction
what are the 2 divisions of meiosis called:
meiosis I and meiosis II
Within each division there are the following stages: prophase, metaphase, anaphase and telophase
prophase I
DNA condenses and becomes visible as chromosomes
DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere
The chromosomes are arranged side by side in homologous pairs
A pair of homologous chromosomes is called a bivalent
As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
In this stage centrioles migrate to opposite poles and the spindle is formed
The nuclear envelope breaks down and the nucleolus disintegrates
Metaphase I
The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
Anaphase I
The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
The centromeres do not divide
Telophase I
The chromosomes arrive at opposite poles
Spindle fibres start to break down
Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
Cytokinesis
This is when the division of the cytoplasm occurs
Cell organelles also get distributed between the two developing cells
In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
The end product of cytokinesis in meiosis I: two haploid cells
These cells are haploid as they contain half the number of centromeres
Meiosis II
There is no interphase between meiosis I and meiosis II so the DNA is not replicated
The second division of meiosis is almost identical to the stages of mitosis
Prophase II
The nuclear envelope breaks down and chromosomes condense
A spindle forms at a right angle to the old one
Metaphase II
Chromosomes line up in a single file along the equator of the spindle
Anaphase II
Centromeres divide and individual chromatids are pulled to opposite poles
This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell
Telophase II
Nuclear membranes form around each group of chromosomes
Cytokinesis in Meiosis II
Cytoplasm divides as new cell surface membranes are formed creating four haploid cells
The cells still contain the same number of centromeres as they did at the start of meiosis I but they now only have half the number of chromosomes (previously chromatids)
Meiosis I or Meiosis II
Homologous chromosomes pair up side by side in meiosis I only
This means if there are pairs of chromosomes in a diagram or photomicrograph meiosis I must be occurring
The number of cells forming can help distinguish between meiosis I and II
If there are two new cells forming it is meiosis I but if there are four new cells forming it is meiosis II