Reproduction Flashcards
what is a gamete
sex cells of an organism
For example, the sperm and egg (ovum) cells in humans
how is zygote formed
Gametes fuse during fertilisation
Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)
how are sex cells formed
formed during meiosis and only have one copy of each chromosome, so they are haploid cells
adaptations of a sperm cell
Have a flagellum (tail) that allows them to swim towards the egg cell
mitochondria which carries out aerobic respiration provides energy for flagellum to move
An acrosome that contains digestive enzymes to break down the protective glycoprotein layer (a jelly-like coating known as the zona pellucida) surrounding the egg cell - sperm cells must penetrate this layer in order to fertilise the egg
Adaptations of egg cells
Are much larger than sperm cells as most of their internal space contains food to nourish a growing embryo
Have follicle cells that form a protective coating
Have a jelly-like glycoprotein layer, known as the zona pellucida, that forms an impenetrable barrier after fertilisation by a sperm cell has occurred, to prevent other sperm nuclei from entering the egg
what does the sperm cell follow
a chemical trail released by the egg cell and travel up through the cervix to reach the uterus. Then travel into the oviduct containing the egg cell
when can fertilisation occur
If a sperm cell meets the egg cell in the oviduct
when is it most likely for fertilisation to occur
1-2 days after the female has ovulated (i.e. released an egg cell from one of her ovaries into an oviduct)
what is the acrosome reaction
the acrosome of a sperm cell releases enzymes that digest a path through the protective outer layer of the egg cell (the zona pellucida), allowing the sperm to pass through the egg cell membrane
what happens after the acrosome reaction has occurred and what is this known as
Cortical reaction
the egg cell immediately releases the contents of vesicles known as cortical granules into the space between the egg cell membrane and the zona pellucida
The chemicals contained within the cortical granules cause the zona pellucida to rapidly thicken and harden, preventing any more sperm cells from entering, ensuring only one sperm cell can fertilise the egg cell
What do every chromosomes consist of
long DNA molecule that contains several hundred or even thousands of different genes coding for different proteins
what is a gene
base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule
what is a locus
The position of a gene on a chromosome
why do genes occupy specific locus
so that the gene for a particular characteristic is always found at the same position on a particular chromosome
what can genes exist as
two or more different forms called alleles
what is an autosome
any chromosome that isn’t a sex chromosome
what is autosomally linked
if two or more genes are located on the same autosome so a non-sex chromosome
do two or more genes on the same autosome assort independently
Two or more genes on the same autosome do not assort independently during meiosis
Instead, these genes are linked and they stay together in the original parental combination
These linked genes are passed on to offspring all together (through the gametes)
what is independent assortment
during independent assortment Genes with loci on different chromosomes are randomly distributed in gametes
what are sex linked genes
when the inheritance of a gene is dependant on the sex of the individual as some genes are only present on one sex chromosome and not the other
why are men more susceptible to sex linked genes
Most often sex-linked genes are found on the longer X chromosome
If the gene is on the X chromosome, males (XY) will only have one copy of the gene, whereas females (XX) will have two
Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions
why are female less likely to show sex linked recessive condition
Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele
what happens when loci of genes are closer
The closer the loci of the genes on the chromosome, the more closely linked they are. This is because they are far less likely to be separated during recombination in meiosis.
what is meiosis
gives rise to cells that are genetically different from each other and is the type of cell division used to produce gametes
how many types does the nucleus of parent cell undergo cell division
two rounds of division. These are:
Meiosis I
Meiosis II
in meiosis I is nucleus diploid or haploid
The nucleus of the original ‘parent’ cell is diploid (2n) i.e. it contains two sets of chromosomes
What happens before and during meiosis
Before meiosis I, these chromosomes replicate
During meiosis I, the homologous pairs of chromosomes are split up, to produce two haploid (n) nuclei
At this point, each chromosome still consists of two chromatids
When does the chromosome number half
in the first division of meiosis (meiosis I), not the second division (meiosis II)
what happens in meiosis II
the chromatids that make up each chromosome separate to produce four haploid (n) nuclei
At this point, each chromosome now consists of a single chromatid
what are benefits of meiosis
Having genetically different offspring can be advantageous for natural selection
Meiosis has several mechanisms that increase the genetic diversity of gametes produced
Both crossing over and independent assortment result in different combinations of alleles in gametes
what is crossing over
non-sister chromatids exchange alleles
how does crossing over work
During meiosis I homologous chromosomes pair up and are in very close proximity to each other
The paired chromosomes are known as bivalents
The non-sister chromatids can cross over and get entangled
These crossing points are called chiasmata
The entanglement places stress on the DNA molecules
As a result of this, a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
what does swapping of alleles lead to
new combination of alleles on the two chromosomes
what is independent assortment meiosis
production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during meiosis I
what increases genetic variation in gametes
different combinations of chromosomes in daughter cells
orientation of homologous pair?
Each pair can be arranged with either chromosome on top, this is completely random
The orientation of one homologous pair is independent/unaffected by the orientation of any other pair
what does combination of allele depend on
how the pairs of homologous chromosomes were lined up
how can you work out number of different chromosome combinations
formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell
what happens by the end of meiosis
each orientation gives two types of gamete so there are four types al together
how does meiosis given rise to genetic variation in gametes
independent / random assortment and crossing-over/chiasmata
independent assortment gives rise to new / different
combinations of (paternal and maternal) chromosomes ;
crossing over involves swapping of sections of chromatids /chromosomes
how does crossing over differ in sex chromosomes
crossovers cannot form between (some sections of) the X and Y chromosome (1)
because ( they are not
homologous chromosomes / the Y chromosome is shorter / there are alleles on the X chromosome that are not on the Y chromosome ](1)
describe the process of crossing over that occurs during meiosis
homologous chromosomes line up
(1)
* chiasmata form
(1)
break in {DNA / chromatid / chromosome } occurs
(1)
* genetic information exchanged between {chromatids} (1)
events following the acrosome reaction
sperm cell {fuses / eg} with egg cell (membrane) ;
reference to { cortical granules / vesicles / lysosomes};
idea of (cortical granules)
{moving towards / fusing with } egg cell (surface) membrane;
reference to exocytosis (of cortical granules / vesicles / lysosomes);
.
idea of contents (of cortical granules) {secreted /released into jelly layer / eq} OR reference to cortical reaction:
idea of { hardening / thickening / eq } of { zona pellucida / jelly layer }
compare and contrast the structure of a unfertilised egg and a zygote
Both made up of cell membrane and cytoplasm
both contain golgi and mitochondria in cytoplasm
unfertillised egg is haploid and zygote dipolid
unfertillised egg contain cortical granules and zygote doesnt
