heredity Flashcards
How does sexual reproduction lead to variation?
Sexual reproduction increases genetic variation in offspring, which in turn increases the genetic variability in species. Sexual reproduction promotes genetic variation by producing different gene combinations. Meiosis is the process by which sex cells or gametes are created. Genetic variation occurs as alleles in gametes are separated and randomly united upon fertilization. The genetic recombination of genes also occurs during crossing over or the swapping of gene segments in homologous chromosomes during meiosis.
Advantages and disadvantages of sexual/asexual reproduction.
Sexual reproduction involves two parents and the joining of male and female gametes during fertilisation. The offspring inherit a mixture of genes from both parents, so are different to each other and their parents.
Sexual reproduction- ADVANTAGES
• produces genetic variation in the offspring
• the species can adapt to new environments due to variation, which gives them a survival advantage
• a disease is less likely to affect all the individuals in a population
DISADVANTAGES
• time and energy are needed to find a mate
• it is not possible for an isolated individual to reproduce
Asexual and sexual reproduction in fungi
Some fungi are able to reproduce both sexually and asexually. Fungi reproduce using spores which they release into the environment. A new fungus will grow from the spore. The Coprinus cinereus fungus can produce spores by sexual reproduction to help create variation in the species. This method of reproduction is advantageous when the environment is changing because the variation introduced leads to an increase in the probability that a variant that can deal with the change. Coprinus cinereus also produces spores by asexual reproduction. These spores can be produced quickly and in large numbers to enable many individual fungi to develop. A disadvantage of these spores is that they generate offspring that are unlikely to be resistant to unfavourable conditions because they are all genetically identical.
What is binary fission?
Binary fission is a single cell that divided into two identical daughter cells. It begins with DNA replication where the genetic information of the bacteria is copied and divided into two. The cell extends and splits into two, producing daughter cells with identical genetic information.
Examples of asexual reproduction with examples
Budding- the nucleus divides and a budge forms in the side of the cell
Spores- mitosis produces genetically identical cells to the parent
Internal(terrestrial) and external (aquatic) fertilization
Advantages/disadvantages
Internal fertilisation- when a male transfers his gametes directly into the females body through the penis. Examples include: Mammals (humans), Birds, reptiles, terrestrial plants
Advantages-
• higher fertilisation chance
• gametes are protected from disease
Disadvantages-
• parental care can be lengthy and demanding
• mating rituals are complex and tough.
Process of fertilisation and implantation in mammals
- sexual reproduction begins with the development of gametes
- in females this is in the ovaries, where ovum (eggs) are produced and released into the fallopian tubes.
- Once fertilisation has occurred, the zygote begins to divide and migrate from the fallopian tube into the uterus.
- The blastocyst embeds itself onto the wall of the uterus (endometrium)
- This is a nutrient dense lining which will provide oxygen and nutrients to the growing embryo. This occurs 7 days after fertilisation, establishing pregnancy
Functions of hormones – oestrogen, progesterone and oxytocin
Luteinising hormone- Triggers ovulation and development of corpus luteum.
Oestrogen- main female sex hormone, triggers ovulation and maintains and stimulates production of other hormones. E.g oxytocin
Progesterone- released by corpus luteum during implantation, first trimester to maintain endometrium, then by the placenta to prevent contractions and miscarriage.
Oxytocin- released towards end of third trimester to stimulate contractions
Prolactin- released during third trimester to stimulate lactation.
Mitosis/Meiosis
How does meiosis lead to variation – crossing over, independent assortment and random segregation
Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA. This produces a unique combination of genes in the resulting zygote. Recombination or crossing over occurs during prophase I. Homologous chromosomes – 1 inherited from each parent – pair along their lengths, gene by gene. Breaks occur along the chromosomes, and they rejoin, trading some of their genes. The chromosomes now have genes in a unique combination. Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis. A gamete will end up with 23 chromosomes after meiosis, but independent assortment means that each gamete will have 1 of many different combinations of chromosomes. This reshuffling of genes into unique combinations increases the genetic variation in a population and explains the variation we see between siblings with the same parents.
Differences- Mitosis
• Involves one cell division
• Results in two daughter cells
• Results in diploid (two sets of chromosomes) daughter cells (chromosome number remains the same as parent cell)
• Daughter cells are genetically identical
• Occurs in all organisms except viruses
• Creates all body cells (somatic) apart from the germ cells (eggs and sperm)
• Prophase is much shorter
• No recombination/crossing over occurs in prophase.
• In metaphase individual chromosomes (pairs of chromatids) line up along the equator.
• During anaphase the sister chromatids are separated to opposite poles.
Meiosis
• Involves two successive cell divisions
• Results in four daughter cells
• Results in haploid (One set of chromosome) daughter cells (chromosome number is halved from the parent cell)
• Daughter cells are genetically different
• Occurs only in animals, plants and fungi
• Creates germ cells (eggs and sperm) only
• Prophase I takes much longer
• Involves recombination/crossing over of chromosomes in prophase I
• In metaphase I pairs of chromosomes line up along the equator.
• During anaphase I the sister chromatids move together to the same pole.
• During anaphase II the sister chromatids are separated to opposite pole
Stages of meiosis :
Meiosis I
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
Prophase-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
Metaphase-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
Anaphase-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
Telophase-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
Meiosis II
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
Protein synthesis
Transcription refers to the first step of gene expression where an RNA polymer is created from a DNA template. This reaction is catalyzed by enzymes called RNA polymerases and the RNA polymer is antiparallel and complementary to the DNA template. The stretch of DNA that codes for an RNA transcript is called a transcription unit and could contain more than one gene.
Patterns of inheritance
Dominant- An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself.
Recessive- Describes the variant of a gene for a particular characteristic which is masked or suppressed in the presence of the dominant variant. A recessive gene will remain dormant unless it is paired with another recessive gene.
Genotype- The alleles that an organism has for a particular characteristic, usually written as letters.
Phenotype- The visible characteristics of an organism which occur as a result of its genes.
A homozygous individual has identical alleles for the same characteristic, for example AA or aa.
A heterozygous individual has two different alleles for the same characteristic, for example Aa.
