genetics: chapter 13 Flashcards
How do genes contribute to heredity, and what is the transmission of traits based on? Why?
Genes are hereditary units holding coded information that is passed from parents to offspring. The transmission of traits is based on precise DNA replication. In order to pass hereditary information, parents must replicate their DNA to pass on.
What are gametes, and the roles they play in reproduction?
- Gametes are reproductive cells (also known as sperm and egg cells) that transmit genes from one generation to the next.
- Gametes are haploid; they contain one set of chromosomes. During fertilization, a sperm and egg cell will unite to form a diploid (zygote), which will eventually undergo mitosis to develop into an embryo.
- In reproduction, the fusion of the gametes (during fertilization) are what pass genes from both parents to offspring.
Where is the DNA located in a eukaryotic cell?
The DNA molecules are linear and packaged into chromosomes within the nucleus. They are associated with various histone proteins.
Small amounts of DNA may also be found in the mitochondria and chloroplasts.
Describe a diploid vs. haploid. How many chromosomes do humans have, and in which cells?
A diploid cell has two sets of chromosomes: one set from each parent. A haploid cell consists of only one set of chromosomes.
Humans have 46 chromosomes. Each cell has two sets of chromosomes, so they inherit 23 chromosomes from each parent. This makes them diploid.
Humans have 46 chromosomes in their somatic cells. They also have haploid cells, which are the gametes and precursors (cells that give rise to the gametes).
Describe which cells are precursors to gametes and explain them.
Germ cells. These are specialized cells that migrate to and differentiate within the gonads.
Gonads are the primary reproductive organs. For males, these are testes, and for females, these are ovaries.
Germ cells will undergo a process called gametogenesis (the formation of gametes). During this process, the diploid germ cells will undergo meiosis to give rise to the gametes, which are haploids.
Describe in detail the chromosome.
A chromosome consists of one long DNA molecule that is coiled tightly in association with various proteins. The DNA molecule carries the genetic information (genes).
One chromosome can have hundreds to thousands of genes.
An unreplicated chromosome looks like one strand. When replicated, it consists of two sister chromatids.
In the cell cycle, a chromosome undergoes interphase, which consists of G1 (chromosomes are relaxed, uncoiled, and not visible under the light microscope), S phase (chromosomes duplicate to form 2 sister chromatids connected at the centromere), and G2 (remain duplicated and prepare for mitosis).
Describe a gene’s locus.
A gene’s specific location along the chromosome. Each gene of an organism’s genome has a specific locus unique to that of the other genes. Alleles are variations of a gene that occupy the same locus on a homologous chromosome pair. One homolog may have a gene with a specific allele, the other homolog may have the gene with the other allele. They occupy the same positions on their respective chromosome.
What is asexual vs. sexual reproduction
Asexual reproduction: one parent produces identical offspring (clones) through mitosis; without the fusion of gametes (fertilization). Genetic variation may occur within offspring if there are genetic mutations.
Sexual reproduction: two parents give rise to genetically varying offspring. This is due to the behavior of the chromosomes during the sexual life cycle.
Describe the behavior of chromosomes during the sexual life cycle.
During mitosis, chromosomes become condensed enough to become visible under the light microscope. A single somatic cell has 23 types of chromosomes and 2 of each type. The two chromosomes making one type (one chromsome from each parent) is similar in size, centromere position, and staining pattern. These homologs carry genes that control the same inherited characters (although they may be varying in the allele; the type of gene)
Discuss the exception to the behavior of chromosomes during the sexual life cycle.
Females have a homologous pair of X chromosomes (XX) but males have one X and Y chromosome (XY). XX chromosomes are homologous, but only small parts of the X and Y chromosome are homologous.
E.g.: The Y chromosome is much smaller than the X, and does not carry most of the genes on the X. In addition, the Y has genes that are not on the X.
Why do homologous chromosomes occur?
They occur due to our sexual origins. We inherit one set of chromosomes from each parent; thus, our somatic cells are inherited half from our fathers and half from our mothers.
The exceptions to this are our gamete cells, which are haploids (one set from our mother or father). An unfertilized egg will always contain an X chromosome. However, as a male consists of both an X and Y, a sperm can contain an X OR a Y chromosome.
Describe the human life cycle
The human life cycle begins with fertilization. The resulting fertilized egg, the zygote (diploid), will undergo mitosis and generate (along with its descendents) all the somatic cells in the body.
The gametes are developed from specialized germ cells (come from the gonads). They form through meiosis, which halve the number of chromosomes from the germ cells (which are diploid).
When the gametes fuse during fertilization, the resulting zygote is combined of both haploids, therefore restoring the diploid number.
What would happen if the gametes were formed by mitosis?
If germ cells went through mitosis, the resulting gametes would be diploids. Then, when they fuse during fertilization, they will have double the number of chromosomes as the diploid germ cells. With each subsequent generation, the diploid chromosome number would continue to double.
By undergoing meiosis, this counterbalances the doubling of chromosomes that would happen with mitosis. Therefore, the human life cycle would continue the same as the previous generation (the same number of chromosomes each time).
Describe the mechanisms of a karytope. What is its difference from a karyogram? What does it screen?
A karyotype describe the number of chromosomes and what they look like (size bands, centromere placement).
A karyogram is an image of the chromosomes arranged in homologous pairs from longest to shortest (decreasing length) and by centromere position. It is prepared with isolated somatic cells, which are stimulated with drugs to undergo mitosis. During metaphase, a cell is arrested and stained. The chromosomes will be seen under a microscope with a digital camera equipped.
A karyogram can screen for defective or abnormal chromosomes associated with certain disorders, such as Down Syndrome
